[generics] constraint for nillable types?

[Bryan C. Mills]

I tried writing a generic “FirstNonNil” function, but got stuck at the "== nil" part.

First I thought to maybe add a type list enumerating all of the nillable types, but I don't have a way to enumerate all of the possible interface types as part of that list, and there is no constraint for “any interface type”.

Then I thought I could use a `var zero T` variable and compare `== zero` instead of `== nil`. But that doesn't work either: I need the `comparable` constraint to get `==`, but not every nillable type is comparable with an arbitrary variable (slices, maps, and functions are not).

So it seems that the best I can do is to write a generic `First` function and make the caller write out `func (x T) bool { return x != nil }` explicitly at every call site. Did I miss a better option?

[roger peppe]

FWIW I also came across this issue recently in a different context and similarly wondered if a nillable constraint might be a good idea.

There is a kinda workaround although you won't like it :)

https://go2goplay.golang.org/p/-nDAUVYWuxo

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[jimmy frasche]

One way is https://github.com/golang/go/issues/26842 which is to
provide some mechanism to compare any value with its type's zero value
(either with a universal zero value or something like isZero(v)). That
removes all the special cases so you could use it even without any
constraints

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