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

[Simon Marlow]

On 18 March 2013 03:08, Simon Peyton-Jones <simonpj at microsoft.com> wrote:
> 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.
>
>
>
> Good idea -- esp if it makes the packaging story simpler.  GHC already uses
> a binary format for interface files, so there’s no good reason to use a
> human-readable format for package data base stuff.  For interface files you
> can read them with ghc --show-iface, and as Ian remarks something similar is
> already true for the package data base.

A bit of background here: the binary serialisation of packages is an
optimisation only (though an important one), and is done independently
of Cabal.  To install a Cabal package you can put the package
description file that Cabal generates into GHC's database directory,
and it is picked up automatically.  The binary cache can be updated
separately with 'ghc-pkg recache'.  It was done this way to make it
easier for Linux distros that want to install packages by moving files
into place and then running comands.

So I don't think you want Cabal to know about the binary serialization
format, it's a GHC-only optimisation.

Cheers,
Simon

>
>
> Simon
>
>
>
> From: ghc-devs-bounces at haskell.org [mailto:ghc-devs-bounces at haskell.org] On
> Behalf Of Mark Lentczner
> Sent: 17 March 2013 16:57
> To: dag.odenhall at gmail.com
> Cc: Haskell Libraries; cabal-devel; Duncan Coutts; ghc-devs at haskell.org;
> Antoine Latter
> Subject: Re: Advance notice that I'd like to make Cabal depend on parsec
>
>
>
> 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, Java.
> 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 gmail.com
> <dag.odenhall at gmail.com> 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 googlemail.com> 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
>
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