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proposal: Go 2: obtain length of array from its type #43483

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AndrewWPhillips opened this issue Jan 4, 2021 · 3 comments
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proposal: Go 2: obtain length of array from its type #43483

AndrewWPhillips opened this issue Jan 4, 2021 · 3 comments
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FrozenDueToAge LanguageChange Proposal v2 A language change or incompatible library change
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@AndrewWPhillips
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The len() function built into Go is evaluated at compile-time when it is given an array. The only way I have found to obtain the length of a type parameter (using the go2go tool) is to create a dummy variable of the type before I call len on it.

func f[T interface{ type [2]int }]() {
	var dummy T
	const c = len(dummy)
}

go2go playground

But sometimes you can't declare a dummy variable. In this case it would be useful for the built-in len function to alternatively take a type parameter so I could do the following:

func f[T interface{ type [2]int }]() {
	const c = len(T)  // WARNING: does not currently compile
}

Perhaps a better approach would be to add a variation of len that takes a type parameter using generics syntax.

func f[T interface{ type [2]int }]() {
	const c = len[T]()  // WARNING: does not currently compile
}

Of course, if this was done then it would also make sense to treat the other built-in functions that take a type parameter (new and make) in the same way.

	i := new[int]() // and deprecate i := new(int)
	make[[]int](2)  // and deprecate make([]int, 2)
@AndrewWPhillips AndrewWPhillips changed the title https://go2goplay.golang.org/p/jWBVa0gbIre proposal: Go 2: obtain length of array from its type Jan 4, 2021
@gopherbot gopherbot added this to the Proposal milestone Jan 4, 2021
@tmthrgd
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tmthrgd commented Jan 4, 2021

You can use T{} and thus len(T{}) to avoid having to declare a variable. Take a look: https://go2goplay.golang.org/p/kr8xdGz0Ecn. It seems like that should cover any use case.

@ianlancetaylor ianlancetaylor added v2 A language change or incompatible library change LanguageChange labels Jan 4, 2021
@griesemer
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There are two issues here: One is to obtain the length of an array from its array type, and one is to obtain the length of an array that is the single type in a type list of a constraint interface.

Regarding the former; what @tmthrgd said: Given an array type A, since it's an array, it is always valid to create an array literal A{} and then take the length of that len(A{}). Note that the compiler doesn't generate any code for that (per the spec: "The expressions len(s) and cap(s) are constants if the type of s is an array or pointer to an array and the expression s does not contain channel receives or (non-constant) function calls; in this case s is not evaluated.").

Regarding the latter: Given our current interpretation of the generics draft design, A{} happens to be valid for a type parameter A if for all types in A's constraint type the expression A{} is valid and produces the same type. That is, such type lists can only contain a single type. In that case, the len "trick" from above works, but it's definitively stretching the interpretation of type parameters a bit. We probably need to be more precise in an eventuell spec and decide explicitly if this should be valid.

Either way, given that there exist reasonable work-arounds, there's definitely not a strong case here for changing the language.

@griesemer griesemer mentioned this issue Jan 6, 2021
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@AndrewWPhillips
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Thanks @tmthrgd. I don't know why that did not occur to me.

@golang golang locked and limited conversation to collaborators Jan 6, 2022
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@tmthrgd @ianlancetaylor @griesemer @gopherbot @AndrewWPhillips