[IMP] compiler: add support for t-for directive

This commit adds support for the `t-for` whose syntax and usage is
similar to the syntax of the for..of loop in JS (and in fact it compiles
to a for..of loop). This looping construct supports looping on arbitrary
iterables and destructuring assignments, which would be difficult to
support in a backward compatible manner on the existing t-foreach
directive.

doc/reference/app.md

@@ -117,7 +117,7 @@ const { loadFile, mount } = owl;
 Dev mode activates some additional checks and developer amenities:
 
 - [Props validation](./props.md#props-validation) is performed
-- [t-foreach](./templates.md#loops) loops check for key unicity
+- [t-for and t-foreach](./templates.md#loops) loops check for key unicity
 - Lifecycle hooks are wrapped to report their errors in a more developer-friendly way
 - onWillStart and onWillUpdateProps will emit a warning in the console when they
   take longer than 3 seconds in an effort to ease debugging the presence of deadlocks

doc/reference/templates.md

@@ -61,7 +61,7 @@ For reference, here is a list of all standard QWeb directives:
 | `t-out`                        | [Outputting value, possibly without escaping](#outputting-data) |
 | `t-set`, `t-value`             | [Setting variables](#setting-variables)                         |
 | `t-if`, `t-elif`, `t-else`,    | [conditionally rendering](#conditionals)                        |
-| `t-foreach`, `t-as`            | [Loops](#loops)                                                 |
+| `t-for/t-of`, `t-foreach/t-as` | [Loops](#loops)                                                 |
 | `t-att`, `t-attf-*`, `t-att-*` | [Dynamic attributes](#dynamic-attributes)                       |
 | `t-call`                       | [Rendering sub templates](#sub-templates)                       |
 | `t-debug`, `t-log`             | [Debugging](#debugging)                                         |
@@ -372,6 +372,24 @@ Like conditions, `t-foreach` applies to the element bearing the directive’s at
 
 is equivalent to the previous example.
 
+Owl also has another pair of directives that can be used for looping that allow
+destructuring the contents of the loop item: `t-for` and `t-of`, which behaves
+much like `for..of` in javascript:
+
+```xml
+<t t-for="[left, right]" t-of="[['a', 1], ['b', 2], ['c', 3]]" t-key="left">
+    <p><t t-esc="left"/>: <t t-esc="right"/></p>
+</t>
+```
+
+will be rendered as:
+
+```xml
+<p>a: 1</p>
+<p>b: 2</p>
+<p>c: 3</p>
+```
+
 An important difference should be made with the usual `QWeb` behaviour: Owl
 requires the presence of a `t-key` directive, to be able to properly reconcile
 renderings.
@@ -380,8 +398,9 @@ renderings.
 and maps, it will expose the key of the current iteration as the contents of the
 `t-as`, and the corresponding value with the same name and the suffix `_value`.
 
-In addition to the name passed via t-as, `t-foreach` provides a few other useful
+In addition to the name passed via t-as, `t-foreach` (but not `t-for`) provides
-variables (note: `$as` will be replaced with the name passed to `t-as`):
+a few other useful variables (note: `$as` will be replaced with the name passed
+to `t-as`):
 
 - `$as_value`: the current iteration value, identical to `$as` for arrays and
   other iterables, but for objects and maps, it provides the value (where `$as`
@@ -393,10 +412,9 @@ variables (note: `$as` will be replaced with the name passed to `t-as`):
   (equivalent to `$as_index + 1 == $as_size`), requires the iteratee’s size be
   available
 
-These extra variables provided and all new variables created into the `t-foreach`
+These variables and all new variables created inside`t-foreach` and `t-for` are
-are only available in the scope of the `t-foreach`. If the variable exists outside
+only available inside of the loop. If a variable existed outside the context of
-the context of the `t-foreach`, the value is copied at the end of the foreach
+the loop, the assignment will affect the outer variable.
-into the global context.
 
 ```xml
 <t t-set="existing_variable" t-value="false"/>
@@ -408,7 +426,7 @@ into the global context.
     <!-- existing_variable and new_variable now true -->
 </p>
 
-<!-- existing_variable always true -->
+<!-- existing_variable still true -->
 <!-- new_variable undefined -->
 ```
 
@@ -471,18 +489,11 @@ are all equivalent:
 </t>
 ```
 
-If there is no `t-key` directive, Owl will use the index as a default key.
+The `t-key` directive is mandatory, and as mentioned should represent the object's
-
+identity. You may be tempted to use the loop index as a key, but keep in mind that
-Note: the `t-foreach` directive only accepts arrays (lists) or objects. It does
+this is only correct if items in the loop cannot be reordered. If this is not the
-not work with other iterables, such as `Set`. However, it is only a matter of
+case, using the index as the key can lead to bugs that are difficult to find, so
-using the `...` javascript operator. For example:
+use the index as the key only if you are sure items cannot be reordered.
-
-```xml
-<t t-foreach="[...items]" t-as="item">...</t>
-```
-
-The `...` operator will convert the `Set` (or any other iterables) into a list,
-which will work with Owl QWeb.
 
 ### Sub Templates
 

src/compiler/code_generator.ts

@@ -18,6 +18,7 @@ import {
   ASTTCallBlock,
   ASTTEsc,
   ASTText,
+  ASTTFor,
   ASTTForEach,
   ASTTif,
   ASTTKey,
@@ -471,6 +472,8 @@ export class CodeGenerator {
         return this.compileTIf(ast, ctx);
       case ASTType.TForEach:
         return this.compileTForeach(ast, ctx);
+      case ASTType.TFor:
+        return this.compileTFor(ast, ctx);
       case ASTType.TKey:
         return this.compileTKey(ast, ctx);
       case ASTType.Multi:
@@ -907,7 +910,7 @@ export class CodeGenerator {
     if (!ast.hasNoValue) {
       this.addLine(`ctx[\`${ast.elem}_value\`] = ${keys}[${loopVar}];`);
     }
-    this.define(`key${this.target.loopLevel}`, ast.key ? compileExpr(ast.key) : loopVar);
+    this.define(`key${this.target.loopLevel}`, compileExpr(ast.key));
     if (this.dev) {
       // Throw error on duplicate keys in dev mode
       this.helpers.add("OwlError");
@@ -954,6 +957,46 @@ export class CodeGenerator {
     return block.varName;
   }
 
+  compileTFor(ast: ASTTFor, ctx: Context): string {
+    let { block } = ctx;
+    if (block) {
+      this.insertAnchor(block);
+    }
+    block = this.createBlock(block, "list", ctx);
+    this.target.loopLevel++;
+    this.addLine(`ctx = Object.create(ctx);`);
+    // Throw errors on duplicate keys in dev mode
+    if (this.dev) {
+      this.define(`keys${block.id}`, `new Set()`);
+    }
+    this.define(`c_block${block.id}`, "[]");
+    const index = `i${this.target.loopLevel}`;
+    this.addLine(`let ${index} = ${0};`);
+    const binding = compileExpr(ast.binding);
+    this.addLine(`for (${binding} of ${compileExpr(ast.iterable)}) {`);
+    this.target.indentLevel++;
+    this.define(`key${this.target.loopLevel}`, compileExpr(ast.key));
+    if (this.dev) {
+      // Throw error on duplicate keys in dev mode
+      this.helpers.add("OwlError");
+      this.addLine(
+        `if (keys${block.id}.has(String(key${this.target.loopLevel}))) { throw new OwlError(\`Got duplicate key in t-for: \${key${this.target.loopLevel}}\`)}`
+      );
+      this.addLine(`keys${block.id}.add(String(key${this.target.loopLevel}));`);
+    }
+    const subCtx = createContext(ctx, { block, index });
+    this.compileAST(ast.body, subCtx);
+    this.addLine(`${index}++;`);
+    this.target.indentLevel--;
+    this.target.loopLevel--;
+    this.addLine(`}`);
+    if (!ctx.isLast) {
+      this.addLine(`ctx = ctx.__proto__;`);
+    }
+    this.insertBlock("l", block, ctx);
+    return block.varName;
