[Haskell-cafe] workarounds for Codec.Compression.Zlib errors in darcs

Don Stewart dons at galois.com
Wed Nov 26 17:30:45 EST 2008

> On Wed, 2008-11-26 at 14:38 +0000, Eric Kow wrote:
> > Hi everybody,
> > 
> > This advisory is for people who have installed darcs 2.1.2 via the
> > Cabal build method.  As you may have noticed, the cabalised darcs
> > sometimes fails with errors like
> > 
> >   Codec.Compression.Zlib: incorrect data check
> > 
> > Why this happens
> > ----------------
> > Older versions of darcs can to produce gzipped files with broken CRCs.
> > We never noticed this because our homegrown wrapper around the C libz
> > library does not pick up these errors.
> I should note that one moral of this story is to check that your FFI
> imports are correct. That is, check they import the foreign functions at
> the right Haskell types. In this case the mistake was that the foreign
> function returned a C int, but the Haskell foreign import declaration
> stated that the C function returned IO () rather than IO CInt.
> This is where a tool really helps. The hsc2hs tool cannot check the
> cross-language type consistency while c2hs can. It reads the C header
> files and generates the FFI imports at the correct Haskell types.
> The downside is that c2hs is not shipped with ghc, it is a bit slower
> and it's not quite so good with structures.
> I think there is a need for a tool like c2hs but that works in a
> checking mode rather than in a generating mode. It would use much of the
> same code as c2hs but it would read the C header files and the .hs file
> (via ghc api) and check that the FFI imports are using the right types.
> That way it could be run to check a package without the checker tool
> being needed at build time on every platform. The downside would be that
> some C header files differ between platforms and c2hs handles this fine
> while a checker tool might say it's ok on one platform and that may not
> carry over to another. Still, it would be an improvement on just using
> raw FFI imports (or hsc2hs, which is really the same thing).

Yes, this plagued xmonad's X11 bindings: almost all bugs in the last 12
months were due to FFI bindings.

I'd love a hsc2hs -Wall mode.

-- Don

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