There is precedent in the arrow literature for calling this function "dup". For example, on page 55 of this paper: http://homepages.inf.ed.ac.uk/wadler/papers/arrows-jfp/arrows-jfp.pdf -- Dan Burton On Sun, Oct 28, 2018 at 12:48 AM David Feuer wrote:

I'm not opposed. Should this be called dup or diagonal? What about larger tuples?

Side note: \x -> (x,x) = join (,)

On Sat, Oct 27, 2018, 6:03 PM Ivan Perez wrote:

...

Dear all,

The function \x -> (x,x) is very convenient when working with arrows.

Would it be appropriate to add it to Data.Tuple?

Cheers,

Ivan

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On 28/10/18 03:51, Dannyu NDos wrote:

If this is for convenience for arrows, isn't it better implemented as?:

dup :: Arrow a => a b (b,b) dup = id &&& id

Hi Performance aside, I'd say your suggestion is better than using a pure function. It's clearer about what it does, and it works for functions as well (since they are arrows). Ivan

According to GHCi,

λ:> import Control.Arrow λ:> :t (id &&& id) (id &&& id) :: b -> (b, b)

That is, this implementation has type a -> (a, a) as well. Yes, yes, that's what I meant by "it works for functions as well (since

On 28/10/18 09:48, Vanessa McHale wrote: they are arrows)" Ivan

jam? Seriously? I'm sure we can do much better wrt names than that. Even dup I'd rather see expanded to "duplicate". The theory might have shown that these are useful combinators, but that doesn't mean they should decide the names. On Sun, Oct 28, 2018 at 10:10 PM Edward Kmett wrote:

If we're going to add this we should add the symmetric operation for

jam :: Either a a -> a

to Data.Either. (Name taken from Conal's compiling to categories code.)

My own code has called these diag and codiag respectively. I happily yield to a nicer convention.

I have no real preference for whether we do the simple version in Data.Tuple (which has a lot of precedent, as Data.Tuple tends to have lots of little simple combinators that could be done more generally as arrows) or moving it into Control.Arrow.

-Edward

On Sun, Oct 28, 2018 at 12:07 PM Ivan Perez wrote:

...

...

According to GHCi,

λ:> import Control.Arrow λ:> :t (id &&& id) (id &&& id) :: b -> (b, b)

That is, this implementation has type a -> (a, a) as well. Yes, yes, that's what I meant by "it works for functions as well (since

On 28/10/18 09:48, Vanessa McHale wrote: they are arrows)"

Ivan _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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dup is nice, but i dont really care what we call it as long as its not annoying or dumbe :) On Sun, Oct 28, 2018 at 6:22 PM Edward Kmett wrote:

I have no strong preference for the name for 'jam', merely the existence of the dual construction if we're going to name one.

However a needless name conflict with duplicate from Control.Comonad would be rather unfortunate.

dup as the name for this operation has a ton of precedent in languages like forth, and its length is comparable to the already smashed fst and snd.

-Edward

On Sun, Oct 28, 2018 at 6:15 PM Oliver Charles wrote:

...

jam? Seriously? I'm sure we can do much better wrt names than that. Even dup I'd rather see expanded to "duplicate". The theory might have shown that these are useful combinators, but that doesn't mean they should decide the names. On Sun, Oct 28, 2018 at 10:10 PM Edward Kmett wrote:

...

If we're going to add this we should add the symmetric operation for

jam :: Either a a -> a

to Data.Either. (Name taken from Conal's compiling to categories code.)

My own code has called these diag and codiag respectively. I happily

yield to a nicer convention.

...

I have no real preference for whether we do the simple version in

Data.Tuple (which has a lot of precedent, as Data.Tuple tends to have lots of little simple combinators that could be done more generally as arrows) or moving it into Control.Arrow.

...

-Edward

On Sun, Oct 28, 2018 at 12:07 PM Ivan Perez

wrote:

...

...

...

According to GHCi,

λ:> import Control.Arrow λ:> :t (id &&& id) (id &&& id) :: b -> (b, b)

That is, this implementation has type a -> (a, a) as well. Yes, yes, that's what I meant by "it works for functions as well (since

On 28/10/18 09:48, Vanessa McHale wrote: they are arrows)"

Ivan _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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Hi, Ivan Perez wrote:

The function \x -> (x,x) is very convenient when working with arrows. Would it be appropriate to add it to Data.Tuple?

David Feuer replied:

I'm not opposed. Should this be called dup or diagonal? What about larger tuples?

Dan Burton replied:

There is precedent in the arrow literature for calling this function "dup"

It would indeed be unfortunate if it was not called "dup". One could argue, as David (I'm guessing) does, that it's a bit of an odd definition to add to Data.Tuple as it only works for pairs (as implied by its name). And the reason it's convenient for Arrows is that (standard) Arrows are defined in a way that relies on pairs. So maybe it should somehow be defined in relation with Arrows. But then again, all other functions in Data.Tuple are at present specific to pairs ("fst", "snd", "swap", ...). So "dup" would certainly not look out of place there. Best, /Henrik This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.