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Shift AST Specification

About

This document defines an interface for an abstract syntax tree that can be used to represent the structure of an ECMAScript program. The interface is intended to be able to represent any structurally valid ECMAScript program (according to the latest version of ECMA-262) while minimising the number of invalid programs that may be represented.

This specification is defined using the Web IDL standard with the following extended attributes:

  1. TypeIndicator: This extended attribute may be applied to a readonly attribute to indicate the following:
    • The type following the attribute keyword must be an identifier.
    • Declare an enum whose identifier is the type that follows the attribute keyword, whose values are the identifiers of all interfaces that inherit the attribute.
    • The value of the attribute is the member of the above-declared enum associated with the interface's identifier.
  2. NonEmpty: This extended attribute may be applied to any attribute with a Sequence or Array type to disallow the zero-length inhabitant of that type.

Status

This specification currently supports ECMAScript 2019 as defined in the latest version of ECMA-262 at the time of this writing. Stability of this data structure is not guaranteed between releases of ECMA-262.

Implementations

This section is reserved for a listing of implementations that conform to this specification as well as tools that operate on those data types. To add your own implementation to this list, please open a Pull Request. Implementations should be ordered by submission date within a category that appropriately describes their purpose.

Conform

Create

Transform

Analyse

Use

Design Decisions

The following design decisions showcase characteristics of the Shift AST design goals.

  • The concepts of Block and VariableDeclaration are separate from BlockStatement and VariableDeclarationStatement respectively in order to avoid usage of those statements where other types of statements are not allowed.
  • AssignmentExpression is separate from BinaryExpression because ES6 restricts the left operand of AssignmentExpression to bindings (12.14.1), and this restriction could be represented with a sufficiently advanced type system. Similarly, ComputedAssignmentExpression further restricts its left operand, and is consequently separated. All other binary expressions are grouped together.
  • RegExps and other exotic values are not used as primitives to allow for the possibility of serialisation to JSON.

Compromises

None right now!

License

Copyright 2014 Shape Security, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.