pegjs-util
This is a small utility class for the excellent
PEG.js parser generator which wraps around PEG.js's
central parse
function and provides three distinct convenience
features: Parser Tree Token Unrolling, Abstract Syntax Tree Node
Generation and Cooked Error Reporting.
Installation
Node environments (with NPM package manager):
$ npm install pegjs
$ npm install pegjs-util
Browser environments (with Bower package manager):
$ bower install pegjs
$ bower install pegjs-util
Usage
sample.pegjs
{
var unroll = options.util.makeUnroll(line, column, offset, SyntaxError)
var ast = options.util.makeAST(line, column, offset)
}
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:$([a-zA-Z_][a-zA-Z0-9_]*) {
return ast("Identifier").set("name", id)
}
_ "blank"
= (co / ws)*
co "comment"
= "//" (![\r\n] .)*
/ "/*" (!"*/" .)* "*/"
ws "whitespaces"
= [ \t\r\n]+
sample.js
var fs = require("fs")
var ASTY = require("asty")
var PEG = require("pegjs")
var PEGUtil = require("pegjs-util")
PEGUtil.makeAST.cb(function (line, column, offset, args) {
return ASTY.apply(null, args).pos(line, column, offset)
})
var parser = PEG.buildParser(fs.readFileSync("sample.pegjs", "utf8"))
var result = PEGUtil.parse(parser, fs.readFileSync(process.argv[2], "utf8"), "start")
if (result.error !== null)
console.log("ERROR: Parsing Failure:\n" +
PEGUtil.errorMessage(result.error, true).replace(/^/mg, "ERROR: "))
else
console.log(result.ast.dump().replace(/\n$/, ""))
Example Session
$ 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.
Description
PEGUtil is a small utility class for the excellent
PEG.js parser generator. It wraps around PEG.js's
central parse
function and provides three distinct convenience features:
Parser Tree Token Unrolling
In many PEG.js 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(line, column, offset, SyntaxError)
}
The options.util
above points to the PEGUtil API and is made available
automatically by using PEGUtil.parse
instead of PEG.js's standard
parser method parse
.
Abstract Syntax Tree Node Generation
Usually the result of PEG.js 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(line, column, offset)
}
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:
PEGUtil.makeAST.cb(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 PEG.js's standard
parser method parse
.
Cooked Error Reporting
Instead of calling the regular PEG.js 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,
column: Number,
message: String,
found: String,
expected: String,
location: {
prolog: String,
token: String,
epilog: String
}
}
}
For convenience reasons you can render a standard human-friendly
error message out of this information with
PEGUtil.errorMessage(result.error)
.
License
Copyright (c) 2014 Ralf S. Engelschall (http://engelschall.com/)
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.