[Haskell-cafe] How to understand the 'forall' ?
ryani.spam at gmail.com
Wed Sep 2 01:17:20 EDT 2009
You can also look at it in terms of the "deconstruction" operation on
this data type.
For example, lists:
> data List a = Nil | Cons a (List a)
> case_list :: r -> (a -> List a -> r) -> List a -> r
> case_list nil_case cons_case Nil = nil_case
> case_list nil_case cons_case (Cons x xs) = cons_case x xs
Now, given the "Branch" type:
> data Branch tok st a = forall b. Branch (PermParser tok st (b -> a)) (GenParser tok st b)
what is the type of its "case" construct? Here's the answer:
> case_branch :: (forall b. PermParser tok st (b -> a) -> GenParser tok st b -> r) -> Branch tok st a -> r
> case_branch k (Branch pparser gparser) = k pparser gparser
Notice the higher-rank type on the argument k; it has to be able to
accept *any* type b, because the constructor hid an object of *some*
type which is no longer known. So unless you can accept *any* b, you
can't operate on this object.
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