That doesn't require existential quantification, but it'll need Rank-2
types if you ever do anything with Job. Unfortunately, a universally
quantified Job like what you wrote (or what Magicloud seems to want) is
only inhabited by the empty Map.
An existentially quantified Job, as you might get with
data Job = forall k a. Job (Map k a)
does let you wrap up any Map containing anything in it, but unfortunately
the only thing you can do with that map afterwards is ask for "structural"
properties about it, like whether it's empty or how many elements it has in
it. You could ask to enumerate the elements in it, but you wouldn't be able
to touch any of them because you wouldn't know what their types were.
So I'm not really sure how to interpret the question. Was the goal to have
a heterogeneous Map, maybe? Or just to avoid having to type type variables
all over the place? Both of those are possible but require a bit more
sophistication with types.
-Dan
On Wed, Jun 13, 2012 at 7:32 AM, Ismael Figueroa Palet wrote: Do you want to hide the specific types of the job? Presumably to then
define a type JobList = [Job] ?
You can do that with the ExistentialQuantification extension. type Job = forall k a. Map k a
type JobList = [Job] ??
Note you can't unpack the types k a once you have hidden them. But the
typechecker can use it to ensure some static property.
Also you could use unsafeCoerce to do some casts, but *only if you are
*sure* that things will go OK*. 2012/6/13 Magicloud Magiclouds Hi,
I've forgotten this.
This is OK:
type Job k a = Map k a
And this is OK:
{-# LANGUAGE RankNTypes #-} -- or LiberalTypeSynonyms?
type Job = forall a. forall k. Map k a Then how to write it like this?
type Job = Map k a
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