What's the difference between a parser and a scanner?

parsing yacc lexical-analysis
neuromancer picture neuromancer · Nov 15, 2009 · Viewed 18.2k times · Source

I already made a scanner, now I'm supposed to make a parser. What's the difference?

Answer

barkmadley picture barkmadley · Nov 16, 2009

A Scanner simply turns an input String (say a file) into a list of tokens. These tokens represent things like identifiers, parentheses, operators etc.

A parser converts this list of tokens into a Tree-like object to represent how the tokens fit together to form a cohesive whole (sometimes referred to as a sentence).

In terms of programming language parsers, the output is usually referred to as an Abstract Syntax Tree (AST). Each node in the AST represents a different construct of the language, e.g. an IF statement would be a node with 2 or 3 sub nodes, a CONDITION node, a THEN node and potentially an ELSE node.

A parser does not give the nodes any meaning beyond structural cohesion. The next thing to do is extract meaning from this structure (sometimes called contextual analysis).

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