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Today's Topics:
1. Re: Type unions (Russ Abbott)
2. Re: Equivalence of Inheritance (Michael Katelman)
3. Re: Equivalence of Inheritance (Bastian Erdn??)
----------------------------------------------------------------------
Message: 1
Date: Tue, 14 Dec 2010 13:43:17 -0800
From: Russ Abbott <russ.abb...@gmail.com>
Subject: Re: [Haskell-beginners] Type unions
To: Hector Guilarte <hector...@gmail.com>
Cc: beginners <beginners@haskell.org>
Message-ID:
<aanlktin78=zsbc-zdgl6qm0lqrbczmo3f=3ztsyvy...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Thanks. I had assumed that in
data A = ...
data B = ...
data AorB = A | B
the A | B in the definition of AorB referred to the types A and B. But now I
gather that they are completely unrelated -- except for the fact that they
are spelled the same.
Thanks.
*
-- Russ *
*
*
On Tue, Dec 14, 2010 at 1:29 PM, Hector Guilarte <hector...@gmail.com>wrote:
> But did you already understand what's happening?
>
> > data AorB = A | B
>
> is pretty much like an enumeration the same as
>
> > data DayOfTheWeek = Monday | Tuesday | Wednesday | ... | Sunday
>
> it stores no values of any type, you could even have DayOfTheWeek as
> a constructor in the same declaration of the data DayOfTheWeek, because
> they are
> in different scopes:
>
> > data DayOfTheWeek = Monday | Tuesday | Wednesday | ... | Sunday
> | DayOfTheWeek
>
> and by no means, that A or B on the right hand side of your data AorB are
> referencing the
> data A nor the data B...
>
> In the end, you could have:
>
> data A = Aconstructor Int
> data B = Bconstructor Int
> data AorB = A A | B B
>
> f :: Int -> AorB
> f x
> | even x = A (Aconstructor x)
> | otherwise = B (Bconstructor x)
>
> I just added a Value to each constructor of the data AorB, which happens
> to be
> named the same as the value they store...
>
> What I'm about to do is something I haven't tried, but I don't see why it
> shouldn't
> compile:
>
> > data Try = Int Int
>
> in data Try you have a constructor named Int, and it has a value of type
> Int,
>
> I'm trying to explain it the best I can, but I don't know if I managed to
> do it
> clearly, please let me know if there's something where I wasn't clear
> enough.
>
> Hector
>
>
> On Tue, Dec 14, 2010 at 4:40 PM, Russ Abbott <russ.abb...@gmail.com>wrote:
>
>> My typo started this most recent confusion. When I wrote
>>
>> data AorB = A | B
>>
>> compiles with error.
>> That raises the question of what it really means!
>>
>>
>> I meant to say
>>
>> data AorB = A | B
>>
>> compiles *without *error.
>> That raises the question of what it really means!
>>
>> *
>> -- Russ *
>>
>>
>> On Tue, Dec 14, 2010 at 1:04 PM, Tobias Brandt <tob.bra...@googlemail.com
>> > wrote:
>>
>>> On 14 December 2010 22:02, Hector Guilarte <hector...@gmail.com> wrote:
>>> > Tobias you replied at the same time I was answering, you did explained
>>> what
>>> > is happening,
>>> > however, you said something which isn't right. As I mentioned before,
>>> "Type
>>> > Constructors" and
>>> > "Value Constructors" are in different scopes, and so, their names can
>>> be
>>> > used again...
>>> > if you don't believe me, give it a try with this example.
>>>
>>> I believe you. As I said, I was talking crap :-)
>>>
>>
>>
>
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Message: 2
Date: Tue, 14 Dec 2010 15:45:46 -0600
From: Michael Katelman <katel...@uiuc.edu>
Subject: Re: [Haskell-beginners] Equivalence of Inheritance
To: russ.abb...@gmail.com
Cc: beginners <beginners@haskell.org>
Message-ID:
<aanlktik5uu1mnr2husnawunknafma4mogxcatpwr3...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Without going too exotic, I see two choices. Keeping the single type
Person makes what you are asking for, as far as I know, impossible
(dependent types). For human readability you could consider
type MensGroup = [Person]
type WomensGroup = [Person]
If you split Person into two types, Man and Woman, there are
repercussions for the aggregate group type
type Group = [Either Man Woman]
-Mike
On Tue, Dec 14, 2010 at 3:30 PM, Russ Abbott <russ.abb...@gmail.com> wrote:
> Now that this is straightened out, I went back to what I was doing in the
> first place and?realized?that I haven't solved my problem.
> Given
>
> data Person =
> ?? ? ?Man {name :: String, age :: Int, prostateCondition :: Condition}
> ??| Woman {name :: String, age :: Int, ovaryCondition ? ?:: Condition}
>
> I'd like to define something like this.
>
> type MensGroup = [Man]
>
> Is there a way to do something like that?
> -- Russ
>
>
> On Tue, Dec 14, 2010 at 1:06 PM, Russ Abbott <russ.abb...@gmail.com> wrote:
>>
>> That's good. (It's more or less the way I was doing it.) ?What I wanted to
>> avoid was this.
>>
>> getGenderSpecificCondition ( ?Man _ _ cond) = cond
>> getGenderSpecificCondition (Woman _ _ cond) = cond
>>
>> I know it seems like a small thing, but I would like to be able to write
>> it like this.
>>
>> getGenderSpecificCondition p
>> ?? | p == (Man _ _ cond) = cond
>> ?? | p == (Woman _ _ cond) = cond
>>
>> But that's not legal syntax. ?A pattern can't appear in that context. But
>> this does the job.