+  }
+
   compileTKey(ast: ASTTKey, ctx: Context): string | null {
     const tKeyExpr = generateId("tKey_");
     this.define(tKeyExpr, compileExpr(ast.expr));

src/compiler/parser.ts

@@ -26,6 +26,7 @@ export const enum ASTType {
   TCallBlock,
   TTranslation,
   TPortal,
+  TFor,
 }
 
 export interface ASTText {
@@ -104,7 +105,15 @@ export interface ASTTForEach {
   hasNoLast: boolean;
   hasNoIndex: boolean;
   hasNoValue: boolean;
-  key: string | null;
+  key: string;
+}
+
+export interface ASTTFor {
+  type: ASTType.TFor;
+  iterable: string;
+  binding: string;
+  body: AST;
+  key: string;
 }
 
 export interface ASTTKey {
@@ -183,6 +192,7 @@ export type AST =
   | ASTTCall
   | ASTTOut
   | ASTTForEach
+  | ASTTFor
   | ASTTKey
   | ASTComponent
   | ASTSlot
@@ -230,6 +240,7 @@ function parseNode(node: Node, ctx: ParsingContext): AST | null {
   return (
     parseTDebugLog(node, ctx) ||
     parseTForEach(node, ctx) ||
+    parseTFor(node, ctx) ||
     parseTIf(node, ctx) ||
     parseTPortal(node, ctx) ||
     parseTCall(node, ctx) ||
@@ -531,6 +542,36 @@ function parseTForEach(node: Element, ctx: ParsingContext): AST | null {
   };
 }
 
+function parseTFor(node: Element, ctx: ParsingContext): AST | null {
+  if (!node.hasAttribute("t-for")) {
+    return null;
+  }
+  const binding = node.getAttribute("t-for")!;
+  node.removeAttribute("t-for");
+  const iterable = node.getAttribute("t-of") || "";
+  node.removeAttribute("t-of");
+  const key = node.getAttribute("t-key");
+  if (!key) {
+    throw new OwlError(
+      `"Directive t-for should always be used with a t-key!" (expression: t-for="${binding}" t-of="${iterable}")`
+    );
+  }
+  node.removeAttribute("t-key");
+  const body = parseNode(node, ctx);
+
+  if (!body) {
+    return null;
+  }
+
+  return {
+    type: ASTType.TFor,
+    iterable,
+    binding,
+    body,
+    key,
+  };
+}
+
 function parseTKey(node: Element, ctx: ParsingContext): AST | null {
   if (!node.hasAttribute("t-key")) {
     return null;

tests/compiler/__snapshots__/t_for.test.ts.snap

@@ -0,0 +1,670 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`t-for destructuring array items 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for ([ctx['key'],ctx['value']] of Object.entries({a:1,b:2})) {
+      const key1 = ctx['key'];
+      const b3 = text(\`(\`);
+      const b4 = text(ctx['key']);
+      const b5 = text(\`: \`);
+      const b6 = text(ctx['value']);
+      const b7 = text(\`)\`);
+      c_block1[i1] = withKey(multi([b3, b4, b5, b6, b7]), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for destructuring array items: rest 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for ([ctx['head'],...ctx['tail']] of [[1,2,3],[4,5,6]]) {
+      const key1 = ctx['head'];
+      const b3 = text(\`(\`);
+      const b4 = text(ctx['head']);
+      const b5 = text(\`;\`);
+      const b6 = text(ctx['tail']);
+      const b7 = text(\`)\`);
+      c_block1[i1] = withKey(multi([b3, b4, b5, b6, b7]), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for destructuring object items 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for ({k:ctx['k'],v:ctx['v']} of [{k:'a',v:1},{k:'b',v:2}]) {
+      const key1 = ctx['k'];
+      const b3 = text(\`(\`);
+      const b4 = text(ctx['k']);
+      const b5 = text(\`: \`);
+      const b6 = text(ctx['v']);
+      const b7 = text(\`)\`);
+      c_block1[i1] = withKey(multi([b3, b4, b5, b6, b7]), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for destructuring object items: rest 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for ({k:ctx['k'],v:ctx['v']} of [{k:'a',v:1},{k:'b',v:2}]) {
+      const key1 = ctx['k'];
+      const b3 = text(\`(\`);
+      const b4 = text(ctx['k']);
+      const b5 = text(\`: \`);
+      const b6 = text(ctx['v']);
+      const b7 = text(\`)\`);
+      c_block1[i1] = withKey(multi([b3, b4, b5, b6, b7]), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for does not pollute the rendering context 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['item'] of [1]) {
+      const key1 = ctx['item'];
+      c_block2[i1] = withKey(text(ctx['item']), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`t-for iterate on items (on a element node) 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  let block3 = createBlock(\`<span><block-text-0/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['item'] of [1,2]) {
+      const key1 = ctx['item'];
+      let txt1 = ctx['item'];
+      c_block2[i1] = withKey(block3([txt1]), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`t-for iterate, Map param 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for ([ctx['key'],ctx['value']] of ctx['map']) {
+      const key1 = ctx['key'];
+      const b3 = text(\` [\`);
+      const b4 = text(ctx['key']);
+      const b5 = text(\`: \`);
+      const b6 = text(ctx['value']);
+      const b7 = text(\`] \`);
+      c_block1[i1] = withKey(multi([b3, b4, b5, b6, b7]), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for iterate, Set param 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['item'] of ctx['set']) {
+      const key1 = ctx['item'];
+      c_block1[i1] = withKey(text(ctx['item']), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for iterate, generator param 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['item'] of ctx['gen']()) {
+      const key1 = ctx['item'];
+      c_block1[i1] = withKey(text(ctx['item']), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for iterate, iterable param 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['item'] of ctx['map'].values()) {
+      const key1 = ctx['item'];
+      c_block1[i1] = withKey(text(ctx['item']), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for nested destructuring 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for ([ctx['key'],{left:ctx['left'],right:ctx['right']}] of Object.entries(ctx['obj'])) {
+      const key1 = ctx['key'];
+      const b3 = text(\`(\`);
+      const b4 = text(ctx['key']);
+      const b5 = text(\`: [\`);
+      const b6 = text(ctx['left']);
+      const b7 = text(\`, \`);
+      const b8 = text(ctx['right']);
+      const b9 = text(\`])\`);
+      c_block1[i1] = withKey(multi([b3, b4, b5, b6, b7, b8, b9]), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for simple iteration (in a node) 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['item'] of [3,2,1]) {
+      const key1 = ctx['item'];
+      c_block2[i1] = withKey(text(ctx['item']), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`t-for simple iteration 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['item'] of [3,2,1]) {
+      const key1 = ctx['item'];
+      c_block1[i1] = withKey(text(ctx['item']), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for simple iteration with two nodes inside 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block3 = createBlock(\`<span>a<block-text-0/></span>\`);
+  let block4 = createBlock(\`<span>b<block-text-0/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['item'] of [3,2,1]) {
+      const key1 = ctx['item'];
+      let txt1 = ctx['item'];
+      const b3 = block3([txt1]);
+      let txt2 = ctx['item'];
+      const b4 = block4([txt2]);
+      c_block1[i1] = withKey(multi([b3, b4]), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for t-call with body in t-for in t-for 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { isBoundary, withDefault, setContextValue, withKey } = helpers;
+  const callTemplate_1 = app.getTemplate(\`sub\`);
+  
+  let block1 = createBlock(\`<div><block-child-0/><span>[<block-text-0/>][<block-text-1/>][<block-text-2/>]</span></div>\`);
+  let block6 = createBlock(\`<span><block-text-0/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    ctx[isBoundary] = 1
