proposal: cmp: Add Cond[T any](cond bool, ifval, elseval T) T

[earthboundkid]

Proposal Details

Recently, #64825 was marked as a likely declined proposal. This is an alternate proposal with a similar goal: to make writing simple, commonly used bool conversions easier. Here is the proposed addition:

func Cond[T any](cond bool, ifval, elseval T) T {
   if cond {
     return ifval
  }
  return elseval
}

I have been using a helper like this in my code since Go 1.18 and I find it convenient to have. The bool conversions mentioned in #64825 would be cmp.Cond(b, 1, 0) with this alternative proposal.

[seh]

This is analogous to a ternary conditional operator, but it evaluates its consequent and alternative values eagerly, which makes it less useful. However, I understand that we're not likely to see such an operator in Go any time soon.

[Merovius]

I'll note that it should be

func Cond[T any, B ~bool](cond B, ifval, elseval T) T

[x0wllaar]

I mean, you can make it lazy if you want: https://go.dev/play/p/D9BDFlGF-Uf

[earthboundkid]

I'll note that it should be

func Cond[T any, B ~bool](cond B, ifval, elseval T) T

I don't think it's worthwhile to make it generic over bool types. If it were builtin, sure.

Of course, if it were builtin, it should be lazy.

[Merovius]

I disagree. bool is a declared type and it would be pretty frustrating, having to convert it if you happen to use a user-declared boolean type (this is different for slices, for example, where type MySlice []T is assignable to []T). Not that declaring boolean types is incredibly common, but it's not unheard of. And the cost seems close to zero. For a standard library function, it seems like a no-brainer to me, that we'd want to make it as general as we can.

[Jorropo]

I mean, you can make it lazy if you want: https://go.dev/play/p/D9BDFlGF-Uf

That is impressive extends to go through to not write a simpler if check.

[zigo101]

#36303 proposal: new builtin to pick from one of two values

#37739 proposal: Go 2: lazy values

Zig's solution:

_ = if (cond) ifval else elseval;

[Jorropo]

Python's solution:

_ = ifval if cond else elseval

Skips () but is not intuitive (it's forced by parser limitations and falls out of the requirements but it's still weird).

[Jorropo]

Of course, if it were builtin, it should be lazy.

👎 this would be weird magic, currently builtins only cheat about the type system (generics before we had generics and ~string → ~[]byte signature overload) but not control flow. The value of adding control flow cheats so you can save a couple of ⏎ presses is low to me.

[earthboundkid]

I had not seen #36303 before. Looks like the builtin version of this has been rejected. I still think the cmp.Cond version is marginally convenient and fits with the general vibe of package cmp.