I'm using Hugs March 1999 version, with extensions (-98).
With extensions turned on, Hugs does not enforce the restriction against
type synonyms in instance declarations. I was exploring a possible way of
implementing Kevin Atkinson's goals, and thought this might work.
import Array
-- one of the multi-parameter type classes suggested by Kevin Atkinson
class Find c i e where
find :: i -> c i e -> Maybe e
-- using a type constructor to shoehorn [(i,e)] into the class Find
data Cont3 i e = Cont3 [(i,e)]
instance Eq i => Find Cont3 i e where
find i (Cont3 ls) = lookup i ls
-- Array types can be put into Find more easily.
type Y = Array
instance Ix a => Find Y a b where -- non-Haskell-98, but Hugs accepts.
find i a = if i `elem` (indices a) then Just (a ! i) else Nothing
-- a synonym for [(i,e)]
type X i e = [(i,e)]
-- The synonym X is meaningful, and can be used to define a function.
foo :: Ix i => i -> X i e -> Maybe e
foo i ls = lookup i ls
-- This is what we _really_ want X for:
instance Ix i => Find X i e where
find = foo
The last instance declaration has the same explicit structure as the
instance declaration for Y, but it is rejected by Hugs with the error message:
Not enough arguments for type synonym "X"
I believe that if nothing else, the error message is wrong.
-----------------
Apologies if this is a duplicate. I submitted a similar report to the bug
form at http://www.haskell.org/hugs/bug-reports.html last night, and
haven't seen a report come through the mailing list.
--
Scott Turner
[EMAIL PROTECTED] http://www.ma.ultranet.com/~pkturner
