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    0.1.2 • Public • Published


    A plugin to produce a "well-pruned" tree from a pegjs grammar


    The idea is to be able to produce a useful object tree using as little embedded javascript in the pegjs grammar as possible. Maybe this is too much messy syntactic magic, I don't know...


    • Rules or expressions with existing actions will not be affected

    • If labels are provided for members of a sequence, then the returned value, instead of being an array, will be an object with the labels being the keys.

      example: temperature = degrees:$([0-9]+) unit:[CF], when parsing 32F, will return {"degrees": "32", "unit": "F" }

    • By default, key-value pairs with values that are null or undefined are not included in the output (this behavior can be changed with the bonsaiPlugin.keepUndefined option).

      example: temperature = degrees:$([0-9]+) unit:[CF]?, when parsing 32, will return {"degrees": "32" } (but leaving out units is a very bad idea...)

    • If there is only one element in a sequence with a label, and if that label is _, then a single value (rather than an object) will be returned, and the other values ignored:

      example: contained = "[" _:[a-z]+ "]", when parsing [bracketed] will return "bracketed"


    1. npm install pegjs pegjs-bonsai
    2. In your javascript file, use something like this:
    var PEG = require('pegjs')
    var bonsai = require('pegjs-bonsai')
    var parser = PEG.buildParser('temperature = degrees:$([0-9]+) unit:[CF]',
                                 { plugins: [bonsai], bonsaiPlugin: { keepUndefined: true } });


    Some configuration is available, passed to PEG.buildParser in the options.bonsaiPlugin object:

    • showTransformed (for debugging purposes) if true, writes the transformed AST of the grammar to console.log after the transformation pass is complete

    • keepUndefined if true, does not eliminate key-value pairs where the value is null or undefined.

    • exposeTextAs if set to a string s, adds a key-value pair (s, text()), where text() is the entire string that was matched


    npm i pegjs-bonsai

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