cell destructuring

Author: mbostock

src/features.js

@@ -2,12 +2,11 @@ import {simple} from "acorn-walk";
 import walk from "./walk.js";
 
 export default function findFeatures(cell, featureName) {
-  const ast = {type: "Program", body: [cell.body]};
   const features = new Map();
   const {references} = cell;
 
   simple(
-    ast,
+    cell,
     {
       CallExpression: node => {
         const {callee, arguments: args} = node;

src/parse.js

@@ -1,4 +1,4 @@
-import {getLineInfo, TokContext, tokTypes as tt, Parser} from "acorn";
+import {getLineInfo, TokContext, Token, tokTypes as tt, Parser} from "acorn";
 import defaultGlobals from "./globals.js";
 import findReferences from "./references.js";
 import findFeatures from "./features.js";
@@ -106,6 +106,7 @@ export class CellParser extends Parser {
     this.O_function = 0;
     this.O_async = false;
     this.O_generator = false;
+    this.O_destructuring = null;
     this.strict = true;
     this.enterScope(SCOPE_FUNCTION | SCOPE_ASYNC | SCOPE_GENERATOR);
   }
@@ -133,8 +134,23 @@ export class CellParser extends Parser {
 
     // A non-empty cell?
     else if (token.type !== tt.eof && token.type !== tt.semi) {
-      // A named cell?
-      if (token.type === tt.name) {
+
+      // A destructuring cell, maybe?
+      // (But not an object expression or arrow function!)
+      if (token.type === tt.parenL) {
+        id = this.parseParenAndDistinguishExpression(true);
+        if (id.type !== "ArrowFunctionExpression" && this.eat(tt.eq)) {
+          id = this.toAssignable(id, true, this.O_destructuring);
+          this.checkCellDeclaration(id);
+        } else {
+          body = id;
+          id = null;
+        }
+        token = new Token(this);
+      }
+
+      // A simple named cell?
+      else if (token.type === tt.name) {
         if (token.value === "viewof" || token.value === "mutable") {
           token = lookahead.getToken();
           if (token.type !== tt.name) {
@@ -152,22 +168,20 @@ export class CellParser extends Parser {
         }
       }
 
-      // A block?
-      if (token.type === tt.braceL) {
-        body = this.parseBlock();
+      // A block or an expression?
+      if (body === null) {
+        body = token.type === tt.braceL
+          ? this.parseBlock()
+          : this.parseExpression();
       }
 
-      // An expression?
-      // Possibly a function or class declaration?
-      else {
-        body = this.parseExpression();
-        if (
-          id === null &&
-          (body.type === "FunctionExpression" ||
-            body.type === "ClassExpression")
-        ) {
-          id = body.id;
-        }
+      // Promote the name of a function or class declaration?
+      if (
+        id === null &&
+        (body.type === "FunctionExpression" ||
+          body.type === "ClassExpression")
+      ) {
+        id = body.id;
       }
     }
 
@@ -184,12 +198,40 @@ export class CellParser extends Parser {
       ? node
       : super.toAssignable(node, isBinding, refDestructuringErrors);
   }
+  checkCellDeclaration(node) {
+    switch (node.type) {
+      case "Identifier":
+        break;
+      case "ObjectPattern":
+        for (const p of node.properties) this.checkCellDeclaration(p);
+        break;
+      case "ArrayPattern":
+        for (const e of node.elements) e && this.checkCellDeclaration(e);
+        break;
+      case "Property":
+        this.checkCellDeclaration(node.value);
+        break;
+      case "RestElement":
+        this.checkCellDeclaration(node.argument);
+        break;
+      case "AssignmentPattern":
+        this.checkCellDeclaration(node.left);
+        break;
+      default:
+        this.unexpected();
+        break;
+    }
+  }
   checkLocal(id) {
     const node = id.id || id;
     if (defaultGlobals.has(node.name) || node.name === "arguments") {
       this.raise(node.start, `Identifier '${node.name}' is reserved`);
     }
   }
+  checkExpressionErrors(refDestructuringErrors, andThrow) {
+    this.O_destructuring = refDestructuringErrors;
+    return super.checkExpressionErrors(refDestructuringErrors, andThrow);
+  }
   checkUnreserved(node) {
     if (node.name === "viewof" || node.name === "mutable") {
       this.raise(node.start, `Unexpected keyword '${node.name}'`);

src/references.js

@@ -26,7 +26,6 @@ function declaresArguments(node) {
 }
 
 export default function findReferences(cell, globals) {
-  const ast = {type: "Program", body: [cell.body]};
   const locals = new Map;
   const globalSet = new Set(globals);
   const references = [];
@@ -81,11 +80,11 @@ export default function findReferences(cell, globals) {
   }
 
   function declareModuleSpecifier(node) {
-    declareLocal(ast, node.local);
+    declareLocal(cell.body, node.local);
   }
 
   ancestor(
-    ast,
+    cell.body,
     {
       VariableDeclaration: (node, parents) => {
         let parent = null;
@@ -154,8 +153,26 @@ export default function findReferences(cell, globals) {
     }
   }
 
