Hal Daume III
class Null a where nullify :: a -> a
Hmm...why not class Null a where null :: a (you are just throwing away the parameter anyway)?
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 =>.
In this case, I'd expect a type (and not a class, like "Null a" is) I'm a bit puzzled by the terminology: What does "constructor" mean in this context? Normally, I'd use (unqualified) "constructor" to mean data constructor, as in Just, Nothing, Left, Right and so on. (From the obvious Prelude stuff: data Maybe a = Just a | Nothing data Either a b = Left a | Right b ) Unless I'm completely confused, perhaps Hugs should expect a "type constructor" instead of just a "constructor" in its error message? Or perhaps it could simply say "type"? -kzm -- If I haven't seen further, it is by standing in the footprints of giants