capacity for seq and string

[juancarlospaco]

Abstract

Allow read-only access to cap of seq and string.
The name is suggested to be capacity instead of cap.
It is a very simple and tiny RFC.

Motivation

It is a nice information to design macros and metaprogramming,
like unrollings, setLen, newStringOfCap, newSeqOfCap, etc.

len and setLen of seq and string is usable, but cap is not.

Examples

I imagine something kinda like

func capacity*(self: string): int

(Pseudocode-ish)

Change would require 3 ~ 5 lines at most (?).

Before

cap is not accessible, it is always unknown.

This can not be implemented with third-party Nimble modules, in a non-hacky way at least.

After

myString.capacity returns an int with the allocated capacity.

Backward incompatibility

No backward-incompatible changes, because it is a new proc.

[Araq]

Since this probably yet another system.nim extension and it is rather rare to come up, can we name it capacity and setCapacity?

[juancarlospaco]

Yes, makes sense, I do not have a suggestion about the naming.

[c-blake]

"space" and "room" might be short, whole word alternatives, but maybe they also get used in video games which, at one time, was a popular Nim arena. :-) (EDIT: but I don't think the name matters much. Transparency to better time/avoid allocations is a good thing.)

[Araq]

Neither "space" nor "room" is as spot on as "capacity" is though. But of course, it's subjective.

[c-blake]

(I was just brainstorming - it's already been "*OfCap" forever which would be the strongest consistency argument in favor of just getCap and setCap or .cap= rather than capacity...)

[Vindaar]

I'd stick to something related to "capacity". Whether it's the short form "cap" or the full word, I personally don't care too much though.

[arnetheduck]

related: nim-lang/Nim#19727

[planetis-m]

One thing to consider that came up with my work, is that when exposing internal state, like capacity to user code, is that when fuzzing code like that it would be impossible to correctly reproduce the conditions for a crash. That's because when copying or serializing the capacity is not saved.