Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
Sorry, the full code is here:
http://hpaste.org/69972
On Fri, Jun 15, 2012 at 7:09 AM, Arlen Cuss wrote:
> Hi Magicloud,
>
> The indentation has been lost in the mail. Could you post your code
> (preferably without line numbers) on hpaste.org or similar?
>
> —A
>
>
> On Thursday, 14 June 2012 at 5:33 PM, Magicloud Magiclouds wrote:
>
>> And line 14, should be JobInfo a e.
>> I must be too sleepy
>>
>> On Thu, Jun 14, 2012 at 3:30 PM, Magicloud Magiclouds
>> mailto:magicloud.magiclo...@gmail.com)>
>> wrote:
>> > Sorry, the last 'a' of line 22 is 'b'.
>> >
>> > On Thu, Jun 14, 2012 at 3:19 PM, Magicloud Magiclouds
>> > mailto:magicloud.magiclo...@gmail.com)>
>> > wrote:
>> > > OK. I am totally confused here. Why "Couldn't match expected type
>> > > `Jobs k e a' with actual type `M.Map k0 b0'"
>> > >
>> > > 9|data JobInfo a e = (Exception e) =>
>> > > 10| JobInfo { jobId :: ThreadId
>> > > 11| , result :: MVar (Either e a) }
>> > > 12|
>> > > 13|type Jobs k e a = (Ord k, Exception e) =>
>> > > 14| M.Map k (JobInfo e a)
>> > > 15|
>> > > 16|type JobArgs k a = (Ord k) =>
>> > > 17| M.Map k a
>> > > 21|
>> > > 22|start :: (Ord k, Exception e) => JobArgs k a -> (a -> IO b) -> IO
>> > > (Jobs k e a)
>> > > 23|start args worker = do
>> > > 24| arg <- newEmptyMVar
>> > > 25| Map.mapM (\a -> do
>> > > 26| putMVar arg a
>> > > 27| result <- newEmptyMVar
>> > > 28| tId <- forkIO $ do
>> > > 29| arg_ <- takeMVar arg
>> > > 30| result_ <- try $ worker arg_
>> > > 31| putMVar result result_
>> > > 32| return $ JobInfo tId result
>> > > 33| ) args
>> > >
>> > > On Thu, Jun 14, 2012 at 1:24 PM, Magicloud Magiclouds
>> > > mailto:magicloud.magiclo...@gmail.com)>
>> > > wrote:
>> > > > I think I need to think this through
>> > > >
>> > > > On Thu, Jun 14, 2012 at 12:28 PM, Ivan Lazar Miljenovic
>> > > > mailto:ivan.miljeno...@gmail.com)> wrote:
>> > > > > On 14 June 2012 14:20, Magicloud Magiclouds
>> > > > > > > > > > (mailto: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 > > > > > > (mailto: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
>> > > > > > > > mailto:pumpkin...@gmail.com)> wrote:
>> > > > > > > > That doesn't require existential quantification, but
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
Hi Magicloud,
The indentation has been lost in the mail. Could you post your code (preferably
without line numbers) on hpaste.org or similar?
—A
On Thursday, 14 June 2012 at 5:33 PM, Magicloud Magiclouds wrote:
> And line 14, should be JobInfo a e.
> I must be too sleepy
>
> On Thu, Jun 14, 2012 at 3:30 PM, Magicloud Magiclouds
> mailto:magicloud.magiclo...@gmail.com)>
> wrote:
> > Sorry, the last 'a' of line 22 is 'b'.
> >
> > On Thu, Jun 14, 2012 at 3:19 PM, Magicloud Magiclouds
> > mailto:magicloud.magiclo...@gmail.com)>
> > wrote:
> > > OK. I am totally confused here. Why "Couldn't match expected type
> > > `Jobs k e a' with actual type `M.Map k0 b0'"
> > >
> > > 9|data JobInfo a e = (Exception e) =>
> > > 10| JobInfo { jobId :: ThreadId
> > > 11| , result :: MVar (Either e a) }
> > > 12|
> > > 13|type Jobs k e a = (Ord k, Exception e) =>
> > > 14| M.Map k (JobInfo e a)
> > > 15|
> > > 16|type JobArgs k a = (Ord k) =>
> > > 17| M.Map k a
> > > 21|
> > > 22|start :: (Ord k, Exception e) => JobArgs k a -> (a -> IO b) -> IO
> > > (Jobs k e a)
> > > 23|start args worker = do
> > > 24| arg <- newEmptyMVar
> > > 25| Map.mapM (\a -> do
> > > 26| putMVar arg a
> > > 27| result <- newEmptyMVar
> > > 28| tId <- forkIO $ do
> > > 29| arg_ <- takeMVar arg
> > > 30| result_ <- try $ worker arg_
> > > 31| putMVar result result_
> > > 32| return $ JobInfo tId result
> > > 33| ) args
> > >
> > > On Thu, Jun 14, 2012 at 1:24 PM, Magicloud Magiclouds
> > > mailto:magicloud.magiclo...@gmail.com)>
> > > wrote:
> > > > I think I need to think this through
> > > >
> > > > On Thu, Jun 14, 2012 at 12:28 PM, Ivan Lazar Miljenovic
> > > > mailto:ivan.miljeno...@gmail.com)> wrote:
> > > > > On 14 June 2012 14:20, Magicloud Magiclouds
> > > > > > > > > (mailto: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 > > > > > (mailto: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
> > > > > > > > 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
> > > > > > > > Magic
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
And line 14, should be JobInfo a e.
