nullify 6 on the Hugs command line:
ERROR - Unresolved overloading *** Type : (Num a, Null a) => a *** Expression : nullify 6
Try 'nullify (6::Int)'. Hugs (not due to any fault of its own) doesn't know that 6 is supposed to be of type Int, so if you also had an instance 'Null Double', what should it do?
I then tried the following:
instance Num a => Null a where nullify x = 0
but got the following error in Hugs
ERROR "C:\zero.hs":5 - Syntax error in instance head (constructor expected)
This is because instance declarations like this are illegal. Basically, there has to be a constructor on the RHS of the =>. You can't just have a type variable. -98 on hugs and -fglasgow-exts on ghc should lift this restrictuion, though I think you need -fallow-undecidable-instances in ghc and something else in Hugs... - Hal -- Hal Daume III | hdaume@isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume