Extended PEG Syntax

Extended PEG Syntax

As we saw in PEG Basics and Declaring a Grammar, Pegged implements the entire PEG syntax, exactly as it was defined by its author.

Now, I felt the need to extend this a little bit. At that time, semantic actions were not implemented in Pegged so now that they are, these extensions are not strictly necessary, but they are useful shortcuts.

Dropping a Node :

The first extensions act on the result of parsing expression. Given 'e' a parsing expression:

• :e will drop e's captures. And, due to the way sequences are implemented in Pegged, the mother expression will forget e result (that's deliberate). It allows one to write:

mixin(grammar("
    JSON   <- :'{' (Pair (:',' Pair)*)? :'}'
    Pair   <- String :':' Value

    # Rest of JSON grammar ...

"));

On the first rule, see the colon before the curly braces literals and the comma. That means that when called on `{"Hello":42, "World!":0}`, `JSON` parse tree will contain only the interesting parts, not the syntactic signs necessary to structure the JSON grammar:

ParseTree("JSON", ParseTree("Pair", ParseTree("String", ...) ParseTree("Number", ...)), ParseTree("Pair", ParseTree("String", ...) ParseTree("Number", ...))
)

Fusing Captures:
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Rule-Level Extensions
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