wren ng thornton wren at freegeek.org
Tue Mar 6 06:37:36 CET 2012

```On 3/5/12 5:13 PM, AntC wrote:
> I've tried that ListFunc wrapping you suggest:
> [...]
> But I can't 'dig out' the H-R function and apply it (not even
> monomorphically):

That's because the suggestion changed it from a universal quantifier to
an existential quantifier.

data Exists f = forall a. Exists (f a)

data Forall f = Forall (forall a. f a)

With Exists, we're essentially storing a pair of the actual type 'a' and
then the f a itself. We can't just pull out the f a and use it, because
we don't know what 'a' is. We can bring it into scope temporarily by
case matching on the Exists f, which allows us to use polymorphic
functions, but we still don't actually know what it is so we can *only*
use polymorphic functions.

Conversely, with Forall we're storing some f a value which is in fact
polymorphic in 'a'. Here, because it's polymorphic, the caller is free
to choose what value of 'a' they would like the f a to use. Indeed, they
can choose multiple different values of 'a' and get an f a for each of them.

--
Live well,
~wren

```