+  function assignmentIdentifier(node, parents) {
+    if (parents.length > 2 && parents[1].type === "AssignmentPattern" && parents[2] === parents[1].right) {
+      identifier(node, parents);
+    }
+  }
+
+  if (cell.id && (cell.id.type === "ArrayPattern" || cell.id.type === "ObjectPattern")) {
+    declarePattern(cell.id, cell.id);
+    ancestor(
+      cell.id,
+      {
+        VariablePattern: assignmentIdentifier,
+        Identifier: assignmentIdentifier
+      },
+      walk
+    );
+  }
+
   ancestor(
-    ast,
+    cell.body,
     {
       VariablePattern: identifier,
       Identifier: identifier
@@ -210,7 +227,7 @@ export default function findReferences(cell, globals) {
   }
 
   ancestor(
-    ast,
+    cell.body,
     {
       AssignmentExpression: checkConstLeft,
       AssignmentPattern: checkConstLeft,

src/walk.js

@@ -1,7 +1,13 @@
 import {make} from "acorn-walk";
 
 export default make({
-  Import() {},
+  Cell(node, st, c) {
+    if (node.id) c(node.id, st);
+    c(node.body, st);
+  },
+  CellTag(node, st, c) {
+    c(node.body, st);
+  },
   ViewExpression(node, st, c) {
     c(node.id, st, "Identifier");
   },

test/input/destructure-array-literal.js

@@ -0,0 +1 @@
+([foo, ...bar]) = [1, 2, 3]

test/input/destructure-block.js

@@ -0,0 +1,3 @@
+({foo, bar}) = {
+  return {foo: 1, bar: 2};
+}

test/input/destructure-computed.js

@@ -0,0 +1 @@
+({["foo"]}) = 42

test/input/destructure-default-value-async.js

@@ -0,0 +1 @@
+([x, y = x + await z]) = [1]

test/input/destructure-default-value-secret.js

@@ -0,0 +1 @@
+([x, y = Secret("foo")]) = [1]

test/input/destructure-default-value.js

@@ -0,0 +1 @@
+([x, y = 2]) = [1]

test/input/destructure-dynamic-import.js

@@ -0,0 +1 @@
+({default: confetti}) = import("https://cdn.skypack.dev/canvas-confetti")

test/input/destructure-keyword.js

@@ -0,0 +1 @@
+([mutable]) = [42]

test/input/destructure-mutable.js

@@ -0,0 +1 @@
+([mutable foo]) = [42]

test/input/destructure-nested-object-literal.js

@@ -0,0 +1 @@
+({foo: {bar: {baz: qux}}}) = ({})

test/input/destructure-object-literal.js

@@ -0,0 +1 @@
+({foo, bar}) = ({foo: 1, bar: 2})

test/output/destructure-array-literal.js.json

@@ -0,0 +1,63 @@
+{
+  "type": "Cell",
+  "start": 0,
+  "end": 28,
+  "id": {
+    "type": "ArrayPattern",
+    "start": 1,
+    "end": 14,
+    "elements": [
+      {
+        "type": "Identifier",
+        "start": 2,
+        "end": 5,
+        "name": "foo"
+      },
+      {
+        "type": "RestElement",
+        "start": 7,
+        "end": 13,
+        "argument": {
+          "type": "Identifier",
+          "start": 10,
+          "end": 13,
+          "name": "bar"
+        }
+      }
+    ]
+  },
+  "body": {
+    "type": "ArrayExpression",
+    "start": 18,
+    "end": 27,
+    "elements": [
+      {
+        "type": "Literal",
+        "start": 19,
+        "end": 20,
+        "value": 1,
+        "raw": "1"
+      },
+      {
+        "type": "Literal",
+        "start": 22,
+        "end": 23,
+        "value": 2,
+        "raw": "2"
+      },
+      {
+        "type": "Literal",
+        "start": 25,
+        "end": 26,
+        "value": 3,
+        "raw": "3"
+      }
+    ]
+  },
+  "async": false,
+  "generator": false,
+  "references": [],
+  "fileAttachments": [],
+  "databaseClients": [],
+  "secrets": []
+}