[Haskell-cafe] Re: problem with cabal for syb-with-class

Claus Reinke claus.reinke at talk21.com
Tue Aug 26 10:53:12 EDT 2008

>> > So when ghc finds that one of your modules needs to import something
>> > that is not in one of the given packages it says that it's in another
>> > package that is 'hidden'. Of course it's only hidden because Cabal told
>> > ghc to hide them.
>> Yes, it is one of those unfortunate error messages that says "I know
>> what the problem is *and* how to fix it, but I'm not going to". ;-)
> Actually it's more like: "I know what the problem is but I cannot fix
> it. He knows how to fix it, but doesn't know there's a problem!" :-)
> GHC knows what the problem is but it's just following orders. Cabal gave
> the orders but doesn't know there is a problem.

Since you said "don't hold your breath for Cabal's dependencies":
Cabal doesn't have to pass on ghc's messages uninterpreted. That's
a lot like implementing a map as a list and complaining about empty
list instead of element not found.

Cabal is the interface here, ghc is the tool. The interface shouldn't
just pass instructions to the tool, it should also interpret and present
the tool's responses. As suggested in this thread:


Hmm, the archive failed to decode the code sketch attached to the
last message there (which demonstrated that some basic help could
be hacked up as a simple pattern->message script wrapping cabal),
so I attach the old code again for reference.

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