OK. I think I understand a little.
I use Job here just wants to simplify the code. And since I provide
the function as library, I cannot decide what exact type k is. What
should I do?
On Thu, Jun 14, 2012 at 11:23 AM, Arlen Cuss <a...@len.me> wrote:
> (resending to café, turns out I wasn't subbed from this address.)
>
> Hi Magicloud,
> This is correct; because you've hidden the type-variables away by universally
> quantifying them, there's no more level of specificity you can get back *out*
> of them than just "some kind of Map" (Job = M.Map k b, where k ≠ k0, b ≠ b0).
>
> If you have a Job type which can store *any* kind of Map (forall k a. Job
> (Map k a)), then that means you could have a Job with a Map Int Bool, and a
> Job with a Map String (Float -> Float), and they'd both have the same type
> "Job". You can't do anything with the values within, because you're being too
> permissive about what a Job is.
>
> You may want "data Job k a = Job (Map k a)", *or* if you do actually use one
> kind of Map only, then why not "data Job = Job (Map Int String)"
> (substituting your real types for Int and String). In this case, you could
> also consider using newtype ("newtype Job = Job { getJob :: Map Int String
> }") to provide the guarantee that you're getting a Job (and not any Map Int
> String) without performance loss.
>
> Let me know if I've been more confusing than helpful;
>
> Arlen
>
>
> On Thursday, 14 June 2012 at 1:16 PM, Magicloud Magiclouds wrote:
>
>> 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 <pumpkin...@gmail.com
>> (mailto:pumpkin...@gmail.com)> wrote:
>> That doesn't require existential quantification, but it'll need Rank-2
>> typesif you ever do anything with Job. Unfortunately, a universally
>> quantifiedJob like what you wrote (or what Magicloud seems to want) is only
>> inhabitedby 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 unfortunatelythe
>> 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 init. You could ask to enumerate the elements in it, but you
>> wouldn't be ableto 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
>> aheterogeneous Map, maybe? Or just to avoid having to type type variables
>> allover the place? Both of those are possible but require a bit
>> moresophistication with types.
>> >
>> -Dan
>> >
>> >
>> On Wed, Jun 13, 2012 at 7:32 AM, Ismael Figueroa Palet<ifiguer...@gmail.com
>> (mailto:ifiguer...@gmail.com)> wrote:
>> > >
>> Do you want to hide the specific types of the job? Presumably to thendefine
>> a type JobList = [Job] ?You can do that with the ExistentialQuantification
>> extension.
>> > >
>> type Job = forall k a. Map k atype JobList = [Job]
>> > >
>> ??Note you can't unpack the types k a once you have hidden them. But
>> thetypechecker 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 <magicloud.magiclo...@gmail.com
>> (mailto:magicloud.magiclo...@gmail.com)>
>> > > >
>> Hi,I've forgotten this.This is OK:type Job k a = Map k aAnd 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
>> > > >
>> And for G+, please use magiclouds#gmail.com (http://gmail.com).
>> > > >
>> _______________________________________________Haskell-Cafe mailing
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>> > >
>> > >
>> > >
>> > >
>> > >
>> > >
>> > >
>> > >
>> --Ismael
>> > >
>> > >
>> _______________________________________________Haskell-Cafe mailing
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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 (http://gmail.com).
>>
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