proposal: reflect: add Value.SetMapIndexOf

[dsnet]

For most reflection operation, there is an allocation free way to walk a value tree. At present, there is no way to obtain the value of a map entry without allocating since reflect.Value.MapIndex allocates the underlying value on the heap (since map entries are unaddressable).

Assuming we can't do #57060, I propose an explicit API for setting an addressable value with the value of a map entry:

// SetMapIndexOf is equivalent to v.Set(m.MapIndex(k)).
func (v Value) SetMapIndexOf(m, k Value)

The method is so named because the SetMapIndex method is already taken up for the operation equivalent to v[key] = elem. Alternative suggest names welcome.

[bcmills]

At present, there is no way to obtain the value of a map entry without allocating

Does the same also hold for the (*reflect.MapIter).Value method?

[bcmills]

The method is so named because the SetMapIndex method is already taken up for the operation equivalent to v[key] = elem.

Are there any other existing reflect.Value methods that set a Value to some composition of operations applied to another Value? (I guess maybe SetIterValue?)

[dsnet]

Some of the notable operations that may allocate unnecessarily are:

• Append (this always allocates a slice header on the heap even if the slice was not grown)
• AppendSlice
• Value.MapKeys
• Value.MapIndex
• Value.Slice
• Value.Slice3

Most of these all have a modern workaround that are more performant:

• Append and AppendSlice can be replaced with a combination of the new Value.Grow method (reflect: add reflect.Value.Grow #48000) and the Value.SetCap method.
• Value.MapKeys can replaced with a combination of Value.MapRange (which has been made allocation free in CL/400675), and Value.SetIterKey and Value.SetIterMap (reflect: add MapIter.SetKey, SetValue, and friends #46131).
• Value.Slice and Value.Slice3 can be replaced with calls to Value.SetLen and Value.SetCap.

Value.MapIndex has no workaround to avoid the allocation.

[bcmills]

I'm not a big fan of the SetMapIndexOf name, since it seems like SetMapIndex (which already exists in the opposite direction) composed with some kind of Of operation. Perhaps SetFromMapIndex? (It's a shame that SetIterKey and SetIterValue didn't provide a better naming precedent. 😞)

[bcmills]

For alternative APIs, one could imagine:

• a method that returns a *MapIter for a specific key, for use with SetIterValue
• or, a new type (MapEntry?) that represents a lazily-evaluated map entry, along with a SetMapEntryValue(*MapEntry) to be used analogous to SetIterValue
• or, maybe a LazyMapIndex(Value) method that returns a Value representing a lazily-evaluated map entry?

But it's not clear to me that any of those are less warty than the proposed Set variant. 🤔