hsc2hs and HsFFI.h

Ian Lynagh igloo at earth.li
Tue Feb 10 12:15:45 EST 2009

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
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?


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