Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?

Hi there, Thanks for the reply. To be clear, all I want is to "avoid having to type type variables all over the place". What should I do? My original code with RankNTypes and ImpredicativeTypes does not work.... The "type Job = forall k a. M.Map k a" works now. But function uses it does not. Compiler complains about "Couldn't match expected type `Job' with actual type `M.Map k0 b0'". On Wed, Jun 13, 2012 at 9:15 PM, Daniel Peebles wrote:

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 -- 竹密岂妨流水过 山高哪阻野云飞

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-- Ismael

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-- 竹密岂妨流水过 山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.