implement opassign, fix bugs in pre/postfix, introduce a let* helper

lib/src/codegen/js_metalet.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library dev_compiler.src.codegen.js_metalet;
+
+// TODO(jmesserly): import from its own package
+import 'package:dev_compiler/src/js/js_ast.dart';
+import 'package:dev_compiler/src/js/precedence.dart';
+
+import 'js_names.dart' show JSTemporary;
+
+/// A synthetic `let*` node, similar to that found in Scheme.
+///
+/// For example, postfix increment can be desugared as:
+///
+///     // psuedocode mix of Scheme and JS:
+///     (let* (x1=expr1, x2=expr2, t=expr1[expr2]) { x1[x2] = t + 1; t })

[Leaf, 2015/04/08 04:08:34]

t=x1[x2] ?

[Jennifer Messerly, 2015/04/08 16:35:46]

Good catch! Yes :)

+///
+/// [JSMetaLet] will simplify itself automatically when [toExpression],
+/// [toStatement], or [toReturn] is called.
+///
+/// * variables used once will be inlined.
+/// * if used in a statement context they can emit as blocks.
+/// * if return value is not used it can be eliminated, see [statelessResult].
+/// * if there are no variables, the codegen will be simplified.
+///
+/// Because this deals with JS AST nodes, it is not aware of any Dart semantics
+/// around statelessness (such as `final` variables). [variables] should not
+/// be created for these Dart expressions.
+///
+class JSMetaLet extends Expression {
+  /// Creates a temporary to contain the value of [expr]. The temporary can be
+  /// used multiple times in the resulting expression. For example:
+  /// `expr ** 2` could be compiled as `expr * expr`. The temporary scope will
+  /// ensure `expr` is only evaluated once: `(x => x * x)(expr)`.
+  ///
+  /// If the expression does not end up using `x` more than once, or if those
+  /// expressions can be treated as [stateless] (e.g. they are non-mutated
+  /// variables), then the resulting code will be simplified automatically.
+  final Map<String, Expression> variables;
+
+  /// A list of expressions in the body.
+  /// Conceptually this is like a comma expression: the last value is returned.
+  final List<Expression> body;
+
+  /// True if the final expression in [body] can be skipped in [toStatement].
+  final bool statelessResult;
+
+  Expression _expression;
+  Statement _statement;
+  Statement _return;
+
+  JSMetaLet(this.variables, this.body, {this.statelessResult: false});
+
+  /// Returns an expression that ignores the result. This is similar to
+  /// [toStatement] but returns an expression.
+  Expression toVoidExpression() {
+    if (!statelessResult) return this;
+    // Return a new MetaLet.
+    // This allows [toStatement] and [toReturn] to compose nicely.
+    return new JSMetaLet(variables, body.toList()..removeLast());
+  }
+
+  Expression toAssignExpression(Expression left) {
+    if (left is Identifier) {
+      var simple = _simplifyAssignment(left);
+      if (simple != null) return simple;
+
+      var exprs = body.toList();
+      exprs.add(exprs.removeLast().toAssignExpression(left));
+      return new JSMetaLet(variables, exprs);
+    }
+    return super.toAssignExpression(left);
+  }
+
+  Statement toVariableDeclaration(Identifier name) {
+    var simple = _simplifyAssignment(name, isDeclaration: true);
+    if (simple != null) return simple.toStatement();
+    return super.toVariableDeclaration(name);
+  }
+
+  Expression toExpression() {
+    if (_expression != null) return _expression;
+
+    var params = [];
+    var values = [];
+    var letBody = _build(params, values, new Expression.binary(body, ','));
+
+    // Note that [precedenceLevel] must be aware of this possibility, because
+    // comma expression has different precedence from call.
+    if (params.isEmpty) return _expression = letBody;
+
+    // Assign parameters inside the body, to get sequential let* behavior

[Jennifer Messerly, 2015/04/07 22:09:52]

for a single variable we could emit:

     (x => x*x)(123)

instead of:

    (x => (x = 123, x*x))()

thoughts?

anyway, it's kind of neat how easy it is to tweak these heuristics now and have
it apply everywhere :D

[Leaf, 2015/04/08 04:08:34]

I like the former, but I suspect it's because I'm used to the pattern.  The
latter is probably more accessible.

+    // (subsequent bindings can refer to previous ones), and also to be a bit
+    // more readable (lexical order matches source order).
+    var vars = [];
+    for (int i = 0; i < params.length; i++) {
+      vars.add(new Assignment(params[i], values[i]));
+    }
+    letBody = new Binary(',', new Expression.binary(vars, ','), letBody);
+
+    // Convert the comma back to a block, for readability.
+    if (vars.length + body.length >= 4) letBody = letBody.toReturn();
+
+    return new Call(new ArrowFun(params, letBody), []);
+  }
+
+  Statement toStatement() {
+    if (_statement != null) return _statement;

[Leaf, 2015/04/08 04:08:34]

I didn't follow the need for caching.  Do these really get emitted multiple
times?

