[Haskell-cafe] Aren't type system extensions fun?
andrewcoppin at btinternet.com
Mon May 26 17:02:06 EDT 2008
Haskell 98 provides a simple and clean type system, which I feel I
understand very well.
GHC provides a vast zoo of strange and perplexing type system
extensions, which I don't understand at all. (Well, some of it is simple
enough - e.g., multiparameter type classes. But GADTs? FDs? ATs? Hmm...)
Anyway, it seems there is a large set of such type system extensions
that involve writing "forall" all over the place. I have by now more or
less managed to comprehend the fact that
data Thing = forall x. Thing x
allows a type variable to appear on the RHS that is *not* present on the
LHS, thus "hiding" the type of something inside the structure. And for
some reason, they call this "existential quantification" [which I can't
spell never mind pronounce].
Today I was reading a potentially interesting paper, and I stumbled
across something referred to as a "rank-2 type". Specifically,
class Typable x => Term x where
gmapT :: (forall y. Term y => y -> y) -> x -> x
At this point, I am at a complete loss as to how this is any different from
gmapT :: Term y => (y -> y) -> x -> x
Can anybody enlighten me?
This is probably the first real use I've ever seen of so-called rank-2
types, and I'm curios to know why people think they need to exist.
[Obviously when somebody vastly more intelligent than me says something
is necessary, they probably know something I don't...]
At this point, I don't think I even wanna *know* what the hell a rank-N
type is... o_O
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