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['a'] of ctx['numbers']) {
+      const key1 = ctx['a'];
+      ctx = Object.create(ctx);
+      const c_block4 = [];
+      let i2 = 0;
+      for (ctx['b'] of ctx['letters']) {
+        const key2 = ctx['b'];
+        ctx = Object.create(ctx);
+        ctx[isBoundary] = 1;
+        setContextValue(ctx, \\"c\\", 'x'+'_'+ctx['a']+'_'+ctx['b']);
+        c_block4[i2] = withKey(callTemplate_1.call(this, ctx, node, key + \`__1__\${key1}__\${key2}\`), key2);
+        ctx = ctx.__proto__;
+        i2++;
+      }
+      ctx = ctx.__proto__;
+      const b4 = list(c_block4);
+      let txt1 = ctx['c'];
+      const b6 = block6([txt1]);
+      c_block2[i1] = withKey(multi([b4, b6]), key1);
+      i1++;
+    }
+    ctx = ctx.__proto__;
+    const b2 = list(c_block2);
+    let txt2 = ctx['a'];
+    let txt3 = ctx['b'];
+    let txt4 = ctx['c'];
+    return block1([txt2, txt3, txt4], [b2]);
+  }
+}"
+`;
+
+exports[`t-for t-call with body in t-for in t-for 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  return function template(ctx, node, key = \\"\\") {
+    const b2 = text(\` [\`);
+    const b3 = text(ctx['a']);
+    const b4 = text(\`] [\`);
+    const b5 = text(ctx['b']);
+    const b6 = text(\`] [\`);
+    const b7 = text(ctx['c']);
+    const b8 = text(\`] \`);
+    return multi([b2, b3, b4, b5, b6, b7, b8]);
+  }
+}"
+`;
+
+exports[`t-for t-call without body in t-for in t-for 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const callTemplate_1 = app.getTemplate(\`sub\`);
+  
+  let block1 = createBlock(\`<div><block-child-0/><span>[<block-text-0/>][<block-text-1/>][<block-text-2/>]</span></div>\`);
+  let block6 = createBlock(\`<span><block-text-0/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['a'] of ctx['numbers']) {
+      const key1 = ctx['a'];
+      ctx = Object.create(ctx);
+      const c_block4 = [];
+      let i2 = 0;
+      for (ctx['b'] of ctx['letters']) {
+        const key2 = ctx['b'];
+        c_block4[i2] = withKey(callTemplate_1.call(this, ctx, node, key + \`__1__\${key1}__\${key2}\`), key2);
+        i2++;
+      }
+      ctx = ctx.__proto__;
+      const b4 = list(c_block4);
+      let txt1 = ctx['c'];
+      const b6 = block6([txt1]);
+      c_block2[i1] = withKey(multi([b4, b6]), key1);
+      i1++;
+    }
+    ctx = ctx.__proto__;
+    const b2 = list(c_block2);
+    let txt2 = ctx['a'];
+    let txt3 = ctx['b'];
+    let txt4 = ctx['c'];
+    return block1([txt2, txt3, txt4], [b2]);
+  }
+}"
+`;
+
+exports[`t-for t-call without body in t-for in t-for 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { isBoundary, withDefault, setContextValue } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    ctx[isBoundary] = 1
+    setContextValue(ctx, \\"c\\", 'x'+'_'+ctx['a']+'_'+ctx['b']);
+    const b2 = text(\` [\`);
+    const b3 = text(ctx['a']);
+    const b4 = text(\`] [\`);
+    const b5 = text(ctx['b']);
+    const b6 = text(\`] [\`);
+    const b7 = text(ctx['c']);
+    const b8 = text(\`] \`);
+    return multi([b2, b3, b4, b5, b6, b7, b8]);
+  }
+}"
+`;
+
+exports[`t-for t-for in t-for 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['number'] of ctx['numbers']) {
+      const key1 = ctx['number'];
+      ctx = Object.create(ctx);
+      const c_block3 = [];
+      let i2 = 0;
+      for (ctx['letter'] of ctx['letters']) {
+        const key2 = ctx['letter'];
+        const b5 = text(\` [\`);
+        const b6 = text(ctx['number']);
+        const b7 = text(ctx['letter']);
+        const b8 = text(\`] \`);
+        c_block3[i2] = withKey(multi([b5, b6, b7, b8]), key2);
+        i2++;
+      }
+      ctx = ctx.__proto__;
+      c_block2[i1] = withKey(list(c_block3), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`t-for t-for in t-foreach 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { prepareList, withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const [k_block2, v_block2, l_block2, c_block2] = prepareList(ctx['numbers']);;
+    for (let i1 = 0; i1 < l_block2; i1++) {
+      ctx[\`number\`] = v_block2[i1];
+      const key1 = ctx['number'];
+      ctx = Object.create(ctx);
+      const c_block3 = [];
+      let i2 = 0;
+      for (ctx['letter'] of ctx['letters']) {
+        const key2 = ctx['letter'];
+        const b5 = text(\` [\`);
+        const b6 = text(ctx['number']);
+        const b7 = text(ctx['letter']);
+        const b8 = text(\`] \`);
+        c_block3[i2] = withKey(multi([b5, b6, b7, b8]), key2);
+        i2++;
+      }
+      ctx = ctx.__proto__;
+      c_block2[i1] = withKey(list(c_block3), key1);
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`t-for t-for with t-if inside (no external node) 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block3 = createBlock(\`<span><block-text-0/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for ({id:ctx['id'],text:ctx['text']} of ctx['elems']) {
+      const key1 = ctx['id'];
+      let b3;
+      if (ctx['id']<3) {
+        let txt1 = ctx['text'];
+        b3 = block3([txt1]);
+      }
+      c_block1[i1] = withKey(multi([b3]), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`t-for t-for with t-if inside 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  let block4 = createBlock(\`<span><block-text-0/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for ({id:ctx['id'],text:ctx['text']} of ctx['elems']) {
+      const key1 = ctx['id'];
+      let b4;
+      if (ctx['id']<3) {
+        let txt1 = ctx['text'];
+        b4 = block4([txt1]);
+      }
+      c_block2[i1] = withKey(multi([b4]), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`t-for t-foreach in t-for 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { prepareList, withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['number'] of ctx['numbers']) {
+      const key1 = ctx['number'];
+      ctx = Object.create(ctx);
+      const [k_block3, v_block3, l_block3, c_block3] = prepareList(ctx['letters']);;
+      for (let i2 = 0; i2 < l_block3; i2++) {
+        ctx[\`letter\`] = v_block3[i2];
+        const key2 = ctx['letter'];
+        const b5 = text(\` [\`);
+        const b6 = text(ctx['number']);
+        const b7 = text(ctx['letter']);
+        const b8 = text(\`] \`);
+        c_block3[i2] = withKey(multi([b5, b6, b7, b8]), key2);
+      }
+      ctx = ctx.__proto__;
+      c_block2[i1] = withKey(list(c_block3), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`t-for t-key on t-for 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  let block3 = createBlock(\`<span/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['thing'] of ctx['things']) {
+      const key1 = ctx['thing'];
+      c_block2[i1] = withKey(block3(), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`t-for throws error if invalid loop expression 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  let block3 = createBlock(\`<span/>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['item'] of ctx['abc']) {
+      const key1 = ctx['item'];
+      const tKey_1 = ctx['item'];
+      c_block2[i1] = withKey(block3(), tKey_1 + key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;

tests/compiler/__snapshots__/t_set.test.ts.snap

@@ -335,6 +335,99 @@ exports[`t-set t-set evaluates an expression only once 1`] = `
 }"
 `;
 
+exports[`t-set t-set outside modified in t-for 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { isBoundary, withDefault, setContextValue, withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/><p>EndLoop: <block-text-0/></p></div>\`);
+  let block3 = createBlock(\`<p>InLoop: <block-text-0/></p>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    ctx[isBoundary] = 1
+    setContextValue(ctx, \\"iter\\", 0);
