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

[Eric Bavier <ericbavier@openmailbox.org>]

On 2015-10-07 11:07, Federico Beffa wrote:
> ericbavier@openmailbox.org writes:
> 
>> From: Eric Bavier <bavier@member.fsf.org>
>> 
>> 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 interested 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.)

I don't know if symlinking is allowed across store directories, though 
if it is, that'd be something to try.

> * 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.

Not every package would end up with an entry in GHC_PACKAGE_PATH, only 
those that are installed, or declared inputs.  Dependent libraries would 
be found when GHC examines the package databases of those packages.

>   This thought made me wonder if there is a
>   maximum length to the value of environment variables that we could
>   possibly hit.

We've already pushed this quite far.  E.g. the hydra package creates a 
rather sizable PER5LIB path.

-- 
`~Eric