recent changes to hsc2hs break wrapping c++

Evan Laforge qdunkan at
Sat Feb 18 06:22:49 CET 2012

On Thu, Feb 16, 2012 at 4:27 AM, Simon Marlow <marlowsd at> wrote:
> I'm not sure why you're using the C++ compiler from hsc2hs, and I don't
> think that is guaranteed to work.  But maybe we can fix it if there's a
> legitimate reason to want it.

Well, consider if you're writing c++ along with haskell.  You can
communicate with c++ by passing structs, which are guaranteed to have
the layout as c, and call functions wrapped in extern "C" {...}.  You
can't call class constructors from haskell or anything, but you can
create structs to pass in and use directly in c++, which is already
quite convenient.

But those C structs are going to be defined in C++ headers with other
C++ cruft in them that a C compiler is not going to understand.  But
if you pass -c g++ to hsc2hs then it works just fine to emit the
struct offsets.

Otherwise, I'm not sure how you'd call C++ from the haskell FFI.  Like
I said, I guess you'd have to split the C structs into separate
headers, or maybe wall off the C++ bits with ifdefs.  Actually, I
guess the ifdef part wouldn't be *so* bad, at last as far as ifdeffery
goes, but it is intrusive on existing headers, which is not going to
be possible in all situations.  Of course if the C++ API was written
without regard to the haskell FFI it is likely to require structs to
be constructed with constructors instead of direct pokes, which is
beyond the FFI's capabilities.  But in the case where it is designed
to use plain structs in the haskell-facing bits, it's very convenient
how you can just pass -c g++ to earlier versions of hsc2hs and have it
just work.

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