I must be too sleepy
On Thu, Jun 14, 2012 at 3:30 PM, Magicloud Magiclouds
wrote:
> Sorry, the last 'a' of line 22 is 'b'.
>
> On Thu, Jun 14, 2012 at 3:19 PM, Magicloud Magiclouds
> wrote:
>> OK. I am totally confused here. Why "Couldn't match expected type
>> `Jobs k e a' with actual type `M.Map k0 b0'"
>>
>> 9|data JobInfo a e = (Exception e) =>
>> 10| JobInfo { jobId :: ThreadId
>> 11| , result :: MVar (Either e a) }
>> 12|
>> 13|type Jobs k e a = (Ord k, Exception e) =>
>> 14| M.Map k (JobInfo e a)
>> 15|
>> 16|type JobArgs k a = (Ord k) =>
>> 17| M.Map k a
>> 21|
>> 22|start :: (Ord k, Exception e) => JobArgs k a -> (a -> IO b) -> IO
>> (Jobs k e a)
>> 23|start args worker = do
>> 24| arg <- newEmptyMVar
>> 25| Map.mapM (\a -> do
>> 26| putMVar arg a
>> 27| result <- newEmptyMVar
>> 28| tId <- forkIO $ do
>> 29| arg_ <- takeMVar arg
>> 30| result_ <- try $ worker arg_
>> 31| putMVar result result_
>> 32| return $ JobInfo tId result
>> 33| ) args
>>
>> On Thu, Jun 14, 2012 at 1:24 PM, Magicloud Magiclouds
>> wrote:
>>> I think I need to think this through
>>>
>>> On Thu, Jun 14, 2012 at 12:28 PM, Ivan Lazar Miljenovic
>>> wrote:
On 14 June 2012 14:20, Magicloud Magiclouds
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 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 >> (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, Ju
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
Sorry, the last 'a' of line 22 is 'b'.
On Thu, Jun 14, 2012 at 3:19 PM, Magicloud Magiclouds
wrote:
> OK. I am totally confused here. Why "Couldn't match expected type
> `Jobs k e a' with actual type `M.Map k0 b0'"
>
> 9|data JobInfo a e = (Exception e) =>
> 10| JobInfo { jobId :: ThreadId
> 11| , result :: MVar (Either e a) }
> 12|
> 13|type Jobs k e a = (Ord k, Exception e) =>
> 14| M.Map k (JobInfo e a)
> 15|
> 16|type JobArgs k a = (Ord k) =>
> 17| M.Map k a
> 21|
> 22|start :: (Ord k, Exception e) => JobArgs k a -> (a -> IO b) -> IO
> (Jobs k e a)
> 23|start args worker = do
> 24| arg <- newEmptyMVar
> 25| Map.mapM (\a -> do
> 26| putMVar arg a
> 27| result <- newEmptyMVar
> 28| tId <- forkIO $ do
> 29| arg_ <- takeMVar arg
> 30| result_ <- try $ worker arg_
> 31| putMVar result result_
> 32| return $ JobInfo tId result
> 33| ) args
>
> On Thu, Jun 14, 2012 at 1:24 PM, Magicloud Magiclouds
> wrote:
>> I think I need to think this through
>>
>> On Thu, Jun 14, 2012 at 12:28 PM, Ivan Lazar Miljenovic
>> wrote:
>>> On 14 June 2012 14:20, Magicloud Magiclouds
>>> 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 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 > (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
>> Paletmailto: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
>>
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
OK. I am totally confused here. Why "Couldn't match expected type
`Jobs k e a' with actual type `M.Map k0 b0'"
9|data JobInfo a e = (Exception e) =>
10| JobInfo { jobId :: ThreadId
11| , result :: MVar (Either e a) }
12|
13|type Jobs k e a = (Ord k, Exception e) =>
14| M.Map k (JobInfo e a)
15|
16|type JobArgs k a = (Ord k) =>
17| M.Map k a
21|
22|start :: (Ord k, Exception e) => JobArgs k a -> (a -> IO b) -> IO
(Jobs k e a)
23|start args worker = do
24| arg <- newEmptyMVar
25| Map.mapM (\a -> do
26| putMVar arg a
27| result <- newEmptyMVar
28| tId <- forkIO $ do
29| arg_ <- takeMVar arg
30| result_ <- try $ worker arg_
31| putMVar result result_
32| return $ JobInfo tId result
33| ) args
On Thu, Jun 14, 2012 at 1:24 PM, Magicloud Magiclouds
wrote:
> I think I need to think this through
>
> On Thu, Jun 14, 2012 at 12:28 PM, Ivan Lazar Miljenovic
> wrote:
>> On 14 June 2012 14:20, Magicloud Magiclouds
>> 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 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 (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
> Paletmailto: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 stat
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
I think I need to think this through
On Thu, Jun 14, 2012 at 12:28 PM, Ivan Lazar Miljenovic
wrote:
> On 14 June 2012 14:20, Magicloud Magiclouds
> 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 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 >>> (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
Paletmailto: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 >>> (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
listhaskell-c...@haskell.org (mailto:Haskell-Cafe@haskell.org)
> > > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >
> >
> >
> >
> >
> >
> >
--Ismael
> >
> >
___Haskell-Cafe mailing
listhaskell-c...@haskell.org (mailto:Haskell-Cafe@haskell.org)
>>
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
On 14 June 2012 14:20, Magicloud Magiclouds
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 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 >> (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>> (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 >> (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
>>> listhaskell-c...@haskell.org (mailto:Haskell-Cafe@haskell.org)
>>> > > > http://www.haskell.org/mailman/listinfo/haskell-cafe
>>> > >
>>> > >
>>> > >
>>> > >
>>> > >
>>> > >
>>> > >
>>> > >
>>> --Ismael
>>> > >
>>> > >
>>> ___Haskell-Cafe mailing
>>> listhaskell-c...@haskell.org (mailto:Haskell-Cafe@haskell.org)
>>> > > http://www.haskell.org/mailman/listinfo/haskell-cafe
>>> >
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> --
>>> 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).
