From: Michael Edgar <adgar@...>
Date: 2011-09-01T12:41:05+09:00
Subject: Re: ripper on array access operation

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On Aug 31, 2011, at 7:00 PM, justincollins@ucla.edu wrote:

> Quoting "Surya G." <sgaddipati@obtiva.com>:
>=20
>> unknown wrote in post #1019486:
>>> Quoting "Surya G." <sgaddipati@obtiva.com>:
>>>=20
>>> I am pretty sure it's the :aref part. Ripper is making [] a special
>>> case.
>>>=20
>>> -Justin
>>=20
>> Thats what it looks like but why not treat aref as any-other method
>> call.
>>=20
>> --
>> Posted via http://www.ruby-forum.com/.

Aref is parsed differently from other method calls. Ripper delivers a =
parse tree, and by providing aref as a different AST node, a program =
consuming the parse tree can distinguish between a.[](1, 2) and a[1,2]. =
There are plenty of reasons (such as pretty-printing) that these two =
should not be conflated.

>=20
> I don't know. Ripper's output looks like a mess to me when compared to =
RubyParser:
>=20
> s(:call, s(:call, nil, :a, s(:arglist)), :[], s(:arglist, s(:lit, 1)))
>=20
> -Justin
>=20

That's because RubyParser doesn't provide an abstract syntax tree, it =
provides a tree interpretation of Ruby programs. While Ripper comes =
close to a concrete syntax tree at times, at least it sticks to syntax. =
t discuss my choice of Ripper over RubyParser in my undergraduate thesis =
[0]; while it is perhaps suitable for an interpreter, its output =
discards so much of the syntax. It even inserts nodes which have nothing =
to do with syntax and everything to do with semantics. In this example, =
if I want my static analyzer to find examples of a.[](1), I can't using =
RubyParser (at least with the default settings).

It adds :scope nodes which don't have anything to do with the syntax, it =
evaluates constant literals, from integers to regular expressions, which =
is not the job of a general-purpose AST. It even inserts a local =
assignment node when it parses "rescue Foo =3D> x" (x =3D $!).

RubyParser is a good library, and its value as a pure-ruby parser cannot =
be understated. It's not suitable for many uses because of the shortcuts =
it takes, the over-interpretation it does (this aref example is a clear =
one), and the overall focus on semantics and not on just *parsing*.

[0] PDF: =
http://www.cs.dartmouth.edu/cms_file/SYS_techReport/532/TR2011-686.pdf=

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