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Today's Topics:
1. Re: Offside rule for function arguments? (John Smith)
2. Classes in Polymorphic Constructors (John Smith)
3. Re: Classes in Polymorphic Constructors (Ertugrul Soeylemez)
4. Re: Classes in Polymorphic Constructors (John Smith)
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Message: 1
Date: Mon, 23 Aug 2010 12:54:25 +0300
From: John Smith <[email protected]>
Subject: [Haskell-beginners] Re: Offside rule for function arguments?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Sorry, I missed the where.
On 23/08/2010 12:52, Jonas Almström Duregård wrote:
> > The indentation on the second line would generate a parse error, the same
> as it does now.
> What parser error is that? Both
>
> function 0 = 0 where
> fun 1 = 1
> function 2 = 2
>
> and
>
> function 0 = 0 where
> fun 1 = 1
> fun 2 = 2
>
> works for me.
>
> /J
>
> On 23 August 2010 11:46, John Smith <[email protected]
> <mailto:[email protected]>> wrote:
>
> The indentation on the second line would generate a parse error, the same
> as it does now.
>
>
> On 23/08/2010 12:32, Jonas Almström Duregård wrote:
>
> Maybe because of this:
>
> function 0 = 0 where
> fun 1 = 1
> 2 = 2
>
> The last declaration (2=2) can define either fun or function. I'm not
> saying this is a major problem, but there
> may be
> other problems like these.
>
> /J
>
> On 23 August 2010 11:15, Brent Yorgey <[email protected]
> <mailto:[email protected]>
> <mailto:[email protected] <mailto:[email protected]>>>
> wrote:
> > On Mon, Aug 23, 2010 at 09:33:13AM +0300, John Smith wrote:
> >> Why doesn't Haskell allow something like this?
> >>
> >> fac 0 = 0
> >> 1 = 1
> >> x = x * fac (x-1)
> >>
> >> This would be clearer than repeating the function name each time,
> >> and follow the same pattern as guards and case.
> >
> > Good question. I don't know of any particular reason.
> >
> > -Brent
------------------------------
Message: 2
Date: Mon, 23 Aug 2010 15:31:42 +0300
From: John Smith <[email protected]>
Subject: [Haskell-beginners] Classes in Polymorphic Constructors
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Why can polymorphic data constructors not be instantiated as a class? (not sure
if I used the right terminology there)
For example,
a = [1,True]::[Show]
b = "foo":a
c = map show b
This would obviate the need for wrapper types
------------------------------
Message: 3
Date: Mon, 23 Aug 2010 17:32:19 +0200
From: Ertugrul Soeylemez <[email protected]>
Subject: [Haskell-beginners] Re: Classes in Polymorphic Constructors
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII
John Smith <[email protected]> wrote:
> Why can polymorphic data constructors not be instantiated as a class?
> (not sure if I used the right terminology there)
>
> For example,
>
> a = [1,True]::[Show]
> b = "foo":a
> c = map show b
>
> This would obviate the need for wrapper types
Your type signature doesn't make sense. You are confusing type classes
with types. Show is a type class.
As far as I see, you will need a wrapper type:
data Object = forall a. Show a => Object { unObject :: a }
a = [Object 1, Object True]
b = Object "foo" : a
c = map (show . unObject) b
I have not tested this code, though. At least I can get the following
list to type-check (using RankNTypes and ImpredicativeTypes extensions):
[1,2,3] :: [forall a. Num a => a]
but I can't use it in any way. And somehow I also can't mix data types
here.
Greets,
Ertugrul
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://ertes.de/
------------------------------
Message: 4
Date: Mon, 23 Aug 2010 19:12:36 +0300
From: John Smith <[email protected]>
Subject: [Haskell-beginners] Re: Classes in Polymorphic Constructors
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
On 23/08/2010 18:32, Ertugrul Soeylemez wrote:
> John Smith<[email protected]> wrote:
>
>> Why can polymorphic data constructors not be instantiated as a class?
>> (not sure if I used the right terminology there)
>>
>> For example,
>>
>> a = [1,True]::[Show]
>> b = "foo":a
>> c = map show b
>>
>> This would obviate the need for wrapper types
>
> Your type signature doesn't make sense. You are confusing type classes
> with types. Show is a type class.
My question was why doesn't the type system allow classes to be used in this
way. It's the equivalent of a base class or
interface in OO.
------------------------------
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