ANTLR with ASTs Abstract Syntax Trees ANTLR can

ANTLR with ASTs

Abstract Syntax Trees • ANTLR can be instructed to produce ASTs for the output of the parser • ANTLR uses a prefix notation for representing ASTs • A tree like this: + / 3 4 Has a text representation as (+ 3 4)

Abstract Syntax Trees • A tree like this: + / 3 * / 4 5 Has the following text representation: (+ 3 (* 4 5)) Every tree can be represented in a linear format using this prefix notation

ASTs • Instead of recognizing a token sequence and firing off an action, we instead generate an AST • By instructing the parser to output ASTs, we can create an input stream called a Common. Tree. Node. Stream that contains a sequence of tokens that capture the tree structure • We can then write a “tree walking” (second) grammar that describes the sequence of input tokens and actions that process the trees

Tree Walkers • The trees emitted by the parser are much more concise than the parse tree used to discover the structure in the original token sequence • Since the structure is simpler, there are usually many fewer rules in a tree walker grammar than in the original grammar

Tree Walkers • Interpreting ASTs is much faster than interpreting the original token sequence • You will want to generate ASTs for most grammars

Translation Data Flow Tokens Characters Lexer Output Parser AST Tree Walker