angular/packages/compiler/src/output/output_ast.ts

/**
 * @license
 * Copyright Google Inc. All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */


import {ParseSourceSpan} from '../parse_util';
import {error} from '../util';

//// Types
export enum TypeModifier {
  Const
}

export abstract class Type {
  constructor(public modifiers: TypeModifier[]|null = null) {
    if (!modifiers) {
      this.modifiers = [];
    }
  }
  abstract visitType(visitor: TypeVisitor, context: any): any;

  hasModifier(modifier: TypeModifier): boolean { return this.modifiers !.indexOf(modifier) !== -1; }
}

export enum BuiltinTypeName {
  Dynamic,
  Bool,
  String,
  Int,
  Number,
  Function,
  Inferred
}

export class BuiltinType extends Type {
  constructor(public name: BuiltinTypeName, modifiers: TypeModifier[]|null = null) {
    super(modifiers);
  }
  visitType(visitor: TypeVisitor, context: any): any {
    return visitor.visitBuiltinType(this, context);
  }
}

export class ExpressionType extends Type {
  constructor(public value: Expression, modifiers: TypeModifier[]|null = null) { super(modifiers); }
  visitType(visitor: TypeVisitor, context: any): any {
    return visitor.visitExpressionType(this, context);
  }
}


export class ArrayType extends Type {
  constructor(public of : Type, modifiers: TypeModifier[]|null = null) { super(modifiers); }
  visitType(visitor: TypeVisitor, context: any): any {
    return visitor.visitArrayType(this, context);
  }
}


export class MapType extends Type {
  public valueType: Type|null;
  constructor(valueType: Type|null|undefined, modifiers: TypeModifier[]|null = null) {
    super(modifiers);
    this.valueType = valueType || null;
  }
  visitType(visitor: TypeVisitor, context: any): any { return visitor.visitMapType(this, context); }
}

export const DYNAMIC_TYPE = new BuiltinType(BuiltinTypeName.Dynamic);
export const INFERRED_TYPE = new BuiltinType(BuiltinTypeName.Inferred);
export const BOOL_TYPE = new BuiltinType(BuiltinTypeName.Bool);
export const INT_TYPE = new BuiltinType(BuiltinTypeName.Int);
export const NUMBER_TYPE = new BuiltinType(BuiltinTypeName.Number);
export const STRING_TYPE = new BuiltinType(BuiltinTypeName.String);
export const FUNCTION_TYPE = new BuiltinType(BuiltinTypeName.Function);

export interface TypeVisitor {
  visitBuiltinType(type: BuiltinType, context: any): any;
  visitExpressionType(type: ExpressionType, context: any): any;
  visitArrayType(type: ArrayType, context: any): any;
  visitMapType(type: MapType, context: any): any;
}

///// Expressions

export enum BinaryOperator {
  Equals,
  NotEquals,
  Identical,
  NotIdentical,
  Minus,
  Plus,
  Divide,
  Multiply,
  Modulo,
  And,
  Or,
  Lower,
  LowerEquals,
  Bigger,
  BiggerEquals
}

export function nullSafeIsEquivalent<T extends{isEquivalent(other: T): boolean}>(
    base: T | null, other: T | null) {
  if (base == null || other == null) {
    return base == other;
  }
  return base.isEquivalent(other);
}

export function areAllEquivalent<T extends{isEquivalent(other: T): boolean}>(
    base: T[], other: T[]) {
  const len = base.length;
  if (len !== other.length) {
    return false;
  }
  for (let i = 0; i < len; i++) {
    if (!base[i].isEquivalent(other[i])) {
      return false;
    }
  }
  return true;
}

export abstract class Expression {
  public type: Type|null;
  public sourceSpan: ParseSourceSpan|null;

  constructor(type: Type|null|undefined, sourceSpan?: ParseSourceSpan|null) {
    this.type = type || null;
    this.sourceSpan = sourceSpan || null;
  }

  abstract visitExpression(visitor: ExpressionVisitor, context: any): any;

  /**
   * Calculates whether this expression produces the same value as the given expression.
   * Note: We don't check Types nor ParseSourceSpans nor function arguments.
   */
  abstract isEquivalent(e: Expression): boolean;

  prop(name: string, sourceSpan?: ParseSourceSpan|null): ReadPropExpr {
    return new ReadPropExpr(this, name, null, sourceSpan);
  }

  key(index: Expression, type?: Type|null, sourceSpan?: ParseSourceSpan|null): ReadKeyExpr {
    return new ReadKeyExpr(this, index, type, sourceSpan);
  }

  callMethod(name: string|BuiltinMethod, params: Expression[], sourceSpan?: ParseSourceSpan|null):
      InvokeMethodExpr {
    return new InvokeMethodExpr(this, name, params, null, sourceSpan);
  }

  callFn(params: Expression[], sourceSpan?: ParseSourceSpan|null): InvokeFunctionExpr {
    return new InvokeFunctionExpr(this, params, null, sourceSpan);
  }

  instantiate(params: Expression[], type?: Type|null, sourceSpan?: ParseSourceSpan|null):
      InstantiateExpr {
    return new InstantiateExpr(this, params, type, sourceSpan);
  }

  conditional(
      trueCase: Expression, falseCase: Expression|null = null,
      sourceSpan?: ParseSourceSpan|null): ConditionalExpr {
    return new ConditionalExpr(this, trueCase, falseCase, null, sourceSpan);
  }

