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

Mark Lentczner mark.lentczner at
Sun Mar 17 17:57:25 CET 2013

This thread is raising all sorts of questions for me:

Is it essential, or even sensical, that the serialization format GHC needs
for storing package info bear any relation to the human authored form? If
not, the split out of the package types could be accomplished in a way
where GHC uses simple show/read(P) style serialization for storage of
package info, where as cabal-lib would use a lovely parsec parser for
humans. I'd like this approach.

The issue of putting the yet one more HP package into GHC's core packages
is increasing the exposure of the difficulty of the current GHC/HP
relationship. See also threads in HP's mailing list for why can't we bump
some packages in GHC's core set for the next HP release. The split
arrangement is strange because we have two groups making up what is in the
HP, but they have different processes and aims. The complex technical
relationship between the moving parts only heightens the difficulty.

Perhaps the major cause is that because GHC is shipped as a library itself,
it exposes all it's package dependencies. And as it is a large, and
growing, piece of software, the list only wants to grow. But I wonder how
often GHC is used as a library itself? If not often, then perhaps GHC
should be shipped as two parts: Just a compiler (plus the small number of
packages that the compiler forces), and ghc-lib as an optional,
even separate, package - perhaps one with even a traditional way of
depending on other packages. In otherwords, users that wanted to
incorporate the ghc-lib into their programs would depend, and download, and
configure, and build, ghc-lib indpenendant of the GHC binaries installed on
their system. Perhaps then, GHC, the compiler, built from ghc-lib, would be
bootstrapped not from the past compiler, but from the past HP.....

Okay, perhaps that is all just fantasy. But, no other programming system
operates the way we do. They all fall into one of two camps:

   - The dominant implementation is maintained, built, and shipped along
   with a large collection of "common packages". Examples: Python, Ruby, PHP,
   - The dominant implementation is shipped as a bare tool, and large
   common libraries are maintained and shipped independently. Examples: C++
   (think g++ and boost), JavaScript (think browsers, and jQuery).

We are in the middle and, I think, experiencing growing pains because of it.

- Mark

On Sat, Mar 16, 2013 at 3:42 PM, dag.odenhall at <
dag.odenhall at> wrote:

> I'd love to have a proper parser and source-location-aware AST for sake of
> editor/IDE tools, so +1 from me. If you don't end up doing this after all,
> I'd still like to see your parser in a separate package, although I
> understand if you don't feel like maintaining two parsers especially given
> the tedious process for verifying they work similarly. I guess it could
> still be useful in the same way we find haskell-src-exts useful despite
> some incompatibilities with GHC.
> On Thu, Mar 14, 2013 at 3:53 PM, Duncan Coutts <
> duncan.coutts at> wrote:
>> 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
>> _______________________________________________
>> cabal-devel mailing list
>> cabal-devel at
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