Re: [Haskell-beginners] multi parameter classes and instance declarations

16 Feb 2009


      On Mon 16.02 15:28, Markus Barenhoff wrote:
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Good Afternoon,
i'am trying to express something like this here:
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-- CI.hs http://pastebin.com/f5b17cf57
{-# OPTIONS -XMultiParamTypeClasses #-}
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module CI(D,f) where
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class B b
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class (B b) => A a b where
    f :: b
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data D = D
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instance B D
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instance A D D where
    f = D
-- CI.hs
...
-- MultiClassTest.hs http://pastebin.com/f1c481e8d
import CI
...
a = f
-- MultiClassTest.hs
I forgot to mention that constrain a :: D does leed to the same
ghc messages.
...
ghc output:
MultiClassTest.hs:4:4:
    No instance for (CI.A a D)
      arising from a use of `f' at MultiClassTest.hs:4:4
    Possible fix: add an instance declaration for (CI.A a D)
    In the expression: f
    In the definition of `a': a = f
Failed, modules loaded: CI.
Markus