  equals(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Equals, this, rhs, null, sourceSpan);
  }
  notEquals(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.NotEquals, this, rhs, null, sourceSpan);
  }
  identical(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Identical, this, rhs, null, sourceSpan);
  }
  notIdentical(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.NotIdentical, this, rhs, null, sourceSpan);
  }
  minus(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Minus, this, rhs, null, sourceSpan);
  }
  plus(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Plus, this, rhs, null, sourceSpan);
  }
  divide(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Divide, this, rhs, null, sourceSpan);
  }
  multiply(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Multiply, this, rhs, null, sourceSpan);
  }
  modulo(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Modulo, this, rhs, null, sourceSpan);
  }
  and(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.And, this, rhs, null, sourceSpan);
  }
  or(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Or, this, rhs, null, sourceSpan);
  }
  lower(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Lower, this, rhs, null, sourceSpan);
  }
  lowerEquals(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.LowerEquals, this, rhs, null, sourceSpan);
  }
  bigger(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.Bigger, this, rhs, null, sourceSpan);
  }
  biggerEquals(rhs: Expression, sourceSpan?: ParseSourceSpan|null): BinaryOperatorExpr {
    return new BinaryOperatorExpr(BinaryOperator.BiggerEquals, this, rhs, null, sourceSpan);
  }
  isBlank(sourceSpan?: ParseSourceSpan|null): Expression {
    // Note: We use equals by purpose here to compare to null and undefined in JS.
    // We use the typed null to allow strictNullChecks to narrow types.
    return this.equals(TYPED_NULL_EXPR, sourceSpan);
  }
  cast(type: Type, sourceSpan?: ParseSourceSpan|null): Expression {
    return new CastExpr(this, type, sourceSpan);
  }

  toStmt(): Statement { return new ExpressionStatement(this, null); }
}

export enum BuiltinVar {
  This,
  Super,
  CatchError,
  CatchStack
}

export class ReadVarExpr extends Expression {
  public name: string|null;
  public builtin: BuiltinVar|null;

  constructor(name: string|BuiltinVar, type?: Type|null, sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
    if (typeof name === 'string') {
      this.name = name;
      this.builtin = null;
    } else {
      this.name = null;
      this.builtin = <BuiltinVar>name;
    }
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof ReadVarExpr && this.name === e.name && this.builtin === e.builtin;
  }

  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitReadVarExpr(this, context);
  }

  set(value: Expression): WriteVarExpr {
    if (!this.name) {
      throw new Error(`Built in variable ${this.builtin} can not be assigned to.`);
    }
    return new WriteVarExpr(this.name, value, null, this.sourceSpan);
  }
}


export class WriteVarExpr extends Expression {
  public value: Expression;
  constructor(
      public name: string, value: Expression, type?: Type|null, sourceSpan?: ParseSourceSpan|null) {
    super(type || value.type, sourceSpan);
    this.value = value;
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof WriteVarExpr && this.name === e.name && this.value.isEquivalent(e.value);
  }

  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitWriteVarExpr(this, context);
  }

  toDeclStmt(type?: Type|null, modifiers?: StmtModifier[]|null): DeclareVarStmt {
    return new DeclareVarStmt(this.name, this.value, type, modifiers, this.sourceSpan);
  }
}


export class WriteKeyExpr extends Expression {
  public value: Expression;
  constructor(
      public receiver: Expression, public index: Expression, value: Expression, type?: Type|null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type || value.type, sourceSpan);
    this.value = value;
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof WriteKeyExpr && this.receiver.isEquivalent(e.receiver) &&
        this.index.isEquivalent(e.index) && this.value.isEquivalent(e.value);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitWriteKeyExpr(this, context);
  }
}


export class WritePropExpr extends Expression {
  public value: Expression;
  constructor(
      public receiver: Expression, public name: string, value: Expression, type?: Type|null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type || value.type, sourceSpan);
    this.value = value;
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof WritePropExpr && this.receiver.isEquivalent(e.receiver) &&
        this.name === e.name && this.value.isEquivalent(e.value);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitWritePropExpr(this, context);
  }
}

export enum BuiltinMethod {
  ConcatArray,
  SubscribeObservable,
  Bind
}

export class InvokeMethodExpr extends Expression {
  public name: string|null;
  public builtin: BuiltinMethod|null;
  constructor(
      public receiver: Expression, method: string|BuiltinMethod, public args: Expression[],
      type?: Type|null, sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
    if (typeof method === 'string') {
      this.name = method;
      this.builtin = null;
    } else {
      this.name = null;
      this.builtin = <BuiltinMethod>method;
    }
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof InvokeMethodExpr && this.receiver.isEquivalent(e.receiver) &&
        this.name === e.name && this.builtin === e.builtin && areAllEquivalent(this.args, e.args);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitInvokeMethodExpr(this, context);
  }
}


export class InvokeFunctionExpr extends Expression {
  constructor(
      public fn: Expression, public args: Expression[], type?: Type|null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof InvokeFunctionExpr && this.fn.isEquivalent(e.fn) &&
        areAllEquivalent(this.args, e.args);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitInvokeFunctionExpr(this, context);
  }
}


export class InstantiateExpr extends Expression {
  constructor(
      public classExpr: Expression, public args: Expression[], type?: Type|null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof InstantiateExpr && this.classExpr.isEquivalent(e.classExpr) &&
        areAllEquivalent(this.args, e.args);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitInstantiateExpr(this, context);
  }
}


export class LiteralExpr extends Expression {
  constructor(
      public value: number|string|boolean|null|undefined, type?: Type|null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof LiteralExpr && this.value === e.value;
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitLiteralExpr(this, context);
  }
}


export class ExternalExpr extends Expression {
  constructor(
      public value: ExternalReference, type?: Type|null, public typeParams: Type[]|null = null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof ExternalExpr && this.value.name === e.value.name &&
        this.value.moduleName === e.value.moduleName && this.value.runtime === e.value.runtime;
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitExternalExpr(this, context);
  }
}

