RFC: changes to -i flag for finding source files

Edward Kmett ekmett at gmail.com
Fri Apr 25 15:22:46 UTC 2014

+1 from me. I have a lot of projects that suffer with 4 levels of vacuous subdirectories just for this.

In theory cabal could support this on older GHC versions by copying all of the files to a working dir in dist with the expected layout on older GHCs.

That would enable this to get much greater penetration more quickly.


> On Apr 25, 2014, at 9:17 AM, Simon Marlow <marlowsd at gmail.com> wrote:
> I want to propose a simple change to the -i flag for finding source files.  The problem we often have is that when you're writing code for a library that lives deep in the module hierarchy, you end up needing a deep directory structure, e.g.
> src/
>   Graphics/
>     UI/
>      Gtk/
>        Button.hs
>        Label.h
>        ...
> where the top few layers are all empty.  There have been proposals of elaborate solutions for this in the past (module grafting etc.), but I want to propose something really simple that would avoid this problem with minimal additional complexity:
>  ghc -iGraphics.UI.Gtk=src
> the meaning of this flag is that when searching for modules, ghc will look for the module Graphics.UI.Gtk.Button in src/Button.hs, rather than src/Graphics/UI/Gtk/Button.hs.  The source file itself is unchanged: it still begins with "module Graphics.UI.Gtk.Button ...".
> The implementation is only a few lines in the Finder (and probably rather more in the manual and testsuite), but I wanted to get a sense of whether people thought this would be a good idea, or if there's a better alternative before I push it.
> Pros:
>  - simple implementation (but Cabal needs mods, see below)
>  - solves the deep directory problem
> Cons:
>  - It makes the rules about finding files a bit more complicated.
>    People need to find source files too, not just compilers.
>  - packages that want to be compatible with older compilers can't
>    use it yet.
>  - you can't use '=' in a source directory name (but we could pick
>    a different syntax if necessary)
>  - It won't work for Cabal packages until Cabal is modified to
>    support it (PreProcess and SrcDist and perhaps Haddock are the only
>    places affected, I think)
>  - Hackage will need to reject packages that use this feature without
>    also specifying ghc >= 7.10 and some cabal-version too.
>  - Are there other tools/libraries that will need changes? Leksah?
> Cheers,
> Simon
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