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Today's Topics:
1. class and instance question (Walck, Scott)
2. Re: class and instance question (Lyndon Maydwell)
3. Re: class and instance question (Brent Yorgey)
----------------------------------------------------------------------
Message: 1
Date: Thu, 20 May 2010 06:35:34 -0400
From: "Walck, Scott" <[email protected]>
Subject: [Haskell-beginners] class and instance question
To: "[email protected]" <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
Hi folks,
I'm trying to make doubles and triples act like vectors, as in
(3,4) <+> (7,8) ==> (10,12)
(3,2,1) <+> (9,8,7) ==> (12,10,8)
6 *> (1,2,3) ==> (6,12,18)
I thought I should make a type class so that I could use <+> for both double
addition and triple addition,
and *> for both double and triple scalar multiplication. (Some of this
functionality is provided by
NumericPrelude, but I didn't need all of that, and I hoped this would be simple
to write.)
The code below gives the error
NewVectorShort.hs:19:0:
Type synonym `Vector2D' should have 1 argument, but has been given 0
In the instance declaration for `BasicVector Vector2D'
Failed, modules loaded: none.
I don't understand how what I'm trying to do is different from, say, the Monad
instance for Maybe.
(Maybe a) is a type, and (Vector2D a) is a type.
Thanks,
Scott
{-# LANGUAGE TypeSynonymInstances #-}
infixl 6 <+>
infixl 6 <->
infixl 7 *>
infixl 7 <*
class BasicVector v where
(<+>) :: v a -> v a -> v a
(<->) :: v a -> v a -> v a
(*>) :: Num a => a -> v a -> v a
(<*) :: Num a => v a -> a -> v a
v1 <-> v2 = v1 <+> fromInteger (-1) *> v2
v1 <* c = c *> v1
c *> v1 = v1 <* c
type Vector2D a = (a,a)
instance BasicVector Vector2D where
(ax,ay) <+> (bx,by) = (ax+bx,ay+by)
c *> (ax,ay) = (c*ax,c*ay)
------------------------------
Message: 2
Date: Thu, 20 May 2010 18:49:22 +0800
From: Lyndon Maydwell <[email protected]>
Subject: Re: [Haskell-beginners] class and instance question
To: "Walck, Scott" <[email protected]>
Cc: "[email protected]" <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset=UTF-8
It looks like you've given the Vector2D type an argument, but not used
one for the BasicVector instance.
On Thu, May 20, 2010 at 6:35 PM, Walck, Scott <[email protected]> wrote:
> Hi folks,
>
> I'm trying to make doubles and triples act like vectors, as in
>
> (3,4) <+> (7,8) ==> (10,12)
> (3,2,1) <+> (9,8,7) ==> (12,10,8)
> 6 *> (1,2,3) ==> (6,12,18)
>
> I thought I should make a type class so that I could use <+> for both double
> addition and triple addition,
> and *> for both double and triple scalar multiplication. Â (Some of this
> functionality is provided by
> NumericPrelude, but I didn't need all of that, and I hoped this would be
> simple to write.)
> The code below gives the error
>
> NewVectorShort.hs:19:0:
> Â Â Type synonym `Vector2D' should have 1 argument, but has been given 0
> Â Â In the instance declaration for `BasicVector Vector2D'
> Failed, modules loaded: none.
>
> I don't understand how what I'm trying to do is different from, say, the
> Monad instance for Maybe.
> (Maybe a) is a type, and (Vector2D a) is a type.
>
> Thanks,
>
> Scott
>
>
>
> {-# LANGUAGE TypeSynonymInstances #-}
>
> infixl 6 <+>
> infixl 6 <->
> infixl 7 *>
> infixl 7 <*
>
> class BasicVector v where
> Â Â (<+>) :: v a -> v a -> v a
> Â Â (<->) :: v a -> v a -> v a
> Â Â (*>) Â :: Num a => a -> v a -> v a
> Â Â (<*) Â :: Num a => v a -> a -> v a
> Â Â v1 <-> v2 = v1 <+> fromInteger (-1) *> v2
> Â Â v1 <* c = c *> v1
> Â Â c *> v1 = v1 <* c
>
> type Vector2D a = (a,a)
>
> instance BasicVector Vector2D where
> Â Â (ax,ay) <+> (bx,by) = (ax+bx,ay+by)
> Â Â c *> (ax,ay) = (c*ax,c*ay)
>
>
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
------------------------------
Message: 3
Date: Thu, 20 May 2010 11:30:33 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] class and instance question
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Thu, May 20, 2010 at 06:35:34AM -0400, Walck, Scott wrote:
> Hi folks,
>
>
> NewVectorShort.hs:19:0:
> Type synonym `Vector2D' should have 1 argument, but has been given 0
> In the instance declaration for `BasicVector Vector2D'
> Failed, modules loaded: none.
The problem is simply that type synonyms must always be fully applied, so given
type Vector2D a = (a,a)
you cannot declare an instance for Vector2D, since Vector2D is not
applied to an argument. The solution is to make Vector2D a newtype:
newtype Vector2D a = V2D (a,a)
Of course, this means you'll need to wrap and unwrap V2D constructors
in various places, which can be a bit annoying, but such is the price
of progress.
For another take on encoding vector stuff in Haskell, see the
vector-space package on Hackage.
-Brent
>
> I don't understand how what I'm trying to do is different from, say, the
> Monad instance for Maybe.
> (Maybe a) is a type, and (Vector2D a) is a type.
>
> Thanks,
>
> Scott
>
>
>
> {-# LANGUAGE TypeSynonymInstances #-}
>
> infixl 6 <+>
> infixl 6 <->
> infixl 7 *>
> infixl 7 <*
>
> class BasicVector v where
> (<+>) :: v a -> v a -> v a
> (<->) :: v a -> v a -> v a
> (*>) :: Num a => a -> v a -> v a
> (<*) :: Num a => v a -> a -> v a
> v1 <-> v2 = v1 <+> fromInteger (-1) *> v2
> v1 <* c = c *> v1
> c *> v1 = v1 <* c
>
> type Vector2D a = (a,a)
>
> instance BasicVector Vector2D where
> (ax,ay) <+> (bx,by) = (ax+bx,ay+by)
> c *> (ax,ay) = (c*ax,c*ay)
>
>
>
>
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
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