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['val'] of ['a','b']) {
+      const key1 = ctx['val'];
+      let txt1 = ctx['iter'];
+      c_block2[i1] = withKey(block3([txt1]), key1);
+      setContextValue(ctx, \\"iter\\", ctx['iter']+1);
+      i1++;
+    }
+    ctx = ctx.__proto__;
+    const b2 = list(c_block2);
+    let txt2 = ctx['iter'];
+    return block1([txt2], [b2]);
+  }
+}"
+`;
+
+exports[`t-set t-set outside modified in t-for increment-after operator 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { isBoundary, withDefault, setContextValue, withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/><p>EndLoop: <block-text-0/></p></div>\`);
+  let block3 = createBlock(\`<p>InLoop: <block-text-0/></p>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    ctx[isBoundary] = 1
+    setContextValue(ctx, \\"iter\\", 0);
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['val'] of ['a','b']) {
+      const key1 = ctx['val'];
+      let txt1 = ctx['iter'];
+      c_block2[i1] = withKey(block3([txt1]), key1);
+      setContextValue(ctx, \\"iter\\", ctx['iter']++);
+      i1++;
+    }
+    ctx = ctx.__proto__;
+    const b2 = list(c_block2);
+    let txt2 = ctx['iter'];
+    return block1([txt2], [b2]);
+  }
+}"
+`;
+
+exports[`t-set t-set outside modified in t-for increment-before operator 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { isBoundary, withDefault, setContextValue, withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/><p>EndLoop: <block-text-0/></p></div>\`);
+  let block3 = createBlock(\`<p>InLoop: <block-text-0/></p>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    ctx[isBoundary] = 1
+    setContextValue(ctx, \\"iter\\", 0);
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['val'] of ['a','b']) {
+      const key1 = ctx['val'];
+      let txt1 = ctx['iter'];
+      c_block2[i1] = withKey(block3([txt1]), key1);
+      setContextValue(ctx, \\"iter\\", ++ctx['iter']);
+      i1++;
+    }
+    ctx = ctx.__proto__;
+    const b2 = list(c_block2);
+    let txt2 = ctx['iter'];
+    return block1([txt2], [b2]);
+  }
+}"
+`;
+
 exports[`t-set t-set outside modified in t-foreach 1`] = `
 "function anonymous(app, bdom, helpers
 ) {
@@ -454,6 +547,35 @@ exports[`t-set t-set should reuse variable if possible 1`] = `
 }"
 `;
 
+exports[`t-set t-set should reuse variable if possible: for..of 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { isBoundary, withDefault, setContextValue, withKey } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  let block3 = createBlock(\`<div><span>v<block-text-0/></span></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    ctx[isBoundary] = 1
+    setContextValue(ctx, \\"v\\", 1);
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['elem'] of ctx['list']) {
+      const key1 = ctx['elem_index'];
+      let txt1 = ctx['v'];
+      setContextValue(ctx, \\"v\\", ctx['elem']);
+      c_block2[i1] = withKey(block3([txt1]), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
 exports[`t-set t-set with content and sub t-esc 1`] = `
 "function anonymous(app, bdom, helpers
 ) {

tests/compiler/t_for.test.ts

@@ -0,0 +1,317 @@
+import {
+  renderToBdom,
+  renderToString,
+  snapshotEverything,
+  TestContext,
+  makeTestFixture,
+} from "../helpers";
+import { mount, patch } from "../../src/runtime/blockdom";
+
+snapshotEverything();
+
+// -----------------------------------------------------------------------------
+// t-for
+// -----------------------------------------------------------------------------
+
+describe("t-for", () => {
+  test("simple iteration", () => {
+    const template = `<t t-for="item" t-of="[3, 2, 1]" t-key="item"><t t-esc="item"/></t>`;
+    expect(renderToString(template)).toBe("321");
+  });
+
+  test("simple iteration with two nodes inside", () => {
+    const template = `
+      <t t-for="item" t-of="[3, 2, 1]" t-key="item">
+        <span>a<t t-esc="item"/></span>
+        <span>b<t t-esc="item"/></span>
+      </t>`;
+    const expected =
+      "<span>a3</span><span>b3</span><span>a2</span><span>b2</span><span>a1</span><span>b1</span>";
+    expect(renderToString(template)).toBe(expected);
+  });
+
+  test("destructuring array items", () => {
+    const template = `<t t-for="[key, value]" t-of="Object.entries({ a: 1, b: 2 })" t-key="key">(<t t-esc="key"/>: <t t-esc="value"/>)</t>`;
+    expect(renderToString(template)).toBe("(a: 1)(b: 2)");
+  });
+
+  test("destructuring array items: rest", () => {
+    const template = `<t t-for="[head, ...tail]" t-of="[[1, 2, 3], [4, 5, 6]]" t-key="head">(<t t-esc="head"/>;<t t-esc="tail"/>)</t>`;
+    expect(renderToString(template)).toBe("(1;2,3)(4;5,6)");
+  });
+
+  test("destructuring object items", () => {
+    const template = `<t t-for="{ k, v }" t-of="[{ k: 'a', v: 1 }, { k: 'b', v: 2 }]" t-key="k">(<t t-esc="k"/>: <t t-esc="v"/>)</t>`;
+    expect(renderToString(template)).toBe("(a: 1)(b: 2)");
+  });
+
+  test("destructuring object items: rest", () => {
+    const template = `<t t-for="{ k, v }" t-of="[{ k: 'a', v: 1 }, { k: 'b', v: 2 }]" t-key="k">(<t t-esc="k"/>: <t t-esc="v"/>)</t>`;
+    expect(renderToString(template)).toBe("(a: 1)(b: 2)");
+  });
+
+  test("nested destructuring", () => {
+    const template = `<t t-for="[key, {left, right}]" t-of="Object.entries(obj)" t-key="key">(<t t-esc="key"/>: [<t t-esc="left"/>, <t t-esc="right"/>])</t>`;
+    expect(
+      renderToString(template, {
+        obj: {
+          a: { left: 1, right: 2 },
+          b: { left: 3, right: 4 },
+        },
+      })
+    ).toBe("(a: [1, 2])(b: [3, 4])");
+  });
+
+  test("t-key on t-for", async () => {
+    const template = `
+        <div>
+          <t t-for="thing" t-of="things" t-key="thing">
+            <span/>
+          </t>
+        </div>`;
+
+    const fixture = makeTestFixture();
+
+    const vnode1 = renderToBdom(template, { things: [1, 2] });
+    mount(vnode1, fixture);
+    let elm = fixture;
+    expect(elm.innerHTML).toBe("<div><span></span><span></span></div>");
+    const first = elm.querySelectorAll("span")[0];
+    const second = elm.querySelectorAll("span")[1];
+
+    const vnode2 = renderToBdom(template, { things: [2, 1] });
+    patch(vnode1, vnode2);
+
+    expect(elm.innerHTML).toBe("<div><span></span><span></span></div>");
+    expect(first).toBe(elm.querySelectorAll("span")[1]);
+    expect(second).toBe(elm.querySelectorAll("span")[0]);
+  });
+
+  test("simple iteration (in a node)", () => {
+    const template = `
+        <div>
+          <t t-for="item" t-of="[3, 2, 1]" t-key="item"><t t-esc="item"/></t>
+        </div>`;
+    expect(renderToString(template)).toBe("<div>321</div>");
+  });
+
+  test("iterate on items (on a element node)", () => {
+    const template = `
+        <div>
+          <span t-for="item" t-of="[1, 2]" t-key="item"><t t-esc="item"/></span>
+        </div>`;
+    const expected = `<div><span>1</span><span>2</span></div>`;
+    expect(renderToString(template)).toBe(expected);
+  });
+
+  test("iterate, Map param", () => {
+    const template = `
+      <t t-for="[key, value]" t-of="map" t-key="key">
+        [<t t-esc="key"/>: <t t-esc="value"/>]
+      </t>`;
+    const expected = ` [a: 1]  [b: 2]  [c: 3] `;
+    const context = {
+      map: new Map([
+        ["a", 1],
+        ["b", 2],
+        ["c", 3],
+      ]),
+    };
+    expect(renderToString(template, context)).toBe(expected);
+  });
+
+  test("iterate, Set param", () => {
+    const template = `
+      <t t-for="item" t-of="set" t-key="item">
+        <t t-esc="item"/>
+      </t>`;
+    const expected = `123`;
+    const context = { set: new Set([1, 2, 3]) };
+    expect(renderToString(template, context)).toBe(expected);
+  });
+
+  test("iterate, iterable param", () => {
+    const template = `