export class ExternalReference {
  constructor(public moduleName: string|null, public name: string|null, public runtime?: any|null) {
  }
  // Note: no isEquivalent method here as we use this as an interface too.
}

export class ConditionalExpr extends Expression {
  public trueCase: Expression;
  constructor(
      public condition: Expression, trueCase: Expression, public falseCase: Expression|null = null,
      type?: Type|null, sourceSpan?: ParseSourceSpan|null) {
    super(type || trueCase.type, sourceSpan);
    this.trueCase = trueCase;
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof ConditionalExpr && this.condition.isEquivalent(e.condition) &&
        this.trueCase.isEquivalent(e.trueCase) && nullSafeIsEquivalent(this.falseCase, e.falseCase);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitConditionalExpr(this, context);
  }
}


export class NotExpr extends Expression {
  constructor(public condition: Expression, sourceSpan?: ParseSourceSpan|null) {
    super(BOOL_TYPE, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof NotExpr && this.condition.isEquivalent(e.condition);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitNotExpr(this, context);
  }
}

export class AssertNotNull extends Expression {
  constructor(public condition: Expression, sourceSpan?: ParseSourceSpan|null) {
    super(condition.type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof AssertNotNull && this.condition.isEquivalent(e.condition);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitAssertNotNullExpr(this, context);
  }
}

export class CastExpr extends Expression {
  constructor(public value: Expression, type?: Type|null, sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof CastExpr && this.value.isEquivalent(e.value);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitCastExpr(this, context);
  }
}


export class FnParam {
  constructor(public name: string, public type: Type|null = null) {}

  isEquivalent(param: FnParam): boolean { return this.name === param.name; }
}


export class FunctionExpr extends Expression {
  constructor(
      public params: FnParam[], public statements: Statement[], type?: Type|null,
      sourceSpan?: ParseSourceSpan|null, public name?: string|null) {
    super(type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof FunctionExpr && areAllEquivalent(this.params, e.params) &&
        areAllEquivalent(this.statements, e.statements);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitFunctionExpr(this, context);
  }

  toDeclStmt(name: string, modifiers: StmtModifier[]|null = null): DeclareFunctionStmt {
    return new DeclareFunctionStmt(
        name, this.params, this.statements, this.type, modifiers, this.sourceSpan);
  }
}


export class BinaryOperatorExpr extends Expression {
  public lhs: Expression;
  constructor(
      public operator: BinaryOperator, lhs: Expression, public rhs: Expression, type?: Type|null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type || lhs.type, sourceSpan);
    this.lhs = lhs;
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof BinaryOperatorExpr && this.operator === e.operator &&
        this.lhs.isEquivalent(e.lhs) && this.rhs.isEquivalent(e.rhs);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitBinaryOperatorExpr(this, context);
  }
}


export class ReadPropExpr extends Expression {
  constructor(
      public receiver: Expression, public name: string, type?: Type|null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof ReadPropExpr && this.receiver.isEquivalent(e.receiver) &&
        this.name === e.name;
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitReadPropExpr(this, context);
  }
  set(value: Expression): WritePropExpr {
    return new WritePropExpr(this.receiver, this.name, value, null, this.sourceSpan);
  }
}


export class ReadKeyExpr extends Expression {
  constructor(
      public receiver: Expression, public index: Expression, type?: Type|null,
      sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof ReadKeyExpr && this.receiver.isEquivalent(e.receiver) &&
        this.index.isEquivalent(e.index);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitReadKeyExpr(this, context);
  }
  set(value: Expression): WriteKeyExpr {
    return new WriteKeyExpr(this.receiver, this.index, value, null, this.sourceSpan);
  }
}


export class LiteralArrayExpr extends Expression {
  public entries: Expression[];
  constructor(entries: Expression[], type?: Type|null, sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
    this.entries = entries;
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof LiteralArrayExpr && areAllEquivalent(this.entries, e.entries);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitLiteralArrayExpr(this, context);
  }
}

export class LiteralMapEntry {
  constructor(public key: string, public value: Expression, public quoted: boolean) {}
  isEquivalent(e: LiteralMapEntry): boolean {
    return this.key === e.key && this.value.isEquivalent(e.value);
  }
}

export class LiteralMapExpr extends Expression {
  public valueType: Type|null = null;
  constructor(
      public entries: LiteralMapEntry[], type?: MapType|null, sourceSpan?: ParseSourceSpan|null) {
    super(type, sourceSpan);
    if (type) {
      this.valueType = type.valueType;
    }
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof LiteralMapExpr && areAllEquivalent(this.entries, e.entries);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitLiteralMapExpr(this, context);
  }
}

export class CommaExpr extends Expression {
  constructor(public parts: Expression[], sourceSpan?: ParseSourceSpan|null) {
    super(parts[parts.length - 1].type, sourceSpan);
  }
  isEquivalent(e: Expression): boolean {
    return e instanceof CommaExpr && areAllEquivalent(this.parts, e.parts);
  }
  visitExpression(visitor: ExpressionVisitor, context: any): any {
    return visitor.visitCommaExpr(this, context);
  }
}

