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Today's Topics:
1. Re: Type unions (aditya siram)
2. Re: Type unions (aditya siram)
3. Re: Type unions (Tobias Brandt)
4. Re: Type unions (Russ Abbott)
5. Re: Type unions (Tobias Brandt)
6. Re: Type unions (Hector Guilarte)
----------------------------------------------------------------------
Message: 1
Date: Tue, 14 Dec 2010 14:29:51 -0600
From: aditya siram <aditya.si...@gmail.com>
Subject: Re: [Haskell-beginners] Type unions
To: russ.abb...@gmail.com
Cc: beginners <beginners@haskell.org>
Message-ID:
<aanlktinp8d=eetcdpvm2bmjn1fm2ueg2=xqxjw4mj...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Does this help?
data A = A Int
f :: Int -> Either A B
f x
| even x = Left $ A x |
| otherwise = Right $ B x |
-deech
On Tue, Dec 14, 2010 at 2:09 PM, Russ Abbott <russ.abb...@gmail.com> wrote:
> Is there a way to get this to work?
>
> data A = Aconstructor Int
> data B = Bconstructor Int
> data AorB = A | B
> f :: Int -> AorB
> f x
> ??| even x ? ? = Aconstructor x
> ??| otherwise = Bconstructor x
>
> ?I get this diagnostic.
>
> Couldn't match expected type `AorB' against inferred type `A'
>
> Since AorB is A or B, why is this not permitted?
> If instead I write
>
> data AorB = Aconstructor Int | Bconstructor Int
>
> everything works out ok. But what if I want separate types for A and B?
> Thanks,
> -- Russ
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>
------------------------------
Message: 2
Date: Tue, 14 Dec 2010 14:30:51 -0600
From: aditya siram <aditya.si...@gmail.com>
Subject: Re: [Haskell-beginners] Type unions
To: russ.abb...@gmail.com
Cc: beginners <beginners@haskell.org>
Message-ID:
<aanlkti=f2i6kfi+9z7dpyfzwjtmvx=1ycthtyh86u...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Sorry I had a typo. Here's working code:
data A = A Int
data B = B Int
f :: Int -> Either A B
f x
| even x = Left $ A x
| otherwise = Right $ B x
-deech
On Tue, Dec 14, 2010 at 2:29 PM, aditya siram <aditya.si...@gmail.com> wrote:
> Does this help?
>
>
> data A = A Int
>
> f :: Int -> Either A B
> f x
> ?| even x = Left $ A x ? ? |
> ?| otherwise = Right $ B x |
>
> -deech
>
> On Tue, Dec 14, 2010 at 2:09 PM, Russ Abbott <russ.abb...@gmail.com> wrote:
>> Is there a way to get this to work?
>>
>> data A = Aconstructor Int
>> data B = Bconstructor Int
>> data AorB = A | B
>> f :: Int -> AorB
>> f x
>> ??| even x ? ? = Aconstructor x
>> ??| otherwise = Bconstructor x
>>
>> ?I get this diagnostic.
>>
>> Couldn't match expected type `AorB' against inferred type `A'
>>
>> Since AorB is A or B, why is this not permitted?
>> If instead I write
>>
>> data AorB = Aconstructor Int | Bconstructor Int
>>
>> everything works out ok. But what if I want separate types for A and B?
>> Thanks,
>> -- Russ
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
------------------------------
Message: 3
Date: Tue, 14 Dec 2010 21:35:21 +0100
From: Tobias Brandt <tob.bra...@googlemail.com>
Subject: Re: [Haskell-beginners] Type unions
To: russ.abb...@gmail.com
Cc: beginners <beginners@haskell.org>
Message-ID:
<aanlktikuiyjseqx3hb+trnghm+yhs3fw22evmb=lx...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
On 14 December 2010 21:26, Russ Abbott <russ.abb...@gmail.com> wrote:
> Isn't "Either" the same thing as AorB in
>
> data AorB = Aconstructor Int | Bconstructor Int
>
> I want two separate types A and B along with a third type which is their
> Union. Is that not possible?
That's exactly what either is:
data A = ...
data B = ...
f :: Int -> Either A B
> In my actual case, I have more than two types. ?So I would like a way to
> take the union of an arbitrarily number of types.
>
> data Union = A1 | A2 | ...
>
> where each of A1, A2, ... has its own data declaration.
