RE: Existentials

17 Apr 2003


      
...
"Simon Peyton-Jones"  writes:
...
GHC and Hugs both do.  In fact, GHC supports arbitrary-rank 
universal
quantification.
The example I tried failed, so I assumed it wasn't supported.
$ cat Exists.hs
        module Exists where
        data T = forall a. Enum a => Foo (a->a)
        f :: T -> (Int,Bool,Char)
        f (Foo g) = (g 0, g False, g 'a')
    $ ghc -fglasgow-exts -c Exists.hs
Exists.hs:4:
        Couldn't match `Int' against `Bool'
            Expected type: Int
            Inferred type: Bool
        In the first argument of `g', namely `False'
        In the definition of `f': (g 0, g False, g 'a')
You've written an existential constructor.  For universal
quantification, write it like this:

   data T = Foo (forall a . Enum a => a -> a)

a good illustration of the confusion caused by the dual use of forall, I
guess :-)

Cheers,
	Simon

RE: Existentials

Simon Marlow