export interface ExpressionVisitor {
  visitReadVarExpr(ast: ReadVarExpr, context: any): any;
  visitWriteVarExpr(expr: WriteVarExpr, context: any): any;
  visitWriteKeyExpr(expr: WriteKeyExpr, context: any): any;
  visitWritePropExpr(expr: WritePropExpr, context: any): any;
  visitInvokeMethodExpr(ast: InvokeMethodExpr, context: any): any;
  visitInvokeFunctionExpr(ast: InvokeFunctionExpr, context: any): any;
  visitInstantiateExpr(ast: InstantiateExpr, context: any): any;
  visitLiteralExpr(ast: LiteralExpr, context: any): any;
  visitExternalExpr(ast: ExternalExpr, context: any): any;
  visitConditionalExpr(ast: ConditionalExpr, context: any): any;
  visitNotExpr(ast: NotExpr, context: any): any;
  visitAssertNotNullExpr(ast: AssertNotNull, context: any): any;
  visitCastExpr(ast: CastExpr, context: any): any;
  visitFunctionExpr(ast: FunctionExpr, context: any): any;
  visitBinaryOperatorExpr(ast: BinaryOperatorExpr, context: any): any;
  visitReadPropExpr(ast: ReadPropExpr, context: any): any;
  visitReadKeyExpr(ast: ReadKeyExpr, context: any): any;
  visitLiteralArrayExpr(ast: LiteralArrayExpr, context: any): any;
  visitLiteralMapExpr(ast: LiteralMapExpr, context: any): any;
  visitCommaExpr(ast: CommaExpr, context: any): any;
}

export const THIS_EXPR = new ReadVarExpr(BuiltinVar.This, null, null);
export const SUPER_EXPR = new ReadVarExpr(BuiltinVar.Super, null, null);
export const CATCH_ERROR_VAR = new ReadVarExpr(BuiltinVar.CatchError, null, null);
export const CATCH_STACK_VAR = new ReadVarExpr(BuiltinVar.CatchStack, null, null);
export const NULL_EXPR = new LiteralExpr(null, null, null);
export const TYPED_NULL_EXPR = new LiteralExpr(null, INFERRED_TYPE, null);

//// Statements
export enum StmtModifier {
  Final,
  Private,
  Exported,
  Static,
}

export abstract class Statement {
  public modifiers: StmtModifier[];
  public sourceSpan: ParseSourceSpan|null;
  constructor(modifiers?: StmtModifier[]|null, sourceSpan?: ParseSourceSpan|null) {
    this.modifiers = modifiers || [];
    this.sourceSpan = sourceSpan || null;
  }
  /**
   * Calculates whether this statement produces the same value as the given statement.
   * Note: We don't check Types nor ParseSourceSpans nor function arguments.
   */
  abstract isEquivalent(stmt: Statement): boolean;

  abstract visitStatement(visitor: StatementVisitor, context: any): any;

  hasModifier(modifier: StmtModifier): boolean { return this.modifiers !.indexOf(modifier) !== -1; }
}


export class DeclareVarStmt extends Statement {
  public type: Type|null;
  constructor(
      public name: string, public value: Expression, type?: Type|null,
      modifiers: StmtModifier[]|null = null, sourceSpan?: ParseSourceSpan|null) {
    super(modifiers, sourceSpan);
    this.type = type || value.type;
  }
  isEquivalent(stmt: Statement): boolean {
    return stmt instanceof DeclareVarStmt && this.name === stmt.name &&
        this.value.isEquivalent(stmt.value);
  }
  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitDeclareVarStmt(this, context);
  }
}

export class DeclareFunctionStmt extends Statement {
  public type: Type|null;
  constructor(
      public name: string, public params: FnParam[], public statements: Statement[],
      type?: Type|null, modifiers: StmtModifier[]|null = null, sourceSpan?: ParseSourceSpan|null) {
    super(modifiers, sourceSpan);
    this.type = type || null;
  }
  isEquivalent(stmt: Statement): boolean {
    return stmt instanceof DeclareFunctionStmt && areAllEquivalent(this.params, stmt.params) &&
        areAllEquivalent(this.statements, stmt.statements);
  }

  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitDeclareFunctionStmt(this, context);
  }
}

export class ExpressionStatement extends Statement {
  constructor(public expr: Expression, sourceSpan?: ParseSourceSpan|null) {
    super(null, sourceSpan);
  }
  isEquivalent(stmt: Statement): boolean {
    return stmt instanceof ExpressionStatement && this.expr.isEquivalent(stmt.expr);
  }

  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitExpressionStmt(this, context);
  }
}


export class ReturnStatement extends Statement {
  constructor(public value: Expression, sourceSpan?: ParseSourceSpan|null) {
    super(null, sourceSpan);
  }
  isEquivalent(stmt: Statement): boolean {
    return stmt instanceof ReturnStatement && this.value.isEquivalent(stmt.value);
  }
  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitReturnStmt(this, context);
  }
}

export class AbstractClassPart {
  public type: Type|null;
  constructor(type: Type|null|undefined, public modifiers: StmtModifier[]|null) {
    if (!modifiers) {
      this.modifiers = [];
    }
    this.type = type || null;
  }
  hasModifier(modifier: StmtModifier): boolean { return this.modifiers !.indexOf(modifier) !== -1; }
}

export class ClassField extends AbstractClassPart {
  constructor(
      public name: string, type?: Type|null, modifiers: StmtModifier[]|null = null,
      public initializer?: Expression) {
    super(type, modifiers);
  }
  isEquivalent(f: ClassField) { return this.name === f.name; }
}


export class ClassMethod extends AbstractClassPart {
  constructor(
      public name: string|null, public params: FnParam[], public body: Statement[],
      type?: Type|null, modifiers: StmtModifier[]|null = null) {
    super(type, modifiers);
  }
  isEquivalent(m: ClassMethod) {
    return this.name === m.name && areAllEquivalent(this.body, m.body);
  }
}


export class ClassGetter extends AbstractClassPart {
  constructor(
      public name: string, public body: Statement[], type?: Type|null,
      modifiers: StmtModifier[]|null = null) {
    super(type, modifiers);
  }
  isEquivalent(m: ClassGetter) {
    return this.name === m.name && areAllEquivalent(this.body, m.body);
  }
}


