BTW, here's a non-contrived example. It's pretty easy to come up with
examples when you try to use type classes instead of a proper module system.
Here we have expressions parametrized over how identifiers and literals are
represented. First a simple instance, and then one where all the types are
parametrized over the string representation. These are the plug-and-play
type of things I'd like to be able to do.
class IsExpr expr id lit | expr -> id lit where
eId :: id -> expr
eLit :: lit -> expr
eApply :: expr -> expr -> expr
data SimpleExpr = SId Char | SLit Int | SApply SimpleExpr SimpleExpr
instance IsExpr SimpleExpr Char Int where
eId = SId
eLit = SLit
eApply = SApply
data FancyExpr str = FId (Id str) | FLit (Lit str) | FApply (FancyExpr str)
(FancyExpr str)
data Id str = Id str
data Lit str = LString str | LInt Int
instance IsExpr (FancyExpr str) (Id str) (Lit str) where
eId = FId
eLit = FLit
eApply = FApply
On Fri, Apr 18, 2008 at 9:26 AM, Martin Sulzmann
Lennart Augustsson wrote:
To reuse a favorite word, I think that any implementation that distinguishes 'a -> b, a -> c' from 'a -> b c' is broken. :) It does not implement FD, but something else. Maybe this something else is useful, but if one of the forms is strictly more powerful than the other then I don't see why you would ever want the less powerful one.
Do you have any good examples, besides the contrived one
class D a b c | a -> b c
instance D a b b => D [a] [b] [b]
where we want to have the more powerful form of multi-range FDs?
Fixing the problem who mention is easy. After all, we know how to derive improvement for multi-range FDs. But it seems harder to find agreement whether multi-range FDs are short-hands for single-range FDs, or certain single-range FDs, eg a -> b and a -> c, are shorthands for more powerful multi-range FDs a -> b c. I clearly prefer the latter, ie have a more powerful form of FDs.
Martin