+      <t t-for="item" t-of="map.values()" t-key="item">
+        <t t-esc="item"/>
+      </t>`;
+    const expected = `123`;
+    const context = {
+      map: new Map([
+        ["a", 1],
+        ["b", 2],
+        ["c", 3],
+      ]),
+    };
+    expect(renderToString(template, context)).toBe(expected);
+  });
+
+  test("iterate, generator param", () => {
+    const template = `
+      <t t-for="item" t-of="gen()" t-key="item">
+        <t t-esc="item"/>
+      </t>`;
+    const expected = `123`;
+    const context = {
+      *gen() {
+        yield 1;
+        yield 2;
+        yield 3;
+      },
+    };
+    expect(renderToString(template, context)).toBe(expected);
+  });
+
+  test("does not pollute the rendering context", () => {
+    const template = `
+        <div>
+          <t t-for="item" t-of="[1]" t-key="item"><t t-esc="item"/></t>
+        </div>`;
+    const context = { __owl__: {} };
+    renderToString(template, context);
+    expect(Object.keys(context)).toEqual(["__owl__"]);
+  });
+
+  test("t-for in t-for", () => {
+    const template = `
+        <div>
+          <t t-for="number" t-of="numbers" t-key="number">
+            <t t-for="letter" t-of="letters" t-key="letter">
+              [<t t-esc="number"/><t t-esc="letter"/>]
+            </t>
+          </t>
+        </div>`;
+
+    const context = { numbers: [1, 2, 3], letters: ["a", "b"] };
+    const expected = "<div> [1a]  [1b]  [2a]  [2b]  [3a]  [3b] </div>";
+    expect(renderToString(template, context)).toBe(expected);
+  });
+
+  test("t-for in t-foreach", () => {
+    const template = `
+        <div>
+          <t t-foreach="numbers" t-as="number" t-key="number">
+            <t t-for="letter" t-of="letters" t-key="letter">
+              [<t t-esc="number"/><t t-esc="letter"/>]
+            </t>
+          </t>
+        </div>`;
+
+    const context = { numbers: [1, 2, 3], letters: ["a", "b"] };
+    const expected = "<div> [1a]  [1b]  [2a]  [2b]  [3a]  [3b] </div>";
+    expect(renderToString(template, context)).toBe(expected);
+  });
+
+  test("t-foreach in t-for", () => {
+    const template = `
+        <div>
+          <t t-for="number" t-of="numbers" t-key="number">
+            <t t-foreach="letters" t-as="letter" t-key="letter">
+              [<t t-esc="number"/><t t-esc="letter"/>]
+            </t>
+          </t>
+        </div>`;
+
+    const context = { numbers: [1, 2, 3], letters: ["a", "b"] };
+    const expected = "<div> [1a]  [1b]  [2a]  [2b]  [3a]  [3b] </div>";
+    expect(renderToString(template, context)).toBe(expected);
+  });
+
+  test("t-call without body in t-for in t-for", () => {
+    const context = new TestContext();
+    const sub = `
+        <t>
+          <t t-set="c" t-value="'x' + '_' + a + '_'+ b" />
+          [<t t-esc="a" />]
+          [<t t-esc="b" />]
+          [<t t-esc="c" />]
+        </t>`;
+
+    const main = `
+        <div>
+          <t t-for="a" t-of="numbers" t-key="a">
+            <t t-for="b" t-of="letters" t-key="b">
+              <t t-call="sub" />
+            </t>
+            <span t-esc="c"/>
+          </t>
+          <span>[<t t-esc="a" />][<t t-esc="b" />][<t t-esc="c" />]</span>
+        </div>`;
+
+    context.addTemplate("sub", sub);
+    context.addTemplate("main", main);
+
+    const ctx = { numbers: [1, 2, 3], letters: ["a", "b"] };
+    const expected =
+      "<div> [1] [a] [x_1_a]  [1] [b] [x_1_b] <span></span> [2] [a] [x_2_a]  [2] [b] [x_2_b] <span></span> [3] [a] [x_3_a]  [3] [b] [x_3_b] <span></span><span>[][][]</span></div>";
+    expect(context.renderToString("main", ctx)).toBe(expected);
+  });
+
+  test("t-call with body in t-for in t-for", () => {
+    const context = new TestContext();
+    const sub = `
+        <t>
+          [<t t-esc="a" />]
+          [<t t-esc="b" />]
+          [<t t-esc="c" />]
+        </t>`;
+
+    const main = `
+        <div>
+          <t t-for="a" t-of="numbers" t-key="a">
+            <t t-for="b" t-of="letters" t-key="b">
+              <t t-call="sub" >
+                <t t-set="c" t-value="'x' + '_' + a + '_'+ b" />
+              </t>
+            </t>
+            <span t-esc="c"/>
+          </t>
+          <span>[<t t-esc="a" />][<t t-esc="b" />][<t t-esc="c" />]</span>
+        </div>`;
+
+    context.addTemplate("sub", sub);
+    context.addTemplate("main", main);
+
+    const ctx = { numbers: [1, 2, 3], letters: ["a", "b"] };
+    const expected =
+      "<div> [1] [a] [x_1_a]  [1] [b] [x_1_b] <span></span> [2] [a] [x_2_a]  [2] [b] [x_2_b] <span></span> [3] [a] [x_3_a]  [3] [b] [x_3_b] <span></span><span>[][][]</span></div>";
+    expect(context.renderToString("main", ctx)).toBe(expected);
+  });
+
+  test("throws error if invalid loop expression", () => {
+    const test = `<div><t t-for="item" t-of="abc" t-key="item"><span t-key="item"/></t></div>`;
+    expect(() => renderToString(test)).toThrow("ctx.abc is not iterable");
+  });
+
+  test("t-for with t-if inside", () => {
+    const template = `
+        <div>
+          <t t-for="{ id, text }" t-of="elems" t-key="id">
+            <span t-if="id lt 3"><t t-esc="text"/></span>
+          </t>
+        </div>`;
+    const ctx = {
+      elems: [
+        { id: 1, text: "a" },
+        { id: 2, text: "b" },
+        { id: 3, text: "c" },
+      ],
+    };
+    expect(renderToString(template, ctx)).toBe("<div><span>a</span><span>b</span></div>");
+  });
+
+  test("t-for with t-if inside (no external node)", () => {
+    const template = `
+          <t t-for="{ id, text }" t-of="elems" t-key="id">
+            <span t-if="id lt 3"><t t-esc="text"/></span>
+          </t>`;
+    const ctx = {
+      elems: [
+        { id: 1, text: "a" },
+        { id: 2, text: "b" },
+        { id: 3, text: "c" },
+      ],
+    };
+    expect(renderToString(template, ctx)).toBe("<span>a</span><span>b</span>");
+  });
+});

tests/compiler/t_set.test.ts

@@ -122,6 +122,19 @@ describe("t-set", () => {
     expect(renderToString(template, { list: ["a", "b"] })).toBe(expected);
   });
 
+  test("t-set should reuse variable if possible: for..of", () => {
+    const template = `
+        <div>
+          <t t-set="v" t-value="1"/>
+          <div t-for="elem" t-of="list" t-key="elem_index">
+              <span>v<t t-esc="v"/></span>
+              <t t-set="v" t-value="elem"/>
+          </div>
+        </div>`;
+    const expected = "<div><div><span>v1</span></div><div><span>va</span></div></div>";
+    expect(renderToString(template, { list: ["a", "b"] })).toBe(expected);
+  });
+
   test("t-set with content and sub t-esc", () => {
     const template = `
         <div>
@@ -232,6 +245,22 @@ describe("t-set", () => {
     );
   });
 
+  test("t-set outside modified in t-for", async () => {
+    const template = `
+      <div>
+        <t t-set="iter" t-value="0"/>
+        <t t-for="val" t-of="['a','b']" t-key="val">
+          <p>InLoop: <t t-esc="iter"/></p>
+          <t t-set="iter" t-value="iter + 1"/>
+        </t>
+        <p>EndLoop: <t t-esc="iter"/></p>
+      </div>
+    `;
+    expect(renderToString(template)).toBe(
+      "<div><p>InLoop: 0</p><p>InLoop: 1</p><p>EndLoop: 2</p></div>"
+    );
+  });
+
   test("t-set outside modified in t-foreach increment-after operator", async () => {
     const template = `
       <div>
@@ -248,6 +277,22 @@ describe("t-set", () => {
     );
   });
 
+  test("t-set outside modified in t-for increment-after operator", async () => {
+    const template = `
+      <div>
+        <t t-set="iter" t-value="0"/>
+        <t t-for="val" t-of="['a','b']" t-key="val">
+          <p>InLoop: <t t-esc="iter"/></p>
+          <t t-set="iter" t-value="iter++"/>
+        </t>
+        <p>EndLoop: <t t-esc="iter"/></p>
+      </div>
+    `;
+    expect(renderToString(template)).toBe(
