Please, what is wrong here?
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class Project a b where prj :: b -> a -> b
data Pol a = Pol [(a,Int)] Char
instance Project (a,Int) (Pol a) where
prj (Pol _ c) (a,p) = Pol [(a,p)] c
f :: a -> Pol a -> Pol a
f a g@(Pol ms _) = prj g (head ms)
f' :: a -> Pol a -> Pol a
f' a g = prj g (a,1)
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ghc-4.02 -c -fglasgow-exts -optC-fallow-overlapping-instances
-optC-fallow-undecidable-instances
understands f, but does not understand f'.
Probably, there is something about a constant type Int placed in the
instance. Changing it to
instance Integral i => Project (a,i) (Pol a)
where
prj (Pol _ c) (a,p) = Pol [(a,toInt p)] c
removes the compiler error message.
But has ghc to allow the initial declarations?
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Sergey Mechveliani
[EMAIL PROTECTED]
