[Haskell-cafe] Re: Duplicate Instance problem

[oleg at pobox.com]

Jason Dagit wrote:

> I tried to create a type class for making instances of Show display a
> custom way.  After using my class for a while I found that sometimes
> RealFloats would display as 'NaN' and this is unacceptable.  So at
> this point I had something like:
>
> class Show a => StringValue a where
>   toString :: a -> String
>
> instance Num a => StringValue a where
>   toString = show
>
> instance RealFloat a => StringValue a where
>   toString x
>     | isNaN x = "" -- as an example
>     | otherwise = show x

The simplest solution: replace the constraint RealFloat with its
members. That is, replace 
	 "instance RealFloat a => StringValue a"
with

 instance StringValue Float where
   toString x
     | isNaN x = "" -- as an example
     | otherwise = show x

and the same for Double. There are currently only two members of
RealFloat, so code duplication should not be so terrible. Or if it is,
then you can do

> {-# OPTIONS -fglasgow-exts #-}
> {-# OPTIONS -fallow-undecidable-instances #-}
> {-# OPTIONS -fallow-overlapping-instances #-}
>
> class Show a => StringValue a where
>   toString :: a -> String
>   toString = show
>
> instance Num a => StringValue a  -- works for all Num except the following
>
> instance StringValue Float where
>   toString = myrealfloatthing
> instance StringValue Double where
>   toString = myrealfloatthing
>
> myrealfloatthing x 
>      | isNaN x = "" -- as an example
>      | otherwise = show x
>
> t1 = toString (1::Int)
> t2 = toString (1::Double)
> t3 = toString ((0.0::Double)/0.0)


A more general solution will use generic programming. I doubt however
it will be much shorter than the above.