Re: Synonym Type Constructors

Ashley Yakeley wrote:

I don't know if this is a bug in Hugs 98, or whether it's a misunderstanding of mine.

The Haskell 98 Report Sec. 4.2.2 claims that 'type' introduces a new type constructor. Yet it doesn't seem possible to declare the type constructor an instance of a class:

-- class MyClass c where foo :: a -> c a

type T m = IO m

instance MyClass T where foo = return --

Hugs gives: (line 6): Not enough arguments for type synonym "T"

So is T a real type constructor or not?

I haven't tried this in Haskell 98 mode yet, but I'm sure with the -98 flag, Hugs accepts: type T = IO instance MyClass T where ... The problem is not what you suspected to be. It is that you always have to fully apply a type synonym. -- Zhanyong Wan