You can create a new data type:
data MyUnion = First A1 | Second A2 | Third A3
and use it like this:
f :: Int -> MyUnion
------------------------------
Message: 4
Date: Tue, 14 Dec 2010 12:44:32 -0800
From: Russ Abbott <russ.abb...@gmail.com>
Subject: Re: [Haskell-beginners] Type unions
To: Tobias Brandt <tob.bra...@googlemail.com>
Cc: beginners <beginners@haskell.org>
Message-ID:
<aanlktinqylsem8xkytu-mymxedxac_tewmpfrf3zr...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I guess the point is that you can't put type names into the declaration of
some other type.
data A = ...
data B = ...
-- No good
data AorB = A | B
f :: Int -> AorB
f x
| even x = Aconstructor x
| otherwise = Bconstructor x
-- OK
data AorB = AType A | BType B
f :: Int -> AorB
f x
| even x = AType $ Aconstructor x
| otherwise = BType $ Bconstructor x
What's confusing is that
data AorB = A | B
compiles with error.
That raises the question of what it really means!
*-- Russ*
***
*
**
On Tue, Dec 14, 2010 at 12:35 PM, Tobias Brandt
<tob.bra...@googlemail.com>wrote:
> On 14 December 2010 21:26, Russ Abbott <russ.abb...@gmail.com> wrote:
> > Isn't "Either" the same thing as AorB in
> >
> > data AorB = Aconstructor Int | Bconstructor Int
> >
> > I want two separate types A and B along with a third type which is their
> > Union. Is that not possible?
>
> That's exactly what either is:
>
> data A = ...
> data B = ...
>
> f :: Int -> Either A B
>
> > In my actual case, I have more than two types. So I would like a way to
> > take the union of an arbitrarily number of types.
> >
> > data Union = A1 | A2 | ...
> >
> > where each of A1, A2, ... has its own data declaration.
>
> You can create a new data type:
>
> data MyUnion = First A1 | Second A2 | Third A3
>
> and use it like this:
>
> f :: Int -> MyUnion
>
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Message: 5
Date: Tue, 14 Dec 2010 21:52:58 +0100
From: Tobias Brandt <tob.bra...@googlemail.com>
Subject: Re: [Haskell-beginners] Type unions
To: russ.abb...@gmail.com
Cc: beginners <beginners@haskell.org>
Message-ID:
<aanlktinp2y2myfeovoik2adno-su4n2xdwa+gfghf...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
On 14 December 2010 21:44, Russ Abbott <russ.abb...@gmail.com> wrote:
> What's confusing is that
>
>
> data AorB = A | B
>
> compiles with error.
> That raises the?question?of what it really means!
You have to distinguish between type and value constructors.
On the left hand side of a data declaration you have
a type constructor (AorB) and possibly some type variables.
On the right hand side you have value constructors followed
by their arguments (types or type variables). E.g.:
data TypeConstr a b c = ValueConstr1 a b | ValueConstr2 c | ValueConstr3 Int
But in your example A and B were already declared as type
constructors, so they can't be used again as value constructors.
That's why you get an error. If you remove
data A = ...
and
data B = ...
then
data AorB = A | B
compiles.
------------------------------
Message: 6
Date: Tue, 14 Dec 2010 16:26:49 -0430
From: Hector Guilarte <hector...@gmail.com>
Subject: Re: [Haskell-beginners] Type unions
To: russ.abb...@gmail.com
Cc: beginners <beginners@haskell.org>
Message-ID:
<aanlktimoq5rn1scuae5gqg421nctox56d5ndz58kb...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hello,
Nobody has explained you why that doesn't compile...
Here's the deal
Suppose you have a data A which has a constructor named B and a Int
> data A = B Int
now suppose you have a data C which has a constructor named A and a Int
> data C = A Int
that compiles because the name of your data type is different from
the constructor,
that is, the names of the data types and the constructors they have are in
different
scopes, so for doing what you want, you would need to do:
> data A = Aconstructor Int
> data B = Bconstructor Int
> data AorB = A A | B B
Where the first A is a constructor named A and the second references a data
type A,
idem for B
Hope that helps you,
H?ctor Guilarte
On Tue, Dec 14, 2010 at 3:39 PM, Russ Abbott <russ.abb...@gmail.com> wrote:
> Is there a way to get this to work?
>
> data A = Aconstructor Int
> data B = Bconstructor Int
> data AorB = A | B
>
> f :: Int -> AorB
> f x
> | even x = Aconstructor x
> | otherwise = Bconstructor x
>
> I get this diagnostic.
>
> Couldn't match expected type `AorB' against inferred type `A'
>
>
> Since AorB is A or B, why is this not permitted?
>
> If instead I write
>
> data AorB = Aconstructor Int | Bconstructor Int
>
>
> everything works out ok. But what if I want separate types for A and B?
>
> Thanks,
> *
> -- Russ *
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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