Hello, I seem to have run into a problem involving functional dependencies. Below is an excerpt of a program I was writing that illustrates the bug.
data N0 newtype Succ n = Succ n
class Plus a b c | a b -> c instance Plus N0 n n instance Plus a b c => Plus (Succ a) b (Succ c)
( # ) :: Plus x y z => x -> y -> z ( # ) = undefined
class BitRep t n | t -> n where toBits :: t -> n
instance BitRep Bool (Succ N0)
instance BitRep (Bool,Bool,Bool) (Succ (Succ (Succ N0))) where toBits (x,y,z) =
The program works with the line below, but if I remove it I get a type error --- Hugs complains that it cannot solve the constraint: Plus (Succ N0) (Succ N0) (Succ (Succ N0))
toBits' where toBits' =
toBits x # toBits y # toBits z
The program works with GHC in either form, and it looks like Hugs should be able to solve the constraint using the instances above. -Iavor