[Haskell-cafe] Control.Exception base-3/base-4 woes
Neil Brown
nccb2 at kent.ac.uk
Fri Sep 11 11:26:47 EDT 2009
Hi,
In my CHP library I need to do some exception catching. I want the
library to work on GHC 6.8 (with base-3 -- this is the current version
in Ubuntu Hardy and Jaunty, for example) and GHC 6.10 (which comes with
base-4). But base-3 and base-4 need different code for exception
catching (whether it's importing Control.OldException or giving a type
to the catch method).
Here's what I currently do -- my Haskell file contains this:
#if __GLASGOW_HASKELL__ >= 609
import qualified Control.OldException as C
#else
import qualified Control.Exception as C
#endif
My cabal file contains this (it used to say just "base,..." but Hackage
complained at me the other day when I tried to upload that):
Build-Depends: base >= 3 && < 5, ...
This works on two machines: one is 6.8+base-3, the other is
6.10+base-3&base-4, where cabal seems to use base-4. However, I have
had a bug report (now replicated) which stems from a different
6.10+base-3&base-4 machine where cabal picks base-3 instead. The real
problem is that the #if is based on GHC version, but really it should be
based on which base-* library is being used. I know the code works with
base-3 (use Control.Exception) and base-4 (use Control.OldException) but
I can't get the build process right to alter the code based on which
base-* library is being used.
Can anyone tell me how to fix this? I don't think that changing to
always use Control.Exception would fix this, because I need to give a
different type for catch in base-3 to base-4, so there's still the
incompatibility to be dealt with.
Thanks,
Neil.
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