>>
>> getGenderSpecificCondition :: Person -> Condition
>> getGenderSpecificCondition p
>> ?? | isMan p = prostateCondition p
>> ?? | isWoman p = ovaryCondition p
>>
>> isMan ( ? ? Man _ _ cond) = True
>> isMan _ = False
>> isWoman (Woman _ _ cond) = True
>> isWoman _ = False
>>
>> That works! prostateCondition and ovaryCondition are both defined on
>> Person. (I'm surprised to see that.)
>>
>> *Person> Group [Man "Harry" 32 OK, Woman "Sally" 29 Good]
>> Harry(32, OK)
>> Sally(29, Good)
>>
>> Also
>>
>> *Person> prostateCondition (Woman "Sally" 29 Good)
>> *** Exception: No match in record selector prostateCondition
>> *Person> prostateCondition (Man "Harry" 29 Good)
>> Good
>>
>> -- Russ
>>
>>
>> On Tue, Dec 14, 2010 at 12:31 PM, Michael Katelman <katel...@uiuc.edu>
>> wrote:
>>>
>>> Perhaps this?
>>>
>>> https://gist.github.com/741048
>>>
>>> -Mike
>>>
>>> On Tue, Dec 14, 2010 at 2:27 PM, Russ Abbott <russ.abb...@gmail.com>
>>> wrote:
>>> > What I'm after is a version of my example that compiles. ?Can you make
>>> > one?
>>> >
>>> > -- Russ
>>> >
>>> >
>>> > On Tue, Dec 14, 2010 at 12:18 PM, Antoine Latter <aslat...@gmail.com>
>>> > wrote:
>>> >>
>>> >> Sorry, I really don't know enough about what you're after to attempt
>>> >> that.
>>> >>
>>> >> But you'll need to change you're signatures of the form:
>>> >>
>>> >> > function :: Person -> Foo
>>> >>
>>> >> to something of the form:
>>> >>
>>> >> > function :: Person p => p -> Foo
>>> >>
>>> >> Because again, a type class can not be used as a type.
>>> >>
>>> >> Antoine
>>> >>
>>> >> On Tue, Dec 14, 2010 at 2:12 PM, Russ Abbott <russ.abb...@gmail.com>
>>> >> wrote:
>>> >> > What got fouled up is all the adjustments I had to make to the other
>>> >> > declarations.
>>> >> > Can you?complete?the example so that it compiles using
>>> >> >
>>> >> > class Person p where ...
>>> >> >
>>> >> > I'd very much like to see an example that actually compiles.
>>> >> >
>>> >> > Thanks.
>>> >> > -- Russ
>>> >> >
>>> >> > On Tue, Dec 14, 2010 at 11:58 AM, Antoine Latter
>>> >> > <aslat...@gmail.com>
>>> >> > wrote:
>>> >> >>
>>> >> >> On Tue, Dec 14, 2010 at 1:52 PM, Russ Abbott
>>> >> >> <russ.abb...@gmail.com>
>>> >> >> wrote:
>>> >> >> > If gender is a field in a Person type, then a Person must have
>>> >> >> > both
>>> >> >> > an
>>> >> >> > ovaryCondition and a prostateCondition. ?That seems awkward.
>>> >> >> > Regarding
>>> >> >> > ?? ? class Person p where
>>> >> >> > I started down that path but got completely fouled up.
>>> >> >>
>>> >> >> How did this get fouled up? Every class declaration must take
>>> >> >> arguments - here, 'p' is the argument for the class.
>>> >> >>
>>> >> >> Thanks,
>>> >> >> Antoine
>>> >> >
>>> >> >
>>> >
>>> >
>>> > _______________________________________________
>>> > Beginners mailing list
>>> > Beginners@haskell.org
>>> > http://www.haskell.org/mailman/listinfo/beginners
>>> >
>>> >
>>
>
>
------------------------------
Message: 3
Date: Tue, 14 Dec 2010 22:46:30 +0100
From: Bastian Erdn?? <earth...@web.de>
Subject: Re: [Haskell-beginners] Equivalence of Inheritance
To: beginners@haskell.org
Message-ID: <23661ea0-aa0e-4d85-920c-93cea6a8b...@web.de>
Content-Type: text/plain; charset=us-ascii
On Dec 14, 2010, at 19:11, Russ Abbott wrote:
> I'm also confused about how to do the equivalence of inheritance in Haskell.
> Here is a complete example below. It doesn't compile. The error message
> is
>
> Class `Person' used as a type
>
>
> If I write "(Person p) =>" instead, I get other diagnostics.
>
> I would very much appreciate seeing how this should be done in Haskell.
What about
> ---------- Example (multiple files) ------------
> --- Group.hs ---
> module Group where
>
> import Person
>
> data Group = Group { members :: [Person] }
>
> instance Show Group where
> show group = unlines $ map show $ members group
>
> --- Person.hs ---
>
> module Person where
>
> data Person = Person {
> nameP :: p -> String ,
> ageP :: p -> Int ,
> getGenderSpecificCondition :: p -> Condition }
>
> instance Show Person where
> show p = nameP p ++ "(" ++ show (ageP p) ++ ", " ++
> show (getGenderSpecificCondition p) ++ ")"
>
> data Condition = Bad | OK | Good
class PersonClass p where
toPerson :: p -> Person
> --- Man.hs ---
> module Man where
>
> import Person
>
> data Man = Man { nameM :: String
> , ageM :: Int
> , prostateCondition :: Condition
> }
>
> instance PersonClass Man where
> toPerson (Man n a c) = Person n a c
>
> --- Woman.hs---
> module Woman where
>
> import Person
>
> data Woman = Woman { nameW :: String
> , ageW :: Int
> , ovaryCondition :: Condition
> }
>
> instance Person Woman where
> toPerson (Woman n a c) = Person n a c
>
> ---------- End example (multiple files) ------------
------------------------------
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