[Haskell-cafe] classes question

[Joe Fredette]

(+) is a name that is already taken by the Num typeclass, you're  
trying to overload it with a different class. It's equivalent to doing:

foo :: Int
foo = 1

foo :: String
foo = "abc"


this would cause an (obvious) namespace collision. If you want to  
redefine (+), you'll have to import a qualified prelude, by using the  
NoImplicitPrelude
pragma and `import qualified Prelude`.

A better alternative is to use a different name, `+`, `++` are both  
taken (for normal addition and list concatenation, resp.).

/Joe


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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