[Haskell-cafe] classes question

[Ross Mellgren]

It's not your class definition that is the problem -- your class  
definition is good.

The problem is that when you interact with it using the REPL, you  
don't specify any particular type you want so it's ambiguous. Usually  
this is not a problem in actual programs that you compile because  
there's enough context for the type inferencer to figure out what type  
'a' should be.

You can give it enough context in the REPL either by annotating the  
value or by using some other typed thing, e.g.

let a = 1:: Int

If you really want it to only work on Int, then you shouldn't use a  
typeclass, instead just write the function directly --

plus :: Int -> Int -> Int
plus = (+)

-Ross

On Nov 9, 2009, at 5:09 PM, Paul Tokarev wrote:

>
> Thanks, that works. But how sould I change my class definition, so  
> that it
> works without (2::Int), but just with 2?
>
>
> Ross Mellgren wrote:
>>
>> You did not specify what type of number it is -- in Haskell numeric
>> constants like "1" are actually typed as forall a. Num a -- that is,
>> can be any type of Num.
>>
>> Try: plus (2::Int) 3
>>
>> -Ross
>>
>> On Nov 9, 2009, at 1:44 PM, Paul Tokarev wrote:
>>
>>>
>>> Hi.
>>>
>>> I am using Hugs 98
>>> I have that piece of code:
>>>
>>> class PlusTimes a where
>>>  plus :: a -> a -> a
>>>
>>> instance PlusTimes Int where
>>>  plus x y = x + y
>>>
>>> when I run : "plus 2 3" I get this error:
>>> ERROR - Unresolved overloading
>>> *** Type       : (Num a, PlusTimes a) => a
>>> *** Expression : plus 2 3
>>>
>>> What am I doing wrong?
>>> Thanks.
>>>
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>
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