I think I need to think this through....
On Thu, Jun 14, 2012 at 12:28 PM, Ivan Lazar Miljenovic
<ivan.miljeno...@gmail.com> wrote:
> On 14 June 2012 14:20, Magicloud Magiclouds
> <magicloud.magiclo...@gmail.com> wrote:
>> 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?
>
> Do you know what the type of `a'? If so:
>
> type Job k = Map k String
>
> Otherwise... do you even need a type alias?
>
>>
>> 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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>>>> > > > http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>> > >
>>>> > >
>>>> > >
>>>> > >
>>>> > >
>>>> > >
>>>> > >
>>>> > >
>>>> --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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>>
>>
>>
>> --
>> 竹密岂妨流水过
>> 山高哪阻野云飞
>>
>> And for G+, please use magiclouds#gmail.com.
>>
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>
>
>
> --
> Ivan Lazar Miljenovic
> ivan.miljeno...@gmail.com
> http://IvanMiljenovic.wordpress.com
--
竹密岂妨流水过
山高哪阻野云飞
And for G+, please use magiclouds#gmail.com.
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