export class ClassStmt extends Statement {
  constructor(
      public name: string, public parent: Expression|null, public fields: ClassField[],
      public getters: ClassGetter[], public constructorMethod: ClassMethod,
      public methods: ClassMethod[], modifiers: StmtModifier[]|null = null,
      sourceSpan?: ParseSourceSpan|null) {
    super(modifiers, sourceSpan);
  }
  isEquivalent(stmt: Statement): boolean {
    return stmt instanceof ClassStmt && this.name === stmt.name &&
        nullSafeIsEquivalent(this.parent, stmt.parent) &&
        areAllEquivalent(this.fields, stmt.fields) &&
        areAllEquivalent(this.getters, stmt.getters) &&
        this.constructorMethod.isEquivalent(stmt.constructorMethod) &&
        areAllEquivalent(this.methods, stmt.methods);
  }
  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitDeclareClassStmt(this, context);
  }
}


export class IfStmt extends Statement {
  constructor(
      public condition: Expression, public trueCase: Statement[],
      public falseCase: Statement[] = [], sourceSpan?: ParseSourceSpan|null) {
    super(null, sourceSpan);
  }
  isEquivalent(stmt: Statement): boolean {
    return stmt instanceof IfStmt && this.condition.isEquivalent(stmt.condition) &&
        areAllEquivalent(this.trueCase, stmt.trueCase) &&
        areAllEquivalent(this.falseCase, stmt.falseCase);
  }
  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitIfStmt(this, context);
  }
}


export class CommentStmt extends Statement {
  constructor(public comment: string, sourceSpan?: ParseSourceSpan|null) {
    super(null, sourceSpan);
  }
  isEquivalent(stmt: Statement): boolean { return stmt instanceof CommentStmt; }
  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitCommentStmt(this, context);
  }
}


export class TryCatchStmt extends Statement {
  constructor(
      public bodyStmts: Statement[], public catchStmts: Statement[],
      sourceSpan?: ParseSourceSpan|null) {
    super(null, sourceSpan);
  }
  isEquivalent(stmt: Statement): boolean {
    return stmt instanceof TryCatchStmt && areAllEquivalent(this.bodyStmts, stmt.bodyStmts) &&
        areAllEquivalent(this.catchStmts, stmt.catchStmts);
  }
  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitTryCatchStmt(this, context);
  }
}


export class ThrowStmt extends Statement {
  constructor(public error: Expression, sourceSpan?: ParseSourceSpan|null) {
    super(null, sourceSpan);
  }
  isEquivalent(stmt: ThrowStmt): boolean {
    return stmt instanceof TryCatchStmt && this.error.isEquivalent(stmt.error);
  }
  visitStatement(visitor: StatementVisitor, context: any): any {
    return visitor.visitThrowStmt(this, context);
  }
}

export interface StatementVisitor {
  visitDeclareVarStmt(stmt: DeclareVarStmt, context: any): any;
  visitDeclareFunctionStmt(stmt: DeclareFunctionStmt, context: any): any;
  visitExpressionStmt(stmt: ExpressionStatement, context: any): any;
  visitReturnStmt(stmt: ReturnStatement, context: any): any;
  visitDeclareClassStmt(stmt: ClassStmt, context: any): any;
  visitIfStmt(stmt: IfStmt, context: any): any;
  visitTryCatchStmt(stmt: TryCatchStmt, context: any): any;
  visitThrowStmt(stmt: ThrowStmt, context: any): any;
  visitCommentStmt(stmt: CommentStmt, context: any): any;
}

export class AstTransformer implements StatementVisitor, ExpressionVisitor {
  transformExpr(expr: Expression, context: any): Expression { return expr; }

  transformStmt(stmt: Statement, context: any): Statement { return stmt; }

  visitReadVarExpr(ast: ReadVarExpr, context: any): any { return this.transformExpr(ast, context); }

  visitWriteVarExpr(expr: WriteVarExpr, context: any): any {
    return this.transformExpr(
        new WriteVarExpr(
            expr.name, expr.value.visitExpression(this, context), expr.type, expr.sourceSpan),
        context);
  }

  visitWriteKeyExpr(expr: WriteKeyExpr, context: any): any {
    return this.transformExpr(
        new WriteKeyExpr(
            expr.receiver.visitExpression(this, context), expr.index.visitExpression(this, context),
            expr.value.visitExpression(this, context), expr.type, expr.sourceSpan),
        context);
  }

  visitWritePropExpr(expr: WritePropExpr, context: any): any {
    return this.transformExpr(
        new WritePropExpr(
            expr.receiver.visitExpression(this, context), expr.name,
            expr.value.visitExpression(this, context), expr.type, expr.sourceSpan),
        context);
  }

  visitInvokeMethodExpr(ast: InvokeMethodExpr, context: any): any {
    const method = ast.builtin || ast.name;
    return this.transformExpr(
        new InvokeMethodExpr(
            ast.receiver.visitExpression(this, context), method !,
            this.visitAllExpressions(ast.args, context), ast.type, ast.sourceSpan),
        context);
  }

  visitInvokeFunctionExpr(ast: InvokeFunctionExpr, context: any): any {
    return this.transformExpr(
        new InvokeFunctionExpr(
            ast.fn.visitExpression(this, context), this.visitAllExpressions(ast.args, context),
            ast.type, ast.sourceSpan),
        context);
  }

  visitInstantiateExpr(ast: InstantiateExpr, context: any): any {
    return this.transformExpr(
        new InstantiateExpr(
            ast.classExpr.visitExpression(this, context),
            this.visitAllExpressions(ast.args, context), ast.type, ast.sourceSpan),
        context);
  }

  visitLiteralExpr(ast: LiteralExpr, context: any): any { return this.transformExpr(ast, context); }

  visitExternalExpr(ast: ExternalExpr, context: any): any {
    return this.transformExpr(ast, context);
  }

