Re: [PATCH 0/2] native-search-paths for GHC

[Federico Beffa]

address@hidden writes:

> From: Eric Bavier <address@hidden>
>
> The first of these patches lets search-path-as-list function correctly when a
> pattern is given and the 'directory file type is specified.
>
> The second adds a native-search-paths field to our ghc package.  It modifies
> our haskell-build-system to install a package-specific package database with
> each haskell library.  GHC insists on a binary package cache file called
> 'package.cache' to be in each directory listed in GHC_PACKAGE PATH, so we
> uniquely name the package database directories, and use a file-pattern to add
> those to GHC_PACKAGE_PATH.
>
> The benefit of this over the current situation is that one gets the benefit of
> `guix package --search-paths`.  Currently, after installing ghc packages, the
> user needs to know to manually add
> ~/.guix-profile/lib/ghc-7.8.4/package.conf.d to their GHC_PACKAGE_PATH.  GHC
> package recipes would no longer need to propagate runtime dependencies, and
> 'guix environment' also works nicely out-of-the-box::
>
> $ guix environment --ad-hoc ghc ghc-attoparsec
> $ ghc-pkg list
> /gnu/store/...-ghc-mtl-2.1.3.1/lib/ghc-7.8.4/ghc-mtl-2.1.3.1.conf.d
>    base-4.7.0.2
>    ghc-prim-0.3.1.0
>    integer-gmp-0.5.1.0
>    mtl-2.1.3.1
>    rts-1.0
>    transformers-0.3.0.0
> /gnu/store/...-ghc-regex-base-0.93.2/lib/ghc-7.8.4/ghc-regex-base-0.93.2.conf.d
>    array-0.5.0.0
>    base-4.7.0.2
>    bytestring-0.10.4.0
> ...
> /gnu/store/4vvmngz1w8ccm7v7mk4f4dxk45834464-ghc-attoparsec-0.13.0.0/lib/ghc-7.8.4/ghc-attoparsec-0.13.0.0.conf.d
>    array-0.5.0.0
>    attoparsec-0.13.0.0
> ...
>
> Though, as you can see in this example, libraries may be listed more than
> once.  As far as I can tell at this point, that is just an aesthetic detail.
>
> Future work might involve filtering build-only library dependencies from the
> generated package database.  We could probably also remove the ghc package
> database creation during profile generation.
>
> I'd be insterested in hearing others' thoughts on this approach.

Hi Eric,

sounds like a good approach to work around the "package.cache" clash. I
have a couple of questions:

* If I understand correctly, the configuration files of dependencies are
  copied in a unique directory for each package.  Instead of copying
  would a symlink work? (There are literally thousands of packages on
  Hackage and hopefully Guix will get more of them.)

* Some Haskell libraries have a rather large list of dependencies. For
  this reason I can imagine that in some situations GHC_PACKAGE_PATH
  could grow rather long. This thought made me wonder if there is a
  maximum length to the value of environment variables that we could
  possibly hit.

Regards,
Fede