GHC 3.02, outa-tha-box. In the program given below, compiled by:
% ghc -fglasgow-exts -c tt.hs -o tt.o
I get the complaint:
tt.hs:22:
No instance for `Conditional Bool [Bool]'
(arising from use of `ifc' at tt.hs:22)
I say there is an instance, given by the last instance decl in the
file. In fact, if I try to add the following, unconditional instance
decl:
instance Conditional Bool [Bool] where
ifc c x y = map (uncurry (ifb c)) (dist2 x y)
then it gives me the additional complaint:
tt.hs:1:
Duplicate or overlapping instance declarations
for `Conditional Bool [Bool]' at tt.hs:23 and tt.hs:20
Oi! There's either no instances, or there's too many -- what's it going
to be! ;-)
--Jeff
------------------
module Buggo where
class Boolean b where
ifb :: b -> b -> b -> b
instance Boolean Bool where
ifb x y z = if x then y else z
class Conditional c a where
ifc :: c -> a -> a -> a
ifc_bogus :: c -> a
class Dist2 f where
dist2 :: f a -> f b -> f (a, b)
instance Dist2 [] where
dist2 = zip
instance (Boolean b, Dist2 f, Functor f) => Conditional b (f b) where
ifc c x y = map (uncurry (ifb c)) (dist2 x y)
t x y z = ifc (x :: Bool) (y :: [Bool]) z
