hsc2hs and HsFFI.h

[Duncan Coutts]

On Tue, 2009-02-10 at 17:15 +0000, Ian Lynagh wrote:
> Hi all,
> 
> Currently, hsc2hs (as shipped with GHC) cannot be used with just
>     hsc2hs Foo.hsc
> as it cannot find HsFFI.h (http://hackage.haskell.org/trac/ghc/ticket/2897).
> To make it work you need to run something like
>     hsc2hs -I /usr/lib/ghc-6.10.1/include Foo.hsc
> (it also works when called by Cabal, as Cabal passes it this flag
> automatically). However, we would like to have it work without needing
> to use any special flags, and without having to use it from within a
> Cabal package.
> 
> The obvious solution to this problem would seem to be to put HsFFI.h in
>     /usr/lib/hsc2hs/include
> and have hsc2hs automatically add that to the include path. However,
> hsc2hs is supposed to be a compiler-independent tool, and HsFFI.h isn't
> a compiler-independent header file; for example, GHC's implementation
> defines HsInt to be a 64-bit integer type on amd64, whereas hugs's
> implementation defines it to be a 32-bit type. We therefore need a
> different HsFFI.h depending on which compiler we are using.
> 
> One option would be to have hsc2hs (when installed with GHC) append
> "-I /usr/lib/ghc-6.10.1/include" to the commandline. If the user gives a
> "-I /usr/lib/hugs/include" flag then this path will be looked at first,
> and the hugs HsFFI.h will be used.
> 
> Another option would be for the user to tell hsc2hs which compiler
> they're using, e.g.
>     hsc2hs --compiler=/usr/bin/ghc Foo.hsc
> (this "compiler" is distinct from the C compiler that hsc2hs will use).
> hsc2hs will then pass the appropriate -I flag, depending on what sort of
> compiler it is told to use. The hsc2hs that comes with GHC would
> probably default to using the GHC that it is installed with, but
> standalone hsc2hs would probably default to searching for /usr/bin/ghc,
> /usr/bin/hugs, etc.
> 
> This last approach would also make it possible for hsc2hs to take
> "-package foo" flags, and add the include paths for the requested
> packages too.
> 
> The downside is that it's pushing a lot more knowledge into hsc2hs,
> which means there is one more thing to keep in sync.
> 
> 
> Has anyone got any other alternatives to consider? Or opinions on which
> solution is best?

I don't see nice solutions here. It's not nice to have each compiler
ship their own variant/wrapper of hsc2hs (which one gets to
be /usr/bin/hsch2s ?). It's also not nice for hsc2hs to have to know
about each different compiler. It's worst to have ghc get taught how to
compile .hsc files.

My suggestion is to avoid the problem. Why does hsc2hs need to know
anything about which Haskell compiler? It's because it #includes HsFFI.h
in its default hsc template. Why does it need to include HsFFI.h? Well,
actually it probably doesn't need to at all.

The HsFFI.h is not needed by code in the hsc template itself and nor is
it needed by other code generated by hsc2hs as far as I can tell. Does
anyone remember why HsFFI.h was included in the default hsc template? My
guess it's there as a convenience for those modules that need things
from HsFFI.h. I speculate that the number of .hsc modules that actually
need this header file is very low.

So my suggestion is that in those few cases where it is needed, it
should be specified explicitly in the .hsc file. In such cases it is the
responsibility of the build system to use the right -I include dir.
Cabal does this ok and others can do it too if needed.

To see if this is viable we'd want to check that building a bunch of
packages from hackage that use hsc2hs works with a modified template
file. This test should be relatively easy to perform, though would take
several hours of building.

Sound plausible?

Duncan