proposal: Go 2: spec: extend comma-ok expressions to slices, array and string

[win-t]

according to https://go.dev/ref/spec#Index_expressions

An index expression on a map a of type map[K]V used in an assignment or initialization of the special form

v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]

yields an additional untyped boolean value. The value of ok is true if the key x is present in the map, and false otherwise.

I think it is nice to have this syntax for slice, array, or string.

we can return zero-value + false when the index is out of bound

wdyt?

EDIT:
the behavior should be as close to map type as possible, so, it will also work on nil slice

[seankhliao]

See also #6815 and #30209 for _, ok := for ints

with generics you could write an index function, perhaps with a user provided default value which may be more useful

Please fill out https://github.com/golang/proposal/blob/master/go2-language-changes.md when proposing language changes

[win-t]

Please fill out https://github.com/golang/proposal/blob/master/go2-language-changes.md when proposing language changes

sure, I will follow the proper proposal process later, just want to know people's reactions first 😄

[randall77]

For maps, x, ok := m[k] is necessary because there's no other way to know if k is in the map.
For slices, it is easy to compute: ok := i >= 0 & i < len(s). So while I'm sure it would be convenient at times to have a shorter form, it isn't necessary.

[ianlancetaylor]

Yes, the guidelines for comma-ok forms is what @randall77 describes: the comma-ok form only exists if it is difficult or impossible to determine the result in some other way.

[ianlancetaylor]

Based on the discussion above, this is a likely decline. Leaving open for four weeks for final comments.

[bcmills]

the comma-ok form only exists if it is difficult or impossible to determine the result in some other way.

Is it otherwise possible to write generic functions that abstract over maps, arrays, and slices? If so, then it would be unfortunate to have to pass in a func(c Container, k Key) (ok bool) just to smooth out the difference between the two key-checking idioms.

[ianlancetaylor]

Currently you can't write a generic function that works for maps, arrays, and slices. We have no plans to support that in the future. Even if we did, it's not clear that it would make sense to use a comma-ok form, even if we made it syntactically valid.

[ianlancetaylor]

No change in consensus.