  visitConditionalExpr(ast: ConditionalExpr, context: any): any {
    return this.transformExpr(
        new ConditionalExpr(
            ast.condition.visitExpression(this, context),
            ast.trueCase.visitExpression(this, context),
            ast.falseCase !.visitExpression(this, context), ast.type, ast.sourceSpan),
        context);
  }

  visitNotExpr(ast: NotExpr, context: any): any {
    return this.transformExpr(
        new NotExpr(ast.condition.visitExpression(this, context), ast.sourceSpan), context);
  }

  visitAssertNotNullExpr(ast: AssertNotNull, context: any): any {
    return this.transformExpr(
        new AssertNotNull(ast.condition.visitExpression(this, context), ast.sourceSpan), context);
  }

  visitCastExpr(ast: CastExpr, context: any): any {
    return this.transformExpr(
        new CastExpr(ast.value.visitExpression(this, context), ast.type, ast.sourceSpan), context);
  }

  visitFunctionExpr(ast: FunctionExpr, context: any): any {
    return this.transformExpr(
        new FunctionExpr(
            ast.params, this.visitAllStatements(ast.statements, context), ast.type, ast.sourceSpan),
        context);
  }

  visitBinaryOperatorExpr(ast: BinaryOperatorExpr, context: any): any {
    return this.transformExpr(
        new BinaryOperatorExpr(
            ast.operator, ast.lhs.visitExpression(this, context),
            ast.rhs.visitExpression(this, context), ast.type, ast.sourceSpan),
        context);
  }

  visitReadPropExpr(ast: ReadPropExpr, context: any): any {
    return this.transformExpr(
        new ReadPropExpr(
            ast.receiver.visitExpression(this, context), ast.name, ast.type, ast.sourceSpan),
        context);
  }

  visitReadKeyExpr(ast: ReadKeyExpr, context: any): any {
    return this.transformExpr(
        new ReadKeyExpr(
            ast.receiver.visitExpression(this, context), ast.index.visitExpression(this, context),
            ast.type, ast.sourceSpan),
        context);
  }

  visitLiteralArrayExpr(ast: LiteralArrayExpr, context: any): any {
    return this.transformExpr(
        new LiteralArrayExpr(
            this.visitAllExpressions(ast.entries, context), ast.type, ast.sourceSpan),
        context);
  }

  visitLiteralMapExpr(ast: LiteralMapExpr, context: any): any {
    const entries = ast.entries.map(
        (entry): LiteralMapEntry => new LiteralMapEntry(
            entry.key, entry.value.visitExpression(this, context), entry.quoted));
    const mapType = new MapType(ast.valueType, null);
    return this.transformExpr(new LiteralMapExpr(entries, mapType, ast.sourceSpan), context);
  }
  visitCommaExpr(ast: CommaExpr, context: any): any {
    return this.transformExpr(
        new CommaExpr(this.visitAllExpressions(ast.parts, context), ast.sourceSpan), context);
  }
  visitAllExpressions(exprs: Expression[], context: any): Expression[] {
    return exprs.map(expr => expr.visitExpression(this, context));
  }

  visitDeclareVarStmt(stmt: DeclareVarStmt, context: any): any {
    return this.transformStmt(
        new DeclareVarStmt(
            stmt.name, stmt.value.visitExpression(this, context), stmt.type, stmt.modifiers,
            stmt.sourceSpan),
        context);
  }
  visitDeclareFunctionStmt(stmt: DeclareFunctionStmt, context: any): any {
    return this.transformStmt(
        new DeclareFunctionStmt(
            stmt.name, stmt.params, this.visitAllStatements(stmt.statements, context), stmt.type,
            stmt.modifiers, stmt.sourceSpan),
        context);
  }

  visitExpressionStmt(stmt: ExpressionStatement, context: any): any {
    return this.transformStmt(
        new ExpressionStatement(stmt.expr.visitExpression(this, context), stmt.sourceSpan),
        context);
  }

  visitReturnStmt(stmt: ReturnStatement, context: any): any {
    return this.transformStmt(
        new ReturnStatement(stmt.value.visitExpression(this, context), stmt.sourceSpan), context);
  }

  visitDeclareClassStmt(stmt: ClassStmt, context: any): any {
    const parent = stmt.parent !.visitExpression(this, context);
    const getters = stmt.getters.map(
        getter => new ClassGetter(
            getter.name, this.visitAllStatements(getter.body, context), getter.type,
            getter.modifiers));
    const ctorMethod = stmt.constructorMethod &&
        new ClassMethod(stmt.constructorMethod.name, stmt.constructorMethod.params,
                        this.visitAllStatements(stmt.constructorMethod.body, context),
                        stmt.constructorMethod.type, stmt.constructorMethod.modifiers);
    const methods = stmt.methods.map(
        method => new ClassMethod(
            method.name, method.params, this.visitAllStatements(method.body, context), method.type,
            method.modifiers));
    return this.transformStmt(
        new ClassStmt(
            stmt.name, parent, stmt.fields, getters, ctorMethod, methods, stmt.modifiers,
            stmt.sourceSpan),
        context);
  }

  visitIfStmt(stmt: IfStmt, context: any): any {
    return this.transformStmt(
        new IfStmt(
            stmt.condition.visitExpression(this, context),
            this.visitAllStatements(stmt.trueCase, context),
            this.visitAllStatements(stmt.falseCase, context), stmt.sourceSpan),
        context);
  }

  visitTryCatchStmt(stmt: TryCatchStmt, context: any): any {
    return this.transformStmt(
        new TryCatchStmt(
            this.visitAllStatements(stmt.bodyStmts, context),
            this.visitAllStatements(stmt.catchStmts, context), stmt.sourceSpan),
        context);
  }

