Am Montag, 21. Februar 2005 17:32 schrieb 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
For the original code, no extensions are necessary (and neither hugs nor ghc does complain). Probably something like ApproxEq> 1.5 ~= 2.4 ERROR - Unresolved overloading *** Type : (Fractional a, ApproxEq a) => Bool *** Expression : 1.5 ~= 2.4 happened, that doesn't mean there is any problem with the code, only that the interpreter doesn't know what type 1.5 and 2.4 have. Adding a type to either resolves: ApproxEq> 1.5 ~= (2.4::Double) True ApproxEq> [1,2,3] ~= [1.6,2.8,3.9999] ERROR - Unresolved overloading *** Type : (Fractional a, ApproxEq a) => Bool *** Expression : [1,2,3] ~= [1.6,2.8,3.9999] ApproxEq> [1,2,3] ~= ([1.6,2.8,3.9999]::[Float]) True That's a very common problem for newbies, so don't panic. In older versions of hugs (November 2002, e.g.), you would have got an unresolved overloading also from entering [] to the prompt (this is no longer so). If such things happen, add an explicit typing, if that doesn't help, then you have a problem. To deepen the confusion, hugs has no problems with Prelude> 1.5 == 1.5 True I'm not quite sure why this works, must be due to type defaulting, however, including default (Double) doesn't help ApproxEq, so there must be more to it in the (==) case. BTW, if you add Christian's instance, for hugs, -98 +o (lower case o, allow overlapping instances) suffices, for ghci, -fallow-incoherent-instances is not necessary. Stay with Haskell, it's great (though not perfect)! Daniel