The Abstract Syntax Definition Language (ASDL) is a domain-specific language designed to describe the tree-like data structures that represent a program’s abstract syntax, known as Abstract Syntax Trees (ASTs). It was created to solve the problem of separating the logical structure of a program (the AST) from the concrete syntax of the programming language. 

ASDL was introduced as part of the Zephyr compiler infrastructure project at Princeton University and the University of Wisconsin in 1997. ASDL specifications are fed into a generator that automatically produces the necessary data types for a given programming language, such as C, C++, Java, or Python. 

Core concepts ASDL’s syntax is intentionally simple and clear, focusing on two primary data structures: 

  • Sum types: These represent a collection of alternative data types. In an object-oriented language, this corresponds to an abstract base class, with each alternative defined as a subclass. For example, an expression could be a sum type that includes alternatives such as Variable, BinOp, and Call.
  • Product types: These represent a data type composed of a fixed number of other types. In an object-oriented language, this corresponds to a class or a struct with specific fields. 

Example: A simple language

To illustrate, consider a simple programming language with variable assignments and binary operations.

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module MyLanguage {
    stm = Assign(var name, exp value)
        | If(exp test, stm body, stm? orelse)
    exp = Var(var name)
        | BinOp(exp left, op op, exp right)
        | Const(int val)
    op = Add | Sub | Mul | Div
}

In this specification:

  • stm is a sum type that can be either an Assign or an If. The stm? orelse denotes an optional field.
  • Assign is a product type with two fields: name of type var and value of type exp.
  • op is a simple sum type representing different binary operators. 

A tool processing this ASDL file could generate object-oriented classes that reflect this structure. For instance, in a language like Python, this might become:

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class stm: pass

class Assign(stm):
    def __init__(self, name, value):
        self.name = name
        self.value = value
# ... and so on for all types and constructors

Key advantages of ASDL

  • Separation of concerns: It cleanly separates the description of the program’s abstract structure (the AST) from the parsing logic for a specific language.
  • Interoperability: By using a language-independent specification, ASDL facilitates the development of multi-language compiler toolchains where different components can be written in different programming languages.
  • Automated code generation: The declarative nature of ASDL allows tools to automatically generate the necessary data-structure definitions and serialization/deserialization code (“pickles”) for various target languages.
  • Conciseness: Compared to manually writing object-oriented data types or C-style tagged unions, ASDL offers a simple and concise way to define complex, tree-like data structures. 

ASDL in practice

ASDL is notably used in the reference implementation of Python (CPython) to define its AST. The asdl file for Python is used to generate the C data structures and processing functions for the compiler. This allows the Python core developers to clearly specify the intermediate representation without tightly coupling it to the parsing implementation. 

Using ASDL to help walk and process the AST 

  1. Generating a Visitor pattern

One of the most common and powerful ways to process an AST is using the Visitor pattern. ASDL-based tools are often designed to automatically generate the necessary boilerplate for implementing this pattern. 

  • The generated infrastructure: For each ASDL type, the generator creates a corresponding class (the node) and a Visitor interface with a visit method for each node type.
  • How it works:
    • You, as the developer, define a concrete visitor class that inherits from the generated base Visitor interface.
    • You then override only the visit methods for the node types you care about.
    • To traverse the tree, you start at the root node and call its accept method, passing your custom visitor. This triggers a traversal of the entire tree, and your overridden methods will be automatically invoked for the specific nodes you want to process.
  • Benefits: This eliminates manual, error-prone instanceof/isinstance checks and recursive traversal logic. The generated code provides a clean, type-safe way to define different processing phases (e.g., semantic analysis, code generation) in separate visitor classes. 
  1. Automating tree traversal logic 

Beyond the standard Visitor pattern, a smart ASDL generator can provide utilities for common traversal tasks. 

  • Pre-order and post-order traversal: The tool can generate functions that perform standard traversal algorithms. These can be used to implement passes that need to execute code before or after visiting a node’s children.
  • Recursive descent helpers: For languages that support it, the generated code can make it easier to write recursive functions that process the tree. For sum types, this might involve a switch or match statement over the node’s possible variants, with the compiler ensuring all cases are handled. 
  1. Enhancing the AST with information 

ASDL can also support the process of enriching the AST with additional information as it is processed. 

  • Decorating nodes: During a visitor pass (like semantic analysis), you can add information like type information, symbol table references, or resolved names to the AST nodes. ASDL’s extensible nature allows for defining these attributes on the node types themselves.
  • Enabling multiple passes: An ASDL-generated AST serves as the central data structure that is passed between multiple compiler passes (each implemented as a Visitor). The output of one pass (e.g., a type-checked AST) becomes the input for the next (e.g., an intermediate representation).