Hi Mark, all: thanks for the swiftly muddle-dispelling response.
> | Here's some of the threatened examples:
> |
> | > data OrdFuncExist = OE (Ord a => Char -> a)
> | > data OrdListExist = OLE (Ord a => [a])
>
> Perhaps this is a GHC/Hugs difference, but the syntax that you've
> used here isn't permitted in Hugs ... and in old versions where it
> might have been allowed, it would suggest *universal* quantification
> rather than existential.
Must be such a difference. This is what ghc accepts, and documents:
the user guide even claims to following hugs, here, so that'll be a
tad out of date, then...
> I just rewrote the definitions to fit with the Hugs syntax for
> existentials as follows:
>
> > data OrdFuncExist = forall a. Ord a => OE (Char -> a)
> > data OrdListExist = forall a. Ord a => OLE [a]
>
> and now the rest of the code that you sent type checks without any
> apparent problem, including emap, blah, and emax.
It does me ego no end of good to find out that I'm right, and the
compiler is wrong (for a change, it must be said...). Unless either
ghc or hbc catch up to the Hugs extension, though, avails me little
in practice, sadly. (Hint, hint.)
> [We keep trying to argue that it makes sense to use the "forall" keyword
> here (because the constructor function that is being defined really does
> have a polymorphic type). But perhaps this is a further indication that
> the time for a "exists" keyword has come!]
The forall looks syntactically _unnecessary_, slap-bang next to a context,
but it's not confusing in and of itself -- well, not beyond my usual
level of confusion about positive and negative quantifier occurrences.
(Different systems and different docs certainly are, though.)
Cheers,
Alex.
