Types depending on tuples.

[Mike Meyer]

I'm doing some geometry in 2 and 3 dimensions. To do this, I've got Vector2d and Vector3d classes, which are basically just wrappers around 2 and 3-tuples of doubles:

type Vector2d = (Double, Double)

type Vector3d = (Double, Double, Double)

I've also got a typeclass Vector so I can do polymorphic things on them:

type Vector3d = (Double, Double, Double)

(Ok, polymorphic thing).

I now have a need to convert lists of Vector's to a list of lists of Doubles. I'd like that to be part of the Vector typeclass, but - well, declaring it is interesting.

I can't just add a method "toLists :: a -> [Double]", because (according to ghc), there's no match between the expected type 'Double' and the actual type '[t1]'. 

Can I declare this typeclass?

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[Mike Izbicki]

You never actually provided the Vector type class, so it's difficult
to tell what exactly you're trying to do.

On Tue, Aug 26, 2014 at 12:46 AM, Mike Meyer <[hidden email]> wrote:

> I'm doing some geometry in 2 and 3 dimensions. To do this, I've got Vector2d
> and Vector3d classes, which are basically just wrappers around 2 and
> 3-tuples of doubles:
>
> type Vector2d = (Double, Double)
> type Vector3d = (Double, Double, Double)
>
> I've also got a typeclass Vector so I can do polymorphic things on them:
>
> type Vector3d = (Double, Double, Double)
>
> (Ok, polymorphic thing).
>
> I now have a need to convert lists of Vector's to a list of lists of
> Doubles. I'd like that to be part of the Vector typeclass, but - well,
> declaring it is interesting.
>
> I can't just add a method "toLists :: a -> [Double]", because (according to
> ghc), there's no match between the expected type 'Double' and the actual
> type '[t1]'.
>
> Can I declare this typeclass?
>
> _______________________________________________
> Haskell-Cafe mailing list
> [hidden email]
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>

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[Mike Izbicki]

This works just fine for me:

class Vector a where
    toLists :: a -> [Double]

instance Vector (Double,Double,Double) where
    toLists (a,b,c)=[a,b,c]

On Tue, Aug 26, 2014 at 1:17 AM, Mike Meyer <[hidden email]> wrote:

> It's there, but in the text:
>
> class Vector a where
>    toLists :: a -> [Double]
>
>
> On Tue, Aug 26, 2014 at 2:59 AM, Mike Izbicki <[hidden email]> wrote:
>>
>> You never actually provided the Vector type class, so it's difficult
>> to tell what exactly you're trying to do.
>>
>> On Tue, Aug 26, 2014 at 12:46 AM, Mike Meyer <[hidden email]> wrote:
>> > I'm doing some geometry in 2 and 3 dimensions. To do this, I've got
>> > Vector2d
>> > and Vector3d classes, which are basically just wrappers around 2 and
>> > 3-tuples of doubles:
>> >
>> > type Vector2d = (Double, Double)
>> > type Vector3d = (Double, Double, Double)
>> >
>> > I've also got a typeclass Vector so I can do polymorphic things on them:
>> >
>> > type Vector3d = (Double, Double, Double)
>> >
>> > (Ok, polymorphic thing).
>> >
>> > I now have a need to convert lists of Vector's to a list of lists of
>> > Doubles. I'd like that to be part of the Vector typeclass, but - well,
>> > declaring it is interesting.
>> >
>> > I can't just add a method "toLists :: a -> [Double]", because (according
>> > to
>> > ghc), there's no match between the expected type 'Double' and the actual
>> > type '[t1]'.
>> >
>> > Can I declare this typeclass?
>> >
>> > _______________________________________________
>> > Haskell-Cafe mailing list
>> > [hidden email]
>> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>> >
>
>

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[Mike Meyer]

Weird. I thought that's what I tried. Must have done something else.

Thanks!

On Tue, Aug 26, 2014 at 3:31 AM, Mike Izbicki <[hidden email]> wrote:

This works just fine for me:

class Vector a where
    toLists :: a -> [Double]

instance Vector (Double,Double,Double) where
    toLists (a,b,c)=[a,b,c]

On Tue, Aug 26, 2014 at 1:17 AM, Mike Meyer <[hidden email]> wrote:
> It's there, but in the text:
>
> class Vector a where
>    toLists :: a -> [Double]
>
>
> On Tue, Aug 26, 2014 at 2:59 AM, Mike Izbicki <[hidden email]> wrote:
>>
>> You never actually provided the Vector type class, so it's difficult
>> to tell what exactly you're trying to do.
>>
>> On Tue, Aug 26, 2014 at 12:46 AM, Mike Meyer <[hidden email]> wrote:
>> > I'm doing some geometry in 2 and 3 dimensions. To do this, I've got
>> > Vector2d
>> > and Vector3d classes, which are basically just wrappers around 2 and
>> > 3-tuples of doubles:
>> >
>> > type Vector2d = (Double, Double)
>> > type Vector3d = (Double, Double, Double)
>> >
>> > I've also got a typeclass Vector so I can do polymorphic things on them:
>> >
>> > type Vector3d = (Double, Double, Double)
>> >
>> > (Ok, polymorphic thing).
>> >
>> > I now have a need to convert lists of Vector's to a list of lists of
>> > Doubles. I'd like that to be part of the Vector typeclass, but - well,
>> > declaring it is interesting.
>> >
>> > I can't just add a method "toLists :: a -> [Double]", because (according
>> > to
>> > ghc), there's no match between the expected type 'Double' and the actual
>> > type '[t1]'.
>> >
>> > Can I declare this typeclass?
>> >
>> > _______________________________________________
>> > Haskell-Cafe mailing list
>> > [hidden email]
>> > http://www.haskell.org/mailman/listinfo/haskell-cafe
>> >
>
>

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[Tom Ellis]

On Tue, Aug 26, 2014 at 03:54:41AM -0500, Mike Meyer wrote:
> Weird. I thought that's what I tried. Must have done something else.

You said "need to convert lists of Vector's to a list of lists of                                                                              
Doubles" ([[Double]]).  This is just list of Doubles ([Double]).
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[John Lato-2]

On Aug 26, 2014 6:16 AM, "Tom Ellis" <[hidden email]> wrote:
>
> On Tue, Aug 26, 2014 at 03:54:41AM -0500, Mike Meyer wrote:
> > Weird. I thought that's what I tried. Must have done something else.
>
> You said "need to convert lists of Vector's to a list of lists of
> Doubles" ([[Double]]).  This is just list of Doubles ([Double]).

This operates on a single Vector. You can just map this function over a list to do the requested conversion.

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[Tom Ellis]

On Tue, Aug 26, 2014 at 12:36:38PM -0700, John Lato wrote:

> On Aug 26, 2014 6:16 AM, "Tom Ellis" <
> [hidden email]> wrote:
> >
> > On Tue, Aug 26, 2014 at 03:54:41AM -0500, Mike Meyer wrote:
> > > Weird. I thought that's what I tried. Must have done something else.
> >
> > You said "need to convert lists of Vector's to a list of lists of
> > Doubles" ([[Double]]).  This is just list of Doubles ([Double]).
>
> This operates on a single Vector. You can just map this function over a
> list to do the requested conversion.

Sure, I'm just saying that given that the original error was apparently

    no match between the expected type 'Double' and the actual type '[t1]'

this may be the source of the problem.
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