  visitThrowStmt(stmt: ThrowStmt, context: any): any {
    return this.transformStmt(
        new ThrowStmt(stmt.error.visitExpression(this, context), stmt.sourceSpan), context);
  }

  visitCommentStmt(stmt: CommentStmt, context: any): any {
    return this.transformStmt(stmt, context);
  }

  visitAllStatements(stmts: Statement[], context: any): Statement[] {
    return stmts.map(stmt => stmt.visitStatement(this, context));
  }
}


export class RecursiveAstVisitor implements StatementVisitor, ExpressionVisitor {
  visitType(ast: Type, context: any): any { return ast; }
  visitExpression(ast: Expression, context: any): any {
    if (ast.type) {
      ast.type.visitType(this, context);
    }
    return ast;
  }
  visitBuiltinType(type: BuiltinType, context: any): any { return this.visitType(type, context); }
  visitExpressionType(type: ExpressionType, context: any): any {
    type.value.visitExpression(this, context);
    return this.visitType(type, context);
  }
  visitArrayType(type: ArrayType, context: any): any { return this.visitType(type, context); }
  visitMapType(type: MapType, context: any): any { return this.visitType(type, context); }
  visitReadVarExpr(ast: ReadVarExpr, context: any): any {
    return this.visitExpression(ast, context);
  }
  visitWriteVarExpr(ast: WriteVarExpr, context: any): any {
    ast.value.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitWriteKeyExpr(ast: WriteKeyExpr, context: any): any {
    ast.receiver.visitExpression(this, context);
    ast.index.visitExpression(this, context);
    ast.value.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitWritePropExpr(ast: WritePropExpr, context: any): any {
    ast.receiver.visitExpression(this, context);
    ast.value.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitInvokeMethodExpr(ast: InvokeMethodExpr, context: any): any {
    ast.receiver.visitExpression(this, context);
    this.visitAllExpressions(ast.args, context);
    return this.visitExpression(ast, context);
  }
  visitInvokeFunctionExpr(ast: InvokeFunctionExpr, context: any): any {
    ast.fn.visitExpression(this, context);
    this.visitAllExpressions(ast.args, context);
    return this.visitExpression(ast, context);
  }
  visitInstantiateExpr(ast: InstantiateExpr, context: any): any {
    ast.classExpr.visitExpression(this, context);
    this.visitAllExpressions(ast.args, context);
    return this.visitExpression(ast, context);
  }
  visitLiteralExpr(ast: LiteralExpr, context: any): any {
    return this.visitExpression(ast, context);
  }
  visitExternalExpr(ast: ExternalExpr, context: any): any {
    if (ast.typeParams) {
      ast.typeParams.forEach(type => type.visitType(this, context));
    }
    return this.visitExpression(ast, context);
  }
  visitConditionalExpr(ast: ConditionalExpr, context: any): any {
    ast.condition.visitExpression(this, context);
    ast.trueCase.visitExpression(this, context);
    ast.falseCase !.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitNotExpr(ast: NotExpr, context: any): any {
    ast.condition.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitAssertNotNullExpr(ast: AssertNotNull, context: any): any {
    ast.condition.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitCastExpr(ast: CastExpr, context: any): any {
    ast.value.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitFunctionExpr(ast: FunctionExpr, context: any): any {
    this.visitAllStatements(ast.statements, context);
    return this.visitExpression(ast, context);
  }
  visitBinaryOperatorExpr(ast: BinaryOperatorExpr, context: any): any {
    ast.lhs.visitExpression(this, context);
    ast.rhs.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitReadPropExpr(ast: ReadPropExpr, context: any): any {
    ast.receiver.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitReadKeyExpr(ast: ReadKeyExpr, context: any): any {
    ast.receiver.visitExpression(this, context);
    ast.index.visitExpression(this, context);
    return this.visitExpression(ast, context);
  }
  visitLiteralArrayExpr(ast: LiteralArrayExpr, context: any): any {
    this.visitAllExpressions(ast.entries, context);
    return this.visitExpression(ast, context);
  }
  visitLiteralMapExpr(ast: LiteralMapExpr, context: any): any {
    ast.entries.forEach((entry) => entry.value.visitExpression(this, context));
    return this.visitExpression(ast, context);
  }
  visitCommaExpr(ast: CommaExpr, context: any): any {
    this.visitAllExpressions(ast.parts, context);
    return this.visitExpression(ast, context);
  }
  visitAllExpressions(exprs: Expression[], context: any): void {
    exprs.forEach(expr => expr.visitExpression(this, context));
  }

  visitDeclareVarStmt(stmt: DeclareVarStmt, context: any): any {
    stmt.value.visitExpression(this, context);
    if (stmt.type) {
      stmt.type.visitType(this, context);
    }
    return stmt;
  }
  visitDeclareFunctionStmt(stmt: DeclareFunctionStmt, context: any): any {
    this.visitAllStatements(stmt.statements, context);
    if (stmt.type) {
      stmt.type.visitType(this, context);
    }
    return stmt;
  }
  visitExpressionStmt(stmt: ExpressionStatement, context: any): any {
    stmt.expr.visitExpression(this, context);
    return stmt;
  }
  visitReturnStmt(stmt: ReturnStatement, context: any): any {
    stmt.value.visitExpression(this, context);
    return stmt;
  }
  visitDeclareClassStmt(stmt: ClassStmt, context: any): any {
    stmt.parent !.visitExpression(this, context);
    stmt.getters.forEach(getter => this.visitAllStatements(getter.body, context));
    if (stmt.constructorMethod) {
      this.visitAllStatements(stmt.constructorMethod.body, context);
    }
    stmt.methods.forEach(method => this.visitAllStatements(method.body, context));
    return stmt;
  }
  visitIfStmt(stmt: IfStmt, context: any): any {
    stmt.condition.visitExpression(this, context);
    this.visitAllStatements(stmt.trueCase, context);
    this.visitAllStatements(stmt.falseCase, context);
    return stmt;
  }
  visitTryCatchStmt(stmt: TryCatchStmt, context: any): any {
    this.visitAllStatements(stmt.bodyStmts, context);
    this.visitAllStatements(stmt.catchStmts, context);
    return stmt;
  }
  visitThrowStmt(stmt: ThrowStmt, context: any): any {
    stmt.error.visitExpression(this, context);
    return stmt;
  }
  visitCommentStmt(stmt: CommentStmt, context: any): any { return stmt; }
  visitAllStatements(stmts: Statement[], context: any): void {
    stmts.forEach(stmt => stmt.visitStatement(this, context));
  }
}