+      "<div><p>InLoop: 0</p><p>InLoop: 0</p><p>EndLoop: 0</p></div>"
+    );
+  });
+
   test("t-set outside modified in t-foreach increment-before operator", async () => {
     const template = `
       <div>
@@ -264,6 +309,22 @@ describe("t-set", () => {
     );
   });
 
+  test("t-set outside modified in t-for increment-before operator", async () => {
+    const template = `
+      <div>
+        <t t-set="iter" t-value="0"/>
+        <t t-for="val" t-of="['a','b']" t-key="val">
+          <p>InLoop: <t t-esc="iter"/></p>
+          <t t-set="iter" t-value="++iter"/>
+        </t>
+        <p>EndLoop: <t t-esc="iter"/></p>
+      </div>
+    `;
+    expect(renderToString(template)).toBe(
+      "<div><p>InLoop: 0</p><p>InLoop: 1</p><p>EndLoop: 0</p></div>"
+    );
+  });
+
   test("t-set can't alter from within callee", async () => {
     const context = new TestContext();
     const sub = `<div><t t-esc="iter"/><t t-set="iter" t-value="'called'"/><t t-esc="iter"/></div>`;

tests/components/__snapshots__/t_for.test.ts.snap

@@ -0,0 +1,493 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`list of components components in a node in a t-for  1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, [\\"item\\"]);
+  
+  let block1 = createBlock(\`<div><ul><block-child-0/></ul></div>\`);
+  let block3 = createBlock(\`<li><block-child-0/></li>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['item'] of ctx['items']) {
+      const key1 = 'li_'+ctx['item'];
+      const b4 = comp1({item: ctx['item']}, key + \`__1__\${key1}\`, node, this, null);
+      c_block2[i1] = withKey(block3([], [b4]), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`list of components components in a node in a t-for  2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><block-text-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['props'].item;
+    return block1([txt1]);
+  }
+}"
+`;
+
+exports[`list of components crash on duplicate key in dev mode 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { OwlError, withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, []);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const keys1 = new Set();
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['item'] of [1,2]) {
+      const key1 = 'child';
+      if (keys1.has(String(key1))) { throw new OwlError(\`Got duplicate key in t-for: \${key1}\`)}
+      keys1.add(String(key1));
+      const props1 = {};
+      helpers.validateProps(\`Child\`, props1, this);
+      c_block1[i1] = withKey(comp1(props1, key + \`__1__\${key1}\`, node, this, null), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`list of components crash on duplicate key in dev mode 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  return function template(ctx, node, key = \\"\\") {
+    return text(\`\`);
+  }
+}"
+`;
+
+exports[`list of components crash when using object as keys that serialize to the same string 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { OwlError, withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, []);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const keys1 = new Set();
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['item'] of [{},{}]) {
+      const key1 = ctx['item'];
+      if (keys1.has(String(key1))) { throw new OwlError(\`Got duplicate key in t-for: \${key1}\`)}
+      keys1.add(String(key1));
+      const props1 = {};
+      helpers.validateProps(\`Child\`, props1, this);
+      c_block1[i1] = withKey(comp1(props1, key + \`__1__\${key1}\`, node, this, null), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`list of components crash when using object as keys that serialize to the same string 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  return function template(ctx, node, key = \\"\\") {
+    return text(\`\`);
+  }
+}"
+`;
+
+exports[`list of components list of sub components inside other nodes 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`SubComponent\`, true, false, false, []);
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  let block3 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['blip'] of ctx['state'].blips) {
+      const key1 = ctx['blip'].id;
+      const b4 = comp1({}, key + \`__1__\${key1}\`, node, this, null);
+      c_block2[i1] = withKey(block3([], [b4]), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`list of components list of sub components inside other nodes 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<span>asdf</span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    return block1();
+  }
+}"
+`;
+
+exports[`list of components order is correct when slots are not of same type 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { capture, markRaw } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, true, false, []);
+  
+  let block2 = createBlock(\`<div>A</div>\`);
+  
+  function slot1(ctx, node, key = \\"\\") {
+    let b2;
+    if (!ctx['state'].active) {
+      b2 = block2();
+    }
+    return multi([b2]);
+  }
+  
+  function slot2(ctx, node, key = \\"\\") {
+    return text(\`B\`);
+  }
+  
+  function slot3(ctx, node, key = \\"\\") {
+    return text(\`C\`);
+  }
+  
+  return function template(ctx, node, key = \\"\\") {
+    const ctx1 = capture(ctx);
+    return comp1({slots: markRaw({'a': {__render: slot1.bind(this), __ctx: ctx1, active: !ctx['state'].active}, 'b': {__render: slot2.bind(this), __ctx: ctx1, active: true}, 'c': {__render: slot3.bind(this), __ctx: ctx1, active: ctx['state'].active}})}, key + \`__1\`, node, this, null);
+  }
+}"
+`;
+
+exports[`list of components order is correct when slots are not of same type 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { callSlot, withKey } = helpers;
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['slotName'] of ctx['slotNames']) {
+      const key1 = ctx['slotName'];
+      const slot1 = (ctx['slotName']);
+      c_block1[i1] = withKey(toggler(slot1, callSlot(ctx, node, key1 + \`__1__\${key1}\`, slot1, true, {})), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`list of components reconciliation alg works for t-for in t-for 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, [\\"blip\\"]);
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['section'] of ctx['state'].s) {
+      const key1 = ctx['section'];
+      ctx = Object.create(ctx);
+      const c_block3 = [];
+      let i2 = 0;
+      for (ctx['blip'] of ctx['section'].blips) {
+        const key2 = ctx['blip'];
+        c_block3[i2] = withKey(comp1({blip: ctx['blip']}, key + \`__1__\${key1}__\${key2}\`, node, this, null), key2);
+        i2++;
+      }
+      ctx = ctx.__proto__;
+      c_block2[i1] = withKey(list(c_block3), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`list of components reconciliation alg works for t-for in t-for 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><block-text-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['props'].blip;
+    return block1([txt1]);
+  }
+}"
+`;
+
+exports[`list of components reconciliation alg works for t-for in t-for, 2 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, [\\"row\\",\\"col\\"]);
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  let block3 = createBlock(\`<p><block-child-0/></p>\`);
+  let block5 = createBlock(\`<p><block-child-0/></p>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['row'] of ctx['state'].rows) {
+      const key1 = ctx['row'];
+      ctx = Object.create(ctx);
+      const c_block4 = [];
+      let i2 = 0;
+      for (ctx['col'] of ctx['state'].cols) {
+        const key2 = ctx['col'];
+        const b6 = comp1({row: ctx['row'],col: ctx['col']}, key + \`__1__\${key1}__\${key2}\`, node, this, null);
+        c_block4[i2] = withKey(block5([], [b6]), key2);
+        i2++;
+      }
