Re: Existential types, save me now?
Hi Fergus. data OrdFuncExist = OE (Ord a = Char - a) That's not the syntax for a existential type, that's the syntax for a universally quantified type with rank-2 polymorphism. With that syntax, the argument of `OE' must be a polymorphic function with type `forall a . Ord a = Char - a'. That would explain a lot. There seems to be a discrepancy between ghc and the ghc user-guide, in that event. Is this supposed to be a heterogenous list, or a homogeneous list? Sorry, I should have been a tad less terse: it's intended to be a homogeneous list, else the maximum wouldn't be well-typed. Hmm, I'm not sure exactly what you're trying to achieve here... but perhaps you just need to add instance Ord AnyOrd where compare (MkAnyOrd x) (MkAnyOrd y) = compare x y to make it work. I haven't tried this, but this is unwrapping two (potentially) _different_ types, and then trying to compare them, isn't it? (Been there, done that, didn't like the type errors, in a previous brush with existential types...) Cheers, Alex.
RE: Existential types, save me now?
Hi Alex, | 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. 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. [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!] Hope this helps! All the best, Mark [EMAIL PROTECTED] Pacific Software Research Center, Oregon Graduate Institute Looking for a PhD or PostDoc? Interested in joining PacSoft? Let us know!
Re: Existential types, save me now?
On 22-Nov-1999, Alex Ferguson [EMAIL PROTECTED] wrote: Here's some of the threatened examples: data OrdFuncExist = OE (Ord a = Char - a) That's not the syntax for a existential type, that's the syntax for a universally quantified type with rank-2 polymorphism. With that syntax, the argument of `OE' must be a polymorphic function with type `forall a . Ord a = Char - a'. I think the syntax for what you want is data OrdFuncExist = Ord a = OE (Char - a) data OrdListExist = OLE (Ord a = [a]) Is this supposed to be a heterogenous list, or a homogeneous list? That is, must the elements all have the same type `a', or is the choice of `a' supposed to be allowed to vary for each element? If you want a homogeneous existentially quantified list, then use data OrdListExist = Ord a = OLE [a] But I suspect you want a heterogenous existentially quantified list. In that case, you should use something like this: data AnyOrd = Ord a = MkAnyOrd a data OrdListExist = [AnyOrd] emap :: OrdFuncExist - [Char] - OrdListExist emap (OE f) l = OLE (map f l) You'll probably need something along the lines of emap (OE f) l = OLE (map (MkAnyOrd . f) l) The other problem: emax :: OrdListExist - OrdListExist emax (OLE l) = OLE [maximum l] Now, I think I see why this doesn't work: but any ideas on how to fix? Hmm, I'm not sure exactly what you're trying to achieve here... but perhaps you just need to add instance Ord AnyOrd where compare (MkAnyOrd x) (MkAnyOrd y) = compare x y to make it work. -- Fergus Henderson [EMAIL PROTECTED] | "I have always known that the pursuit WWW: http://www.cs.mu.oz.au/~fjh | of excellence is a lethal habit" PGP: finger [EMAIL PROTECTED]| -- the last words of T. S. Garp.
RE: Existential types, save me now?
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.
Re: Existential types, save me now?
On 22-Nov-1999, Alex Ferguson [EMAIL PROTECTED] wrote: Is this supposed to be a heterogenous list, or a homogeneous list? Sorry, I should have been a tad less terse: it's intended to be a homogeneous list, else the maximum wouldn't be well-typed. Oh, of course -- you're quite correct. -- Fergus Henderson [EMAIL PROTECTED] | "I have always known that the pursuit WWW: http://www.cs.mu.oz.au/~fjh | of excellence is a lethal habit" PGP: finger [EMAIL PROTECTED]| -- the last words of T. S. Garp.
