[Haskell] [Haskell-cafe] Help with polymorphic functions

[Ross Mellgren]

You gave test a type signature which gives "a" universal  
quantification, which means in this case that "a" is something, but  
you can't do anything in particular to it (since you don't know  
anything about it).

shift has the signature a -> Int -> a, but it's within the type class  
Bits:

Prelude> import Data.Bits
Prelude Data.Bits> :i shift
class (Num a) => Bits a where
...
shift :: a -> Int -> a
...
	-- Defined in Data.Bits
infixl 8 shift

So in this case, "a" is actually the "a" from "Bits a" above. Your  
function, test, does not say that "a" is a bits, and that's what the  
compiler is telling you.

Change your type signature to

shift :: Bits a => a -> Int -> a

and it should be good to go!

-Ross

P.S. I'm something of a haskell newbie, so if I got any of this wrong,  
please someone more knowledgeable correct me!

On May 8, 2008, at 3:10 PM, Wei Yuan Cai wrote:
> Hello,
>
> I'm having some trouble with a polymorphic function using another  
> polymorphic function within. A simplified code of what I'm trying to  
> do is as follows:
>
> main = print $ test 1 8
>
> test :: a -> Int -> a
> test x n = shift x n
>
> I get the following compilation error:
>
> Could not deduce (Data.Bits.Bits a) from the context ()
>       arising from a use of `shift' at test.hs:8:11-19
>     Possible fix:
>       add (Data.Bits.Bits a) to the context of
>         the type signature for `test'
>     In the expression: shift x n
>     In the definition of `test': test x n = shift x n
>
>
> shift is defined as "a -> Int -> a"
>
> What am I doing wrong here?
>
> Thanks,
> Weiyuan
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