[Jennifer Messerly, 2015/04/08 16:35:46]

That's fair. It's only really needed for toExpression(), because we do that
implicitly whenever the tree is walked. I'll add a comment.

+
+    // Skip return value if not used.
+    var statements = body.map((e) => e.toStatement()).toList();
+    if (statelessResult) statements.removeLast();
+    return _statement = _finishStatement(statements);
+  }
+
+  Statement toReturn() {
+    if (_return != null) return _return;
+    var statements = body
+        .map((e) => e == body.last ? e.toReturn() : e.toStatement())
+        .toList();
+    return _return = _finishStatement(statements);
+  }
+
+  accept(NodeVisitor visitor) => toExpression().accept(visitor);
+
+  void visitChildren(NodeVisitor visitor) {
+    toExpression().visitChildren(visitor);
+  }
+
+  /// This generates as either a comma expression or a call.
+  int get precedenceLevel => variables.isEmpty ? EXPRESSION : CALL;
+
+  Statement _finishStatement(List<Statement> statements) {
+    var params = [];
+    var values = [];
+    var block = _build(params, values, new Block(statements));
+    if (params.isEmpty) return _return = block;
+
+    var vars = [];
+    for (int i = 0; i < params.length; i++) {
+      vars.add(new VariableInitialization(params[i], values[i]));
+    }
+
+    return new Block(
+        [new VariableDeclarationList('let', vars).toStatement(), block]);
+  }
+
+  Node _build(List<JSTemporary> params, List<Expression> values, Node node) {
+    // Visit the tree and count how many times each temp was used.
+    var counter = new _VariableUseCounter();
+    node.accept(counter);
+    // Also count the init expressions.
+    for (var init in variables.values) init.accept(counter);
+
+    var substitutions = {};
+    _substitute(node) => new Template(null, node).safeCreate(substitutions);
+
+    variables.forEach((name, init) {
+      // Since this is let*, subsequent variables can refer to previous ones,
+      // so we need to substitute here.
+      init = _substitute(init);
+      int n = counter.counts[name];
+      if (n == null || n < 2) {
+        substitutions[name] = _substitute(init);
+      } else {
+        params.add(substitutions[name] = new JSTemporary(name));
+        values.add(init);
+      }
+    });
+
+    // Interpolate the body:
+    // Replace interpolated exprs with their value, if it only occurs once.
+    // Otherwise replace it with a temp, which will be assigned once.
+    return _substitute(node);
+  }
+
+  /// If we finish with an assignment to an identifier, try to simplify the
+  /// block. For example:
+  ///
+  ///     ((_) => _.add(1), _.add(2), result = _)([])
+  ///
+  /// Can be transformed to:
+  ///
+  ///     (result = [], result.add(1), result.add(2), result)
+  ///
+  /// However we should not simplify in this case because `result` is read:
+  ///
+  ///     ((_) => _.addAll(result), _.add(2), result = _)([])
+  ///
+  JSMetaLet _simplifyAssignment(Identifier left, {bool isDeclaration: false}) {
+    // See if the result value is a let* temporary variable.
+    if (body.last is! InterpolatedExpression) return null;
+
+    InterpolatedExpression last = body.last;
+    String name = last.nameOrPosition;
+    if (!variables.containsKey(name)) return null;
+
+    // Variables declared can't be used inside their initializer.
+    if (!isDeclaration) {
+      var finder = new _IdentFinder(left.name);
+      for (var expr in body) {
+        if (finder.found) break;
+        expr.accept(finder);
+      }
+      // If the identifier was used elsewhere, bail, because we're going to change

[Jennifer Messerly, 2015/04/07 22:00:28]

oops, long line.

+      // the order of when the assignment happens.
+      if (finder.found) return null;
+    }
+
+    var vars = new Map<String, Expression>.from(variables);
+    var value = vars.remove(name);
+    Expression assign;
+    if (isDeclaration) {
+      // Technically, putting one of these in a comma expression is not
+      // legal. However when isDeclaration is true, toStatement will be
+      // called immediately on the JSMetaLet, which results in legal JS.
+      assign = new VariableDeclarationList(
+          'let', [new VariableInitialization(left, value)]);
+    } else {
+      assign = value.toAssignExpression(left);
+    }
+
+    var newBody = new Expression.binary([assign]..addAll(body), ',');
+    Binary comma = new Template(null, newBody).safeCreate({name: left});
+    return new JSMetaLet(vars, comma.commaToExpressionList(),
+        statelessResult: statelessResult);
+  }
+}
+
+class _VariableUseCounter extends BaseVisitor {
+  final counts = <String, int>{};
+  visitInterpolatedExpression(InterpolatedExpression node) {
+    int n = counts[node.nameOrPosition];
+    counts[node.nameOrPosition] = n == null ? 1 : n + 1;
+  }
+}
+
+class _IdentFinder extends BaseVisitor {
+  final String name;
+  bool found = false;
+  _IdentFinder(this.name);
+
+  visitIdentifier(Identifier node) {
+    if (node.name == name) found = true;
+  }
+  visitNode(Node node) {
+    if (!found) super.visitNode(node);
+  }
+}