Synonym Type Constructors
Tue, 20 Feb 2001 01:34:52 +1100
On 19-Feb-2001, Ashley Yakeley <email@example.com> 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
Right. So by definition, it does.
> Yet it doesn't seem possible to declare the type constructor
> an instance of a class:
> Hugs gives:
> (line 6): Not enough arguments for type synonym "T"
That is also correct.
As the Haskell Report section 4.2.2 says:
| Type constructor symbols T introduced by type synonym declarations
| cannot be partially applied; it is a static error to use T without the
| full number of arguments.
> So is T a real type constructor or not?
The Haskell 98 Report does not define the term "real type constructor".
So this is one of those existential philosphical debates about terminology ;-)
Personally I would describe the type constructors introduced by type
synonym declarations as "real", but not "first class".
Fergus Henderson <firstname.lastname@example.org> | "I have always known that the pursuit
| of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.