+      ctx = ctx.__proto__;
+      const b4 = list(c_block4);
+      c_block2[i1] = withKey(block3([], [b4]), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`list of components reconciliation alg works for t-for in t-for, 2 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><block-text-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['props'].row+'_'+ctx['props'].col;
+    return block1([txt1]);
+  }
+}"
+`;
+
+exports[`list of components simple list 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, [\\"value\\"]);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block1 = [];
+    let i1 = 0;
+    for (ctx['elem'] of ctx['state'].elems) {
+      const key1 = ctx['elem'].id;
+      c_block1[i1] = withKey(comp1({value: ctx['elem'].value}, key + \`__1__\${key1}\`, node, this, null), key1);
+      i1++;
+    }
+    return list(c_block1);
+  }
+}"
+`;
+
+exports[`list of components simple list 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<span><block-text-0/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['props'].value;
+    return block1([txt1]);
+  }
+}"
+`;
+
+exports[`list of components sub components rendered in a loop 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, [\\"n\\"]);
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['number'] of ctx['state'].numbers) {
+      const key1 = ctx['number'];
+      c_block2[i1] = withKey(comp1({n: ctx['number']}, key + \`__1__\${key1}\`, node, this, null), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`list of components sub components rendered in a loop 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<p><block-text-0/></p>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['props'].n;
+    return block1([txt1]);
+  }
+}"
+`;
+
+exports[`list of components sub components with some state rendered in a loop 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, []);
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['number'] of ctx['state'].numbers) {
+      const key1 = ctx['number'];
+      c_block2[i1] = withKey(comp1({}, key + \`__1__\${key1}\`, node, this, null), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`list of components sub components with some state rendered in a loop 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<p><block-text-0/></p>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['state'].n;
+    return block1([txt1]);
+  }
+}"
+`;
+
+exports[`list of components switch component position 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, [\\"key\\"]);
+  
+  let block1 = createBlock(\`<span><block-child-0/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['c'] of ctx['clist']) {
+      const key1 = ctx['c'];
+      c_block2[i1] = withKey(comp1({key: ctx['c']}, key + \`__1__\${key1}\`, node, this, null), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`list of components switch component position 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><block-text-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['props'].key;
+    return block1([txt1]);
+  }
+}"
+`;
+
+exports[`list of components t-for with t-component, and update 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { withKey } = helpers;
+  const comp1 = app.createComponent(\`Child\`, true, false, false, [\\"val\\"]);
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    ctx = Object.create(ctx);
+    const c_block2 = [];
+    let i1 = 0;
+    for (ctx['n'] of [0,1]) {
+      const key1 = ctx['n'];
+      c_block2[i1] = withKey(comp1({val: ctx['n']}, key + \`__1__\${key1}\`, node, this, null), key1);
+      i1++;
+    }
+    const b2 = list(c_block2);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`list of components t-for with t-component, and update 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  
+  let block1 = createBlock(\`<span><block-text-0/><block-text-1/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let txt1 = ctx['state'].val;
+    let txt2 = ctx['props'].val;
+    return block1([txt1, txt2]);
+  }
+}"
+`;

tests/components/t_for.test.ts

@@ -0,0 +1,391 @@
+import { App, Component, mount, onMounted, useState, xml } from "../../src/index";
+import {
+  makeTestFixture,
+  nextAppError,
+  nextTick,
+  snapshotEverything,
+  useLogLifecycle,
+} from "../helpers";
+
+snapshotEverything();
+
+let originalconsoleWarn = console.warn;
+let mockConsoleWarn: any;
+
+let fixture: HTMLElement;
+
+beforeEach(() => {
+  fixture = makeTestFixture();
+  mockConsoleWarn = jest.fn(() => {});
+  console.warn = mockConsoleWarn;
+});
+
+afterEach(() => {
+  console.warn = originalconsoleWarn;
+});
+
+describe("list of components", () => {
+  test("simple list", async () => {
+    class Child extends Component {
+      static template = xml`<span><t t-esc="props.value"/></span>`;
+    }
+
+    class Parent extends Component {
+      static template = xml`
+            <t t-for="elem" t-of="state.elems" t-key="elem.id">
+                <Child value="elem.value"/>
+            </t>`;
+      static components = { Child };
+
+      state = useState({
+        elems: [
+          { id: 1, value: "a" },
+          { id: 2, value: "b" },
+        ],
+      });
+    }
+
+    const parent = await mount(Parent, fixture);
+    expect(fixture.innerHTML).toBe("<span>a</span><span>b</span>");
+    parent.state.elems.push({ id: 4, value: "d" });
+    await nextTick();
+    expect(fixture.innerHTML).toBe("<span>a</span><span>b</span><span>d</span>");
+
+    parent.state.elems.pop();
+
+    await nextTick();
+    expect(fixture.innerHTML).toBe("<span>a</span><span>b</span>");
+  });
+
+  test("components in a node in a t-for ", async () => {
+    class Child extends Component {
+      static template = xml`<div><t t-esc="props.item"/></div>`;
+      setup() {
+        useLogLifecycle();
+      }
+    }
+
+    class Parent extends Component {
+      static template = xml`
+              <div>
+                  <ul>
+                      <t t-for="item" t-of="items" t-key="'li_'+item">
+                          <li>
+                              <Child item="item"/>
+                          </li>
+                      </t>
+                  </ul>
+              </div>`;
+      static components = { Child };
+
+      setup() {
+        useLogLifecycle();
+      }
+
+      get items() {
+        return [1, 2];
+      }
+    }
+
+    await mount(Parent, fixture);
+    expect(fixture.innerHTML).toBe(
+      "<div><ul><li><div>1</div></li><li><div>2</div></li></ul></div>"
+    );
+    expect([
+      "Parent:setup",
+      "Parent:willStart",
+      "Parent:willRender",
+      "Child:setup",
+      "Child:willStart",
+      "Child:setup",
+      "Child:willStart",
+      "Parent:rendered",
+      "Child:willRender",
+      "Child:rendered",
+      "Child:willRender",
+      "Child:rendered",
+      "Child:mounted",
+      "Child:mounted",
+      "Parent:mounted",
+    ]).toBeLogged();
+  });
+
+  test("reconciliation alg works for t-for in t-for", async () => {
+    class Child extends Component {
+      static template = xml`<div><t t-esc="props.blip"/></div>`;
+    }
+
+    class Parent extends Component {
+      static template = xml`
+        <div>
+            <t t-for="section" t-of="state.s" t-key="section">
+                <t t-for="blip" t-of="section.blips" t-key="blip">
+                  <Child blip="blip"/>
+                </t>
+            </t>
+        </div>`;
+      static components = { Child };
+      state = { s: [{ blips: ["a1", "a2"] }, { blips: ["b1"] }] };
+    }
+
+    await mount(Parent, fixture);
+    expect(fixture.innerHTML).toBe("<div><div>a1</div><div>a2</div><div>b1</div></div>");
+  });
+
+  test("reconciliation alg works for t-for in t-for, 2", async () => {
+    class Child extends Component {
