On Mar 17, 2015, at 6:40 PM, Tom Ellis wrote:

On Tue, Mar 17, 2015 at 05:47:11PM -0400, Richard Eisenberg wrote:

...

PS: Of course, this instance is directly hurting my use of the package. [...]

Missing a "not"?

Indeed! And what a terrible word to miss. Thanks for the correction. Richard

On Tue, Mar 17, 2015 at 10:41 PM, Brent Yorgey wrote:

I have EXACTLY the same question. We recently switched from vector-space to linear for diagrams. There are quite a lot of reasons why this works really well for us. But I was very sad to find that we can now add points, which indeed we do not want to be able to do. At least there are still different types for points and vectors, which allows transformations like 'translate' to act on them differently, which I actually find to be much more critical from a correctness point of view.

I have had a brief explanation, but I forget the exact details. Something about 'Additive' not really being about 'additive groups' but instead being a superclass of Applicative. I remember being convinced at least that it's a "pick your poison" sort of choice, i.e. removing the Additive instance for Point would make certain other things ugly/annoying. Hopefully someone else can chime in with more detail.

The name Additive is a bit of a misnomer, like you said. The real reason it exists is to support matrix operations on both sparse/dense matrices. Matrix multiplication looks like: (!*!) :: (Functor m, Foldable t, Additive t, Additive n, Num a) => m (t a) -> t (n a) -> m (n a) Without that instance you'd be unable to build matrices with Point's as one or both of the dimensions, which are intended to transform points. If we can decide that that doesn't make sense (as after all points aren't additive), so such matrices _are_ kinda weird, we can remove the instance. I'm happy to bend the way Point works, as I stick to the projective machinery almost exclusively, myself. Thoughts? -Edward On Tue, Mar 17, 2015 at 5:47 PM Richard Eisenberg wrote:

Hi café,

...

I'm in the middle of responding to https://github.com/goldfirere/ units/pull/45 and trying to learn the `linear` package, which I have yet to use.

I have what may be a basic question: why is there an `instance Additive f => Additive (Point f)` (in the Affine module)? It would seem that the whole point of Point is that it is *not* Additive. We don't want to add Points!

Could someone enlighten me?

Thanks! Richard

PS: Of course, this instance is directly hurting my use of the package. But it is hurting my understanding of the package, because it disagrees with the mental model I've built up of the definitions. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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Then, tentatively, I'll file an issue and make the change for next time we push out linear. -Edward On Mon, Mar 30, 2015 at 12:16 PM, Brent Yorgey wrote:

Sorry for the delay in responding. I am not sure what it would mean to multiply matrices where one or both dimensions are given by Points. Indeed, such things seem very strange, precisely because Points are not additive. I am certainly in favor of removing the instance, and I doubt anyone will miss being able to make Point-structured matrices.

-Brent

On Wed, Mar 18, 2015 at 1:23 PM Edward Kmett wrote:

...

On Tue, Mar 17, 2015 at 10:41 PM, Brent Yorgey wrote:

...

I have EXACTLY the same question. We recently switched from vector-space to linear for diagrams. There are quite a lot of reasons why this works really well for us. But I was very sad to find that we can now add points, which indeed we do not want to be able to do. At least there are still different types for points and vectors, which allows transformations like 'translate' to act on them differently, which I actually find to be much more critical from a correctness point of view.

I have had a brief explanation, but I forget the exact details. Something about 'Additive' not really being about 'additive groups' but instead being a superclass of Applicative. I remember being convinced at least that it's a "pick your poison" sort of choice, i.e. removing the Additive instance for Point would make certain other things ugly/annoying. Hopefully someone else can chime in with more detail.

The name Additive is a bit of a misnomer, like you said. The real reason it exists is to support matrix operations on both sparse/dense matrices.

Matrix multiplication looks like:

(!*!) :: (Functor m, Foldable t, Additive t, Additive n, Num a) => m (t a) -> t (n a) -> m (n a)

Without that instance you'd be unable to build matrices with Point's as one or both of the dimensions, which are intended to transform points.

If we can decide that that doesn't make sense (as after all points aren't additive), so such matrices _are_ kinda weird, we can remove the instance.

I'm happy to bend the way Point works, as I stick to the projective machinery almost exclusively, myself.

Thoughts?

-Edward

On Tue, Mar 17, 2015 at 5:47 PM Richard Eisenberg wrote:

...

Hi café,

...

I'm in the middle of responding to https://github.com/goldfirere/ units/pull/45 and trying to learn the `linear` package, which I have yet to use.

I have what may be a basic question: why is there an `instance Additive f => Additive (Point f)` (in the Affine module)? It would seem that the whole point of Point is that it is *not* Additive. We don't want to add Points!

Could someone enlighten me?

Thanks! Richard

PS: Of course, this instance is directly hurting my use of the package. But it is hurting my understanding of the package, because it disagrees with the mental model I've built up of the definitions. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe