[Haskell-beginners] typeclass error

[Amitava Shee]

Thank you.

I get the following error after making the suggested changes

-- foo.hs
{-# LANGUAGE FlexibleInstances #-}
module Foo where

class Foo a where
    bar :: a -> a
--
-- instance Foo Int where
--     bar i = i + 10
--
-- instance Foo [Char] where
--     bar m = "foo" ++ m
--

instance Num a => Foo a where
    bar x = x + 10

-----

foo.hs:14:0:
    Constraint is no smaller than the instance head
      in the constraint: Num a
    (Use -XUndecidableInstances to permit this)
    In the instance declaration for `Foo a'
Failed, modules loaded: none.
Prelude>

...Amitava

On Tue, Apr 21, 2009 at 9:42 AM, Joe Fredette <jfredett at gmail.com> wrote:

> If I'm not mistaken, changing:
>
>   bar i = i + 10
>
> to
>
>   bar i = i + 10::Int
>
> ought to fix it.
>
> The issue is that the compiler is being to general wrt the type of "10",
> it's inferring that it's just of type (Num a => a) -- the type of all
> numbers (sortof.)
>
> Alternately, you could implement this as:
>
>   instance Num a => Foo a where
>      bar x = x + 10
>
> which similarly ought to fix it. Do you understand why?
>
> Amitava Shee wrote:
>
>> I have the following code
>>
>> -- foo.hs
>> module Foo where
>>
>> class Foo a where
>>    bar :: a -> a
>>   instance Foo Int where
>>    bar i = i + 10
>>   ------------------
>>
>> It fails to compile withe following error
>> *Foo> bar 10
>>
>> <interactive>:1:4:
>>    Ambiguous type variable `t' in the constraints:
>>      `Num t' arising from the literal `10' at <interactive>:1:4-5
>>      `Foo t' arising from a use of `bar' at <interactive>:1:0-5
>>    Probable fix: add a type signature that fixes these type variable(s)
>> *Foo>
>>
>> - Thanks
>> Amitava Shee
>> ------------------------------------------------------------------------
>>
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>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
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