export function findReadVarNames(stmts: Statement[]): Set<string> {
  const visitor = new _ReadVarVisitor();
  visitor.visitAllStatements(stmts, null);
  return visitor.varNames;
}

class _ReadVarVisitor extends RecursiveAstVisitor {
  varNames = new Set<string>();
  visitDeclareFunctionStmt(stmt: DeclareFunctionStmt, context: any): any {
    // Don't descend into nested functions
    return stmt;
  }
  visitDeclareClassStmt(stmt: ClassStmt, context: any): any {
    // Don't descend into nested classes
    return stmt;
  }
  visitReadVarExpr(ast: ReadVarExpr, context: any): any {
    if (ast.name) {
      this.varNames.add(ast.name);
    }
    return null;
  }
}

export function collectExternalReferences(stmts: Statement[]): ExternalReference[] {
  const visitor = new _FindExternalReferencesVisitor();
  visitor.visitAllStatements(stmts, null);
  return visitor.externalReferences;
}

class _FindExternalReferencesVisitor extends RecursiveAstVisitor {
  externalReferences: ExternalReference[] = [];
  visitExternalExpr(e: ExternalExpr, context: any) {
    this.externalReferences.push(e.value);
    return super.visitExternalExpr(e, context);
  }
}

export function applySourceSpanToStatementIfNeeded(
    stmt: Statement, sourceSpan: ParseSourceSpan | null): Statement {
  if (!sourceSpan) {
    return stmt;
  }
  const transformer = new _ApplySourceSpanTransformer(sourceSpan);
  return stmt.visitStatement(transformer, null);
}

export function applySourceSpanToExpressionIfNeeded(
    expr: Expression, sourceSpan: ParseSourceSpan | null): Expression {
  if (!sourceSpan) {
    return expr;
  }
  const transformer = new _ApplySourceSpanTransformer(sourceSpan);
  return expr.visitExpression(transformer, null);
}

class _ApplySourceSpanTransformer extends AstTransformer {
  constructor(private sourceSpan: ParseSourceSpan) { super(); }
  private _clone(obj: any): any {
    const clone = Object.create(obj.constructor.prototype);
    for (let prop in obj) {
      clone[prop] = obj[prop];
    }
    return clone;
  }

  transformExpr(expr: Expression, context: any): Expression {
    if (!expr.sourceSpan) {
      expr = this._clone(expr);
      expr.sourceSpan = this.sourceSpan;
    }
    return expr;
  }

  transformStmt(stmt: Statement, context: any): Statement {
    if (!stmt.sourceSpan) {
      stmt = this._clone(stmt);
      stmt.sourceSpan = this.sourceSpan;
    }
    return stmt;
  }
}

export function variable(
    name: string, type?: Type | null, sourceSpan?: ParseSourceSpan | null): ReadVarExpr {
  return new ReadVarExpr(name, type, sourceSpan);
}

export function importExpr(
    id: ExternalReference, typeParams: Type[] | null = null,
    sourceSpan?: ParseSourceSpan | null): ExternalExpr {
  return new ExternalExpr(id, null, typeParams, sourceSpan);
}

export function importType(
    id: ExternalReference, typeParams: Type[] | null = null,
    typeModifiers: TypeModifier[] | null = null): ExpressionType|null {
  return id != null ? expressionType(importExpr(id, typeParams, null), typeModifiers) : null;
}

export function expressionType(
    expr: Expression, typeModifiers: TypeModifier[] | null = null): ExpressionType {
  return new ExpressionType(expr, typeModifiers);
}

export function literalArr(
    values: Expression[], type?: Type | null,
    sourceSpan?: ParseSourceSpan | null): LiteralArrayExpr {
  return new LiteralArrayExpr(values, type, sourceSpan);
}

export function literalMap(
    values: {key: string, quoted: boolean, value: Expression}[],
    type: MapType | null = null): LiteralMapExpr {
  return new LiteralMapExpr(
      values.map(e => new LiteralMapEntry(e.key, e.value, e.quoted)), type, null);
}

export function not(expr: Expression, sourceSpan?: ParseSourceSpan | null): NotExpr {
  return new NotExpr(expr, sourceSpan);
}

export function assertNotNull(
    expr: Expression, sourceSpan?: ParseSourceSpan | null): AssertNotNull {
  return new AssertNotNull(expr, sourceSpan);
}

export function fn(
    params: FnParam[], body: Statement[], type?: Type | null, sourceSpan?: ParseSourceSpan | null,
    name?: string | null): FunctionExpr {
  return new FunctionExpr(params, body, type, sourceSpan, name);
}

export function ifStmt(condition: Expression, thenClause: Statement[], elseClause?: Statement[]) {
  return new IfStmt(condition, thenClause, elseClause);
}

export function literal(
    value: any, type?: Type | null, sourceSpan?: ParseSourceSpan | null): LiteralExpr {
  return new LiteralExpr(value, type, sourceSpan);
}