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Chapter 15. Assembling Your System with the Interpreter > Creating the AST
Creating the AST
Surprisingly, the Interpreter pattern as defined by the GoF is silent on the origins of the AST. The pattern simply assumes that you have somehow gotten your hands on an AST and works from there. Of course, the AST has to come from somewhere. In practice, there are a surprising number of alternative ways to create an AST.
A Simple Parser
Perhaps the most obvious way of coming up with an AST is to build a parser. Building a parser for our little file-searching language is not especially difficult. Suppose we define the syntax to look something like this:
