Related Issues and Documentation

• Package maps > func Clone ¶
• maps: make Clone more efficient by adding support in the runtime package #58740 (closed)

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What if it were

package maps

// Init returns a new empty map if m is nil, otherwise it returns m.
func Init[Map ~map[K]V, K comparable, V any](m Map) Map {
    if m == nil {
        return make(M)
    }
    return m
}

If you need a clone, do maps.Init(maps.Clone(m)). Another possible name would be Ensure.

It is fairly common if you write a type that has a private map to just do s.init() at the start of each method, where all init does is make sure m is not nil.

I did a quick search of just golang/go for if \w+ == nil \{\n\s+.+ = make\( and got 91 hits. Some of them are for slices rather than maps, which suggests this could have a package slices equivalent as well. Seems like a pretty frequent thing.

I did a quick search of just golang/go for if \w+ == nil \{\n\s+.+ = make\( and got 91 hits. Some of them are for slices rather than maps, which suggests this could have a package slices equivalent as well. Seems like a pretty frequent thing.

Thanks for the search. I think slices are different, usually it needs length or capacity to make in this situation.

For slices, you can do slices.Grow for non-zero length already. Grow doesn't ensure 0 length slices are non-nil, however.

Seems like a pretty frequent thing.

Yeah, I just got bit by this last week, but I'm not sure a CloneOrMake function is a solution: if you know about the hazard, you know to call:

m2 := make(M2)
maps.Copy(m2, m1)

and if you don't know about the hazard, you're going to call Clone anyway.

76e44f4 and bc7c35a are clean-ups by using maps.Clone.
I'd say it's not easy to do, I have to check the possibility of the new map being written and the old map being nil.

What if it were

package maps

// Init returns a new empty map if m is nil, otherwise it returns m.
func Init[Map ~map[K]V, K comparable, V any](m Map) Map {
    if m == nil {
        return make(M)
    }
    return m
}

If you need a clone, do maps.Init(maps.Clone(m)). Another possible name would be Ensure.

It is fairly common if you write a type that has a private map to just do s.init() at the start of each method, where all init does is make sure m is not nil.

I like this idea, closing this because it seems nobody wants a CloneOrMake.