Advance notice that I'd like to make Cabal depend on parsec

[Duncan Coutts]

Hi folks,

I want to give you advance notice that I would like to make Cabal depend
on parsec. The implication is that GHC would therefore depend on parsec
and thus it would become a core package, rather than just a HP package.
So this would affect both GHC and the HP, though I hope not too much.

The rationale is that Cabal needs to parse things, like .cabal files and
currently we do not have a decent parser in the core libraries. By
decent I mean one that can produce error messages with source locations
and that doesn't have unpredictable memory use. The only parser in the
core libraries at the moment is Text.ParserCombinators.ReadP from the
base package and that fails my "decent" criteria on both counts. Its
idea of an error message is (), and on some largish .cabal files we take
100s of MB to parse (I realise that the ReadP in the base package is a
cutdown version so I don't mean to malign all ReadP-style libs out
there).

Partly due to the performance problem, the terrible .cabal file error
messages, and partly because Doaitse Swierstra keeps asking me if .cabal
files have a grammar, I've been writing a new .cabal parser. It uses an
alex lexer and a parsec parser. It's fast and the error messages are
pretty good. I have reverse engineered a grammar that closely matches
the existing parser and .cabal files in the wild, though I'm not sure
Doaitse will be satisfied with the approach I've taken to handling
layout.

Why did I choose parsec? Practicality dictates that I can only use
things in the core libraries, and the nearest thing we have to that is
the parser lib that is in the HP. I tried to use happy but I could not
construct a grammar/lexer combo to handle the layout (also, happy is not
exactly known for its great error messages).

I've been doing regression testing against hackage and I'm satisfied
that the new parser matches close enough. I've uncovered all kinds of
horrors with .cabal files in the wild relying on quirks of the old
parser. I've made adjustments for most of them but I will be breaking a
half dozen old packages (most of those don't actually build correctly
because though their syntax errors are not picked up by the parser, they
do cause failure eventually).

So far I've just done the outline parser, not the individual field
parsers. I'll be doing those next and then integrate. So this change is
still a bit of a ways off, but I thought it'd be useful to warn people
now.

Duncan