Introduce a shadow hierarchy and factory class for use by fasta's BodyBuilder.

pkg/front_end/lib/src/fasta/builder/shadow_ast.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file

[ahe, 2017/03/24 12:00:40]

This comment is so last year ;-)

[Paul Berry, 2017/03/24 14:55:23]

Done.

+// 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.
+
+/// This file declares mixins which can be used to create a shadow hierarchy
+/// of either the kernel or the analyzer AST representations.
+///
+/// These classes are intended to be used by [BodyBuilder] as a form of
+/// indirection so that it can manipulate either kernel or analyzer ASTs.
+///
+/// All methods, getters, and setters defined in this file start with the prefix
+/// "shadow" in order to avoid naming conflicts with code in kernel or analyzer.

[ahe, 2017/03/24 12:00:40]

It would probably help debugging if we also add Shadow prefix to the classes
defined here.

[Paul Berry, 2017/03/24 14:55:23]

Fair enough.  Done.

+///
+/// Note that the analyzer AST representation closely parallels Dart syntax,
+/// whereas the kernel AST representation is desugared.  The classes in the
+/// shadow hierarchy represent the full language (prior to desugaring).
+import 'package:kernel/ast.dart' show DartType;
+
+/// Shadow mixin representing a statement block.
+abstract class Block implements Statement {
+  /// Iterates through the statements contained in the block.
+  Iterable<Statement> get shadowStatements;
+}
+
+/// Common interface for shadow mixins representing expressions.
+///
+/// TODO(paulberry): add an abstract `shadowInfer` method here to do type
+/// inference.
+abstract class Expression {}
+
+/// Shadow mixin representing a function expression.
+abstract class FunctionExpression implements Expression {
+  /// Gets the body of the function expression.
+  Statement get shadowBody;
+
+  /// Indicates whether the function is asynchronous (`async` or `async*`)
+  bool get shadowIsAsync;
+
+  /// Indicates whether the function was declared using `=>` syntax.
+  bool get shadowIsExpressionFunction;
+
+  /// Indicates whether the function is a generator (`sync*` or `async*`)
+  bool get shadowIsGenerator;
+
+  /// Sets the return type of the function expression.
+  ///
+  /// Intended for use by type inference.
+  void set shadowReturnType(DartType type);
+
+  /// Creates a [DartType] representing the type of the function expression.
+  ///
+  /// If type inference has already been performed, returns the inferred type.
+  /// Otherwise returns the declared type.
+  DartType shadowMakeFunctionType();

[scheglov, 2017/03/24 03:19:17]

Why not getter?

[Paul Berry, 2017/03/24 14:55:23]

Done.

+}
+
+/// Shadow mixin representing an integer literal.
+abstract class IntLiteral implements Expression {}
+
+/// Shadow mixin representing a list literal.
+abstract class ListLiteral implements Expression {
+  /// Iterates through the expressions contained in the list literal.
+  Iterable<Expression> get shadowExpressions;
+
+  /// Gets the type argument of the list literal.  If type inference has not
+  /// been performed and no explicit type argument was specified, returns
+  /// `null`.
+  DartType get shadowTypeArgument;
+
+  /// Sets the type argument of the list literal.
+  ///
+  /// Intended for use by type inference.
+  void set shadowTypeArgument(DartType type);
+}
+
+/// Shadow mixin representing a null literal.
+abstract class NullLiteral implements Expression {}
+
+/// Shadow mixin representing a return statement.
+abstract class ReturnStatement implements Statement {
+  /// Gets the expression being returned, or `null` if this is a bare "return"
+  /// statement.
+  Expression get shadowExpression;
+}
+
+/// Common interface for shadow mixins representing statements.
+///
+/// TODO(paulberry): add an abstract `shadowInfer` method here to do type
+/// inference.
+abstract class Statement {}
+
+/// Shadow mixin representing a declaration of a single variable.
+abstract class VariableDeclaration implements Statement {
+  /// Gets the initializer expression for the variable, or `null` if the
+  /// variable has no initializer.
+  Expression get shadowInitializer;
+
+  /// Gets the type of the variable.  If type inference has not been performed
+  /// and no explicit type was specified, returns `null`.
+  DartType get shadowType;
+
+  /// Sets the type of the variable.
+  ///
+  /// Intended for use by type inference.
+  void set shadowType(DartType type);
+}
+
+/// Shadow mixin representing a "read" reference to a variable.

[scheglov, 2017/03/24 03:19:16]

I don't understand this yet.
Is it a node or element in Analyzer terms?
Is it partially resolved, so it knows the referenced variable?
Are these variables limited to locals, or can be also top-levels and fields?
If it also includes not locals, are we going to use shadowInitializer or only
shadowType for them?

[Paul Berry, 2017/03/24 14:55:23]

I haven't prototyped this yet, but here's my current thinking:

(Note that I'm following Peter's advice and renaming the classes in this file to
start with "Shadow...", and the derived classes in fasta/kernel/shadow_ast to
"Kernel...".  I'll follow the new conventions in this explanation, and I'll use
"Analyzer..." to talk about the derived/wrapper classes I hope to introduce in
the future for analyzer.)

- This will be used for locals, top-level variables, and fields.

- For kernel there really isn't a separate notion of element and AST--everything
is a kernel tree node.  So when a KernelVariableGet is constructed, it will just
store the pointer to the KernelVariableDeclaration.

- For analyzer, the equivalent of a VariableGet is SimpleIdentifier (or
sometimes PrefixedIdentifier), and that points to an element, not an AST node. 
The way this will work is that AnalyzerVariableDeclaration will be a wrapper
that stores both a pointer to an AST node and an element.  For local variables,
both will be populated.  For non-local variables, whatever Fasta code creates
the element model will also create an AnalyzerVariableDeclaration, and that will
point just to the element.  In both cases, when an AnalyzerVariableGet is
created, it will extract the element from the AnalyzerVariableDeclaration, and
point the Identifier to it.

+abstract class VariableGet implements Expression {
+  /// Gets the variable declaration which is being referenced.
+  VariableDeclaration get shadowDeclaration;
+}