proposal: x/exp/constraints: move into math, sort

[earthboundkid]

This is follow up to #50792.

Most of the remaining constraints after #48424 are purely numeric: Complex, Float, Integer, Signed, Unsigned. They can all just be added to package math. Ordered is the exception because it also involves strings. It can be added to package sort. There's no longer any need for a top level constraints package.

[ianlancetaylor]

I'm not sure that math.Integer or sort.Ordered convey quite the right feel. They don't sound like constraints.

[earthboundkid]

I think sort.Orderable reads a little better than sort.Ordered.

I think the math ones read fine though: func Sum[N math.Integer|math.Float](ns ...N) N.

[zephyrtronium]

I'm not sure that math.Integer or sort.Ordered convey quite the right feel. They don't sound like constraints.

Conversely, as I mentioned in #45458 (comment), constraints might not always remain only constraints. Committing to names like constraints.Float instead of math.Float could make the future more awkward.

[changkun]

I think this is a good one to avoid the lengthy constraints to appear in the function signature.

I have been experimenting with roughly the same idea that uses self-defined math.Float to replace constraints.Float (also intended to avoid import x/exp/constraints just for a simple interface definition). My overall feeling on the usage of math.Float is positive. See how the usage is distributed in a math package.

The key questions: do we really need a centralized place to define "all" constraints? or how many constraints are expected to appear but not yet in x/exp/constraints?

[changkun]

One addition but less problematic: the dependencies of math and sort won't be able to use these constraints (i.e. math/bits and runtime), but they can also define these constraints internally to avoid circular import easily.

[mpx]

Perhaps it's a coincidence that the current constaints can be considered for the math package? Other future constaints might not have a convient pre-existing package to slot them into. There might be a need for the remainder to fit into a separate "constraints" package again?

This is hypothetical, but it's hard to predict where type parameters and constraints will evolve over time - developers have only started exploring this space.

Is this mostly a workaround for constraints being too long? (I don't think that is a good reason). Or do people think merging into existing packages is more appropriate for a reason outside package name length?

[earthboundkid]

I think experience is showing that you don't really need that many constraints, except for Ordered or (and this isn't in the exp/constraints anyway) byteseq.

"Constraints" is like "utils" but for types. It's better to have something specific than a bunch of general stuff crammed in one place.

[beoran]

The main problem with a constraints package is that is is very likely to become a dependency of all generic packages, even though it doesn't have any functionality, only types. But "a little copying is better than a little dependency", so it is probably better for each generic package to define its own constraints.

[earthboundkid]

Ordered is the only constraint that’s non-trivial to define for yourself. It would be convenient to have it in a canonical place, like sort, if only to copy it elsewhere.

[ianlancetaylor]

@beoran It would be interesting to see how many packages that use generics do in fact import constraints. I would be surprised if most do. I checked some packages in Google internal code that define generics, and none of them import constraints.

[kylelemons]

My team just went down the rabbit hole of history today trying to track down the new home of x/exp/slices.SortFunc (after discovering a major performance win from moving from sort.Slice to sort.Sort and wondering if a generic version of sort.Slice would've been faster), and if I'm reading the closed proposals correctly it seems like this issue is the current "blocker" for that functionality going into the standard library -- we need constraints.Ordered or sort.Ordered or something equivalent before we can get slices.Sort. Is that correct? If so, can we get an update on the status of this proposal?

[zephyrtronium]

@kylelemons #60091

(Interesting that there wasn't already a link between these issues.)

[jimmyfrasche]

As long as we're cross-indexing:

#59488 is for cmp.Ordered instead of sort.Ordered and that's likely to succeed.

Much less officially, in #60274 (comment) there's an alternative to putting the numeric constraints in math.

[earthboundkid]

Closing this issue because #59488 was accepted and implemented. Math constraints are part of #60274 or they can have a separate proposal.