[Haskell-cafe] Re: New to haskell: unresolved overloading question

[Christian Maeder]

Blair Fraser wrote:
> I'm new to haskell and I'm working through the class section of the
> gentle tutorial.  I decided to implement an ApproxEq class (just for
> fun), but have run into problems.  The code below works fine for units
> and bools, but gives an unresolved overloading error for Floats and
> Doubles.  What's going on here?  What does the error message mean and
> what did I do wrong?  (I'm in Hugs.)
> 
> -- ApproxEq: My first class.
> class (Eq a) => ApproxEq a where
>   (~=) :: a -> a -> Bool
>   x ~= y = x == y -- By default, check equality
> 
> -- These two override default with same, just checking if it works
> instance ApproxEq () where () ~= () = True        
> instance ApproxEq Bool where x ~= y = x == y
> 
> -- These two override default with different
> --   consider floating points equal if they differ by one or less
> instance ApproxEq Float where x ~= y = (abs (x-y) <= 1)
> instance ApproxEq Double where x ~= y = (abs (x-y) <= 1)

More elegant seems to be:

instance (Ord a, Num a) => ApproxEq a where x ~= y = (abs (x-y) < 1)

However, this requires extensions to "Allow unsafe overlapping instances":

hugs -98 +O

ghci -fglasgow-exts -fallow-overlapping-instances 
-fallow-undecidable-instances -fallow-incoherent-instances

> -- This one dosn't work either, but it just depends on the other two
> instance ApproxEq a => ApproxEq [a] where
>   [] ~= [] = True
>   (x:xs) ~= (y:ys) = x ~= y && xs ~= ys
>   _ ~= _ = False
> 
> Thanks,
> Blair