Synonym Type Constructors
Mon, 19 Feb 2001 09:31:51 -0500
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