What's the use of abstract syntax trees?

Question

I am learning on my own about writing an interpreter for a programming language, and I have read about Abstract Syntax Trees. I have an idea of what they are, but I do not see their use.

Why are ASTs useful?

Answer 1

You need "syntax trees" to represent the structure of most programming langauges, in order to carry out analysis or transformation on documents that contain programming language text. (You can see some fancy examples of this via my bio).

Whether that tree is abstract (AST) or concrete (CST) is a matter of taste, convenience, and engineering sweat. The term CST is specially used to describe the parse derivation tree when a grammar is used to deconstruct source code; it usually contains tree elements for lots of concrete syntax such as statement terminator semicolons. AST is used to mean "something simpler than the CST", e.g., leaving out semicolon tree nodes because they don't affect program analysis much, and thus writing analyzers that process ASTs is less conceptual and engineering effort than writing the same analyzer on a CST. A better way to understand this is to realize that the AST is usually as isomorphic equivalent of the CST, that is, you should be able to regenerate the CST from it. If you want to transform the source text and regenerate it, then the CST is often a better choice as it loses less information from the original program (and my fancy example uses this approach).

I think you will find the SO discussion on abstract vs. concrete syntax trees pretty helpful.

Answer 2

The main benefit os using an AST is that you separate the parsing and validation logic from the implementation piece. Interpreters implemented as ASTs really are easier to understand and maintain. If you have a problem parsing some strange syntax you look at the AST parser , if a pices of code is not producing the expected results than you look at the code that interprets the AST.

The other great advantage is when you syntax requires "lookahead" e.g. if your syntax allows a subroutine to be used before it is defined it is trivial to validate the existence of a subroutine when you are using an AST - its much more difficult with an "on the fly" parser.