This is a small utility class for the excellent Peggy (formerly PEG.js) parser generator which wraps around Peggy's central parse function and provides three distinct convenience features: Parser Tree Token Unrolling, Abstract Syntax Tree Node Generation and Cooked Error Reporting.

$ npm install peggy pegjs-util

{
    var unroll = options.util.makeUnroll(location, options)
    var ast    = options.util.makeAST   (location, options)
}

start
    = _ seq:id_seq _ {
          return ast("Sample").add(seq)
      }

id_seq
    = id:id ids:(_ "," _ id)* {
          return ast("IdentifierSequence").add(unroll(id, ids, 3))
      }

id
    = id:$([a-zA-Z_][a-zA-Z0-9_]*) {
          return ast("Identifier").set("name", id)
      }

_ "blank"
    = (co / ws)*

co "comment"
    = "//" (![\r\n] .)*
    / "/*" (!"*/" .)* "*/"

ws "whitespaces"
    = [ \t\r\n]+

var fs      = require("fs")
var ASTY    = require("asty")
var PEG     = require("peggy")
var PEGUtil = require("pegjs-util")

var asty = new ASTY()
var parser = PEG.generate(fs.readFileSync("sample.pegjs", "utf8"))
var result = PEGUtil.parse(parser, fs.readFileSync(process.argv[2], "utf8"), {
    startRule: "start",
    makeAST: function (line, column, offset, args) {
        return asty.create.apply(asty, args).pos(line, column, offset)
    }
})
if (result.error !== null)
    console.log("ERROR: Parsing Failure:\n" +
        PEGUtil.errorMessage(result.error, true).replace(/^/mg, "ERROR: "))
else
    console.log(result.ast.dump().replace(/\n$/, ""))

$ cat sample-input-ok.txt
/*  some ok input  */
foo, bar, quux

$ node sample.js sample-input-ok.txt
Sample [1/1]
    IdentifierSequence [2/1]
        Identifier (name: "foo") [2/1]
        Identifier (name: "bar") [2/6]
        Identifier (name: "quux") [2/11]

$ cat sample-input-bad.txt
/*  some bad input  */
foo, bar, quux baz

$ node sample.js sample-input-bad.txt
ERROR: Parsing Failure:
ERROR: line 2 (column 16):   */\nfoo, bar, quux baz\n
ERROR: -----------------------------------------^
ERROR: Expected "," or end of input but "b" found.

PEGUtil is a small utility class for the excellent Peggy (formerly PEG.js) parser generator. It wraps around Peggy's central parse function and provides three distinct convenience features:

In many Peggy gammar rule actions you have to concatenate a first token and a repeated sequence of tokens, where from the sequence of tokens only relevant ones should be picked:

id_seq
    = id:id ids:(_ "," _ id)* {
          return unroll(id, ids, 3)
      }

Here the id_seq rule returns an array of ids, consisting of the first token id and then all 4th tokens from each element of the ids repetition.

The unroll function has the following signature:

unroll(first: Token, list: Token[], take: Number): Token[]
unroll(first: Token, list: Token[], take: Number[]): Token[]

It accepts first to be also null (and then skips this) and take can be either just a single position (counting from 0) or a list of positions.

To make the unroll function available to your rule actions code, place the following at the top of your grammar definition:

{
    var unroll = options.util.makeUnroll(location, options)
}

The options.util above points to the PEGUtil API and is made available automatically by using PEGUtil.parse instead of Peggy's standard parser method parse.

Usually the result of Peggy grammar rule actions should be the generation of an Abstract Syntax Tree (AST) node. For this libraries like e.g. ASTy can be used.

id_seq
    = id:id ids:(_ "," _ id)* {
          return ast("IdentifierSequence").add(unroll(id, ids, 3))
      }

Here the result is an AST node of type IdentifierSequence which contains no attributes but all identifiers as child nodes.

To make the ast function available to your rule actions code, place the following at the top of your grammar definition:

{
    var ast = options.util.makeAST(location, options)
}

Additionally, before performing the parsing step, your application has to tell PEGUtil how to map this call onto the underlying AST implementation. For ASTy you can use a makeAST function like:

function (line, column, offset, args) {
    return ASTY.apply(null, args).pos(line, column, offset)
}

The args argument is an array containing all arguments you supply to the generated ast() function. For ASTy this would be at least the type of the AST node.

The options.util above again points to the PEGUtil API and is made available automatically by using PEGUtil.parse instead of Peggy's standard parser method parse.

Instead of calling the regular Peggy parser.parse(source[, startRule]) you now should call PEGUtil.parse(parser, source[, startRule]). The result then is always an object consisting of either an ast field (in case of success) or an error field (in case of an error).

In case of an error, the error field provides cooked error information which allow you to print out reasonable human-friendly error messages (especially because of the detailed location field):

result = {
    error: {
        line:     Number, /* line number */
        column:   Number, /* column number */
        message:  String, /* parsing error message */
        found:    String, /* found token during parsing */
        expected: String, /* expected token during parsing */
        location: {
            prolog: String, /* text before the error token */
            token:  String, /* error token */
            epilog: String  /* text after the error token */
        }
    }
}

For convenience reasons you can render a standard human-friendly error message out of this information with PEGUtil.errorMessage(result.error).

Copyright (c) 2014-2024 Dr. Ralf S. Engelschall (http://engelschall.com/)

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