+      static template = xml`<div><t t-esc="props.row + '_' + props.col"/></div>`;
+    }
+
+    class Parent extends Component {
+      static template = xml`
+        <div>
+          <p t-for="row" t-of="state.rows" t-key="row">
+            <p t-for="col" t-of="state.cols" t-key="col">
+                <Child row="row" col="col"/>
+              </p>
+            </p>
+        </div>`;
+      static components = { Child };
+      state = useState({ rows: [1, 2], cols: ["a", "b"] });
+    }
+
+    const parent = await mount(Parent, fixture);
+    expect(fixture.innerHTML).toBe(
+      "<div><p><p><div>1_a</div></p><p><div>1_b</div></p></p><p><p><div>2_a</div></p><p><div>2_b</div></p></p></div>"
+    );
+    parent.state.rows = [2, 1];
+    await nextTick();
+    expect(fixture.innerHTML).toBe(
+      "<div><p><p><div>2_a</div></p><p><div>2_b</div></p></p><p><p><div>1_a</div></p><p><div>1_b</div></p></p></div>"
+    );
+  });
+
+  test("sub components rendered in a loop", async () => {
+    class Child extends Component {
+      static template = xml`<p><t t-esc="props.n"/></p>`;
+    }
+
+    class Parent extends Component {
+      static template = xml`
+        <div>
+          <t t-for="number" t-of="state.numbers" t-key="number" >
+            <Child n="number"/>
+          </t>
+        </div>`;
+      static components = { Child };
+
+      state = useState({ numbers: [1, 2, 3] });
+    }
+    await mount(Parent, fixture);
+
+    expect(fixture.innerHTML).toBe(`<div><p>1</p><p>2</p><p>3</p></div>`);
+  });
+
+  test("sub components with some state rendered in a loop", async () => {
+    let n = 1;
+
+    class Child extends Component {
+      static template = xml`<p><t t-esc="state.n"/></p>`;
+      state: any;
+      setup() {
+        this.state = useState({ n });
+        n++;
+      }
+    }
+
+    class Parent extends Component {
+      static template = xml`
+        <div>
+          <t t-for="number" t-of="state.numbers" t-key="number">
+            <Child/>
+          </t>
+        </div>`;
+      static components = { Child };
+
+      state = useState({
+        numbers: [1, 2, 3],
+      });
+    }
+    const parent = await mount(Parent, fixture);
+
+    parent.state.numbers = [1, 3];
+    await nextTick();
+    expect(fixture.innerHTML).toBe(`<div><p>1</p><p>3</p></div>`);
+  });
+
+  test("list of sub components inside other nodes", async () => {
+    // this confuses the patching algorithm...
+    class SubComponent extends Component {
+      static template = xml`<span>asdf</span>`;
+    }
+    class Parent extends Component {
+      static template = xml`
+      <div>
+          <div t-for="blip" t-of="state.blips" t-key="blip.id">
+              <SubComponent />
+          </div>
+      </div>`;
+      static components = { SubComponent };
+      state = useState({
+        blips: [
+          { a: "a", id: 1 },
+          { b: "b", id: 2 },
+          { c: "c", id: 4 },
+        ],
+      });
+    }
+    const parent = await mount(Parent, fixture);
+    expect(fixture.innerHTML).toBe(
+      "<div><div><span>asdf</span></div><div><span>asdf</span></div><div><span>asdf</span></div></div>"
+    );
+    parent.state.blips.splice(0, 1);
+    await nextTick();
+    expect(fixture.innerHTML).toBe(
+      "<div><div><span>asdf</span></div><div><span>asdf</span></div></div>"
+    );
+  });
+
+  test("t-for with t-component, and update", async () => {
+    class Child extends Component {
+      static template = xml`
+          <span>
+            <t t-esc="state.val"/>
+            <t t-esc="props.val"/>
+          </span>`;
+      state = useState({ val: "A" });
+      setup() {
+        onMounted(() => {
+          this.state.val = "B";
+        });
+      }
+    }
+    class Parent extends Component {
+      static components = { Child };
+      static template = xml`
+          <div>
+            <t t-for="n" t-of="[0, 1]" t-key="n">
+              <Child val="n"/>
+            </t>
+          </div>`;
+    }
+
+    await mount(Parent, fixture);
+    expect(fixture.innerHTML).toBe("<div><span>A0</span><span>A1</span></div>");
+
+    await nextTick(); // wait for changes triggered in mounted to be applied
+    expect(fixture.innerHTML).toBe("<div><span>B0</span><span>B1</span></div>");
+  });
+
+  test("switch component position", async () => {
+    const childInstances = [];
+    class Child extends Component {
+      static template = xml`<div t-esc="props.key"></div>`;
+      setup() {
+        childInstances.push(this);
+      }
+    }
+
+    class Parent extends Component {
+      static components = { Child };
+      static template = xml`<span>
+        <t t-for="c" t-of="clist" t-key="c">
+          <Child key="c"/>
+        </t>
+      </span>`;
+
+      clist = [1, 2];
+    }
+
+    const parent = await mount(Parent, fixture);
+    expect(fixture.innerHTML).toBe("<span><div>1</div><div>2</div></span>");
+    parent.clist = [2, 1];
+    parent.render();
+    await nextTick();
+    expect(fixture.innerHTML).toBe("<span><div>2</div><div>1</div></span>");
+    expect(childInstances.length).toBe(2);
+  });
+
+  test("crash on duplicate key in dev mode", async () => {
+    const consoleInfo = console.info;
+    console.info = jest.fn();
+    class Child extends Component {
+      static template = xml``;
+    }
+
+    class Parent extends Component {
+      static template = xml`
+        <t t-for="item" t-of="[1, 2]" t-key="'child'">
+          <Child/>
+        </t>
+      `;
+      static components = { Child };
+    }
+
+    const app = new App(Parent, { test: true });
+    const mountProm = expect(app.mount(fixture)).rejects.toThrow(
+      "Got duplicate key in t-for: child"
+    );
+    await expect(nextAppError(app)).resolves.toThrow("Got duplicate key in t-for: child");
+    await mountProm;
+    console.info = consoleInfo;
+    expect(mockConsoleWarn).toBeCalledTimes(1);
+  });
+
+  test("crash when using object as keys that serialize to the same string", async () => {
+    const consoleInfo = console.info;
+    console.info = jest.fn();
+    class Child extends Component {
+      static template = xml``;
+    }
+
+    class Parent extends Component {
+      static template = xml`
+        <t t-for="item" t-of="[{}, {}]" t-key="item">
+          <Child/>
+        </t>
+      `;
+      static components = { Child };
+    }
+
+    const app = new App(Parent, { test: true });
+    const mountProm = expect(app.mount(fixture)).rejects.toThrow(
+      "Got duplicate key in t-for: [object Object]"
+    );
+    await expect(nextAppError(app)).resolves.toThrow("Got duplicate key in t-for: [object Object]");
+    await mountProm;
+    console.info = consoleInfo;
+    expect(mockConsoleWarn).toBeCalledTimes(1);
+  });
+
+  test("order is correct when slots are not of same type", async () => {
+    class Child extends Component {
+      static template = xml`
+          <t t-slot="{{ slotName }}" t-for="slotName" t-of="slotNames" t-key="slotName"/>
+      `;
+      get slotNames() {
+        return Object.entries(this.props.slots)
+          .filter((entry: any) => entry[1].active)
+          .map((entry) => entry[0]);
+      }
+    }
+
+    class Parent extends Component {
+      static template = xml`
+        <Child>
+          <t t-set-slot="a" active="!state.active"><div t-if="!state.active">A</div></t>
+          <t t-set-slot="b" active="true">B</t>
+          <t t-set-slot="c" active="state.active">C</t>
+        </Child>
+      `;
+      static components = { Child };
+      state = useState({ active: false });
+    }
+
+    const parent = await mount(Parent, fixture);
+    expect(fixture.textContent).toBe("AB");
+    parent.state.active = true;
+    await nextTick();
+    expect(fixture.textContent).toBe("BC");
+  });
+});