>>>
>>>
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
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 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 > (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> (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 > (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
>> listhaskell-c...@haskell.org (mailto:Haskell-Cafe@haskell.org)
>> > > > http://www.haskell.org/mailman/listinfo/haskell-cafe
>> > >
>> > >
>> > >
>> > >
>> > >
>> > >
>> > >
>> > >
>> --Ismael
>> > >
>> > >
>> ___Haskell-Cafe mailing
>> listhaskell-c...@haskell.org (mailto:Haskell-Cafe@haskell.org)
>> > > http://www.haskell.org/mailman/listinfo/haskell-cafe
>> >
>>
>>
>>
>>
>>
>>
>>
>> --
>> 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 list
>> Haskell-Cafe@haskell.org (mailto:Haskell-Cafe@haskell.org)
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
>
> ___
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
(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 (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 (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 (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
> listhaskell-c...@haskell.org (mailto:Haskell-Cafe@haskell.org)
> > > > http://www.haskell.org/mailman/listinfo/haskell-cafe
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> --Ismael
> > >
> > >
> ___Haskell-Cafe mailing
> listhaskell-c...@haskell.org (mailto:Haskell-Cafe@haskell.org)
> > > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
>
>
>
>
>
>
>
> --
> 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 list
> Haskell-Cafe@haskell.org (mailto:Haskell-Cafe@haskell.org)
> http://www.haskell.org/mailman/listinfo/haskell-cafe
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
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
>>>
>>> And for G+, please use magiclouds#gmail.com.
>>>
>>> ___
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe@haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>>
>>
>>
>> --
>> Ismael
>>
>>
>> ___
>> Haskell-Cafe mailing list
>> Haskell-Cafe@haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>
--
竹密岂妨流水过
山高哪阻野云飞
And for G+, please use magiclouds#gmail.com.
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
Thank you all. I just want to wrap some complex types.
So I learn from all info above, I still have to use forall explicitly
On Wed, Jun 13, 2012 at 9:19 PM, Yves Parès wrote:
> Mmmmh... no, to do that you need ImpredicativeTypes (which is I believe
> about to be deprecated).
> You have to declare Job a data, not a type, and use
> ExistentialQuantification.
>
>
> 2012/6/13 Ismael Figueroa Palet
>>
>> 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
>>>
>>> And for G+, please use magiclouds#gmail.com.
>>>
>>> ___
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe@haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>>
>>
>>
>> --
>> Ismael
>>
>>
>> ___
>> Haskell-Cafe mailing list
>> Haskell-Cafe@haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>
--
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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Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
Mmmmh... no, to do that you need ImpredicativeTypes (which is I believe
about to be deprecated).
You have to declare Job a data, not a type, and use
ExistentialQuantification.
2012/6/13 Ismael Figueroa Palet
> 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
>>
>> And for G+, please use magiclouds#gmail.com.
>>
>> ___
>> Haskell-Cafe mailing list
>> Haskell-Cafe@haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>
>
>
> --
> Ismael
>
>
> ___
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
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Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
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
>>
>> And for G+, please use magiclouds#gmail.com.
>>
>> ___
>> Haskell-Cafe mailing list
>> Haskell-Cafe@haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>
>
>
>
> --
> Ismael
>
>
> ___
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
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Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
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
>
> And for G+, please use magiclouds#gmail.com.
>
> ___
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
--
Ismael
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Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
On 13 June 2012 19:59, Magicloud Magiclouds
wrote:
> 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
Does that even make sense? What are the types of `k' and `a' in Job?
--
Ivan Lazar Miljenovic
ivan.miljeno...@gmail.com
http://IvanMiljenovic.wordpress.com
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[Haskell-cafe] What extension do I need to write "type Job = Map k a"?
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
And for G+, please use magiclouds#gmail.com.
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