Grammars as D Modules

Grammars a D Modules

Before being mixed-in, a grammar output by grammar is just a string. If you do not want to generate your grammar anew each time your module is compiled, save the grammar code as a D module and import it. I found I needed this frequently enough to put the functionality in Pegged:

asModule("moduleName", "my/project/hierarchy/fileName", myGrammar);

For example:

asModule("arithmetic","arithmetic",
"Arithmetic:
    Expr     <- Factor AddExpr*
    AddExpr  <- ('+'/'-') Factor
    Factor   <- Primary MulExpr*
    MulExpr  <- ('*'/'/') Primary
    Primary  <- Parens / Number / Variable / '-' Primary

    Parens   <- '(' Expr ')'
    Number   <~ [0-9]+
    Variable <- identifier
");

This will create the arithmetic.d file in the current directory, with the necessary infrastructure.

Once you're done, you can use your grammar:

import arithmetic;

enum input = "2/(8*7988+1*6196-y)";
enum parseTree = Arithmetic(input);

That should speed up the compiling, since the grammar is already generated.

As another example Pegged's own parser, the one used by grammar to parse the grammar-as-a-string the user provides, is generated like this:

import pegged.examples.peggedgrammar; // contains Pegged grammar as a string, name PEGGEDgrammar
asModule("pegged.parser", "pegged/parser", PEGGEDgrammar);

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Next Lesson: Grammar Composition.

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Pegged Tutorial