applying functions to sets

[Paul Zinn-Justin]

I'm not sure if this is a bug or a feature, but I find the following behavior odd.

if I try

apply(set {0,1,2},x->x+1)

then M2 complains

stdio:1:1:(3): error: expected argument 1 to be a list, sequence, or an integer

and rightly so.

but now if I try

(set {0,1,2})/(x->x+1)

then the answer is

set {1, 2, 3}

which is nice but unexpected. 

unfortunately things go wrong when we try

(set {0,1,2})/(x->x//2)

instead of producing set {0,1} as one might hope, the result is

stdio:2:14:(3): error: collision of keys in hash table

because 0//2 == 1//2.

any comments?

[Daniel R. Grayson]

The method for Tally/Function involves applying "applyKeys", which doesn't accommodate collisions of
keys, but the related functions "merge" and "combine" do.  Providing an optional third argument to applyKeys

to be called when collisions occur would allow desirable implementations of Tally/Function and Set/Function;

in the former case, the counts would be added, and in the latter case, the count would always be 1.  If that

feature is desired, make an issue for it on github.  (Does anyone want to implement it, and learn the "D" language?)

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[Paul Zinn-Justin]

Thanks for your answer. I might just do that. Is there any reference / documention for the "D" language?

[Daniel R. Grayson]

Here is the documentation on it: https://github.com/Macaulay2/M2/blob/master/M2/Macaulay2/c/README

[Paul Zinn-Justin]

all right, implemented as you suggested. it's already in my experimental branch (which contains many other features/improvements I'm not yet ready to share with the world), but I'll create an issue and move to my main branch as soon as we've settled the current pull request.