When to Use ~[]E and []E Type Parameters for Slice Arguments in Go?

Differences Between Generic Slice Arguments

In Go's experimental package slices, two functions, Contains and Grow, have different type parameters for their first arguments. Contains takes a slice of comparable elements ([]E), while Grow takes an argument constrained to slice types (~[]E).

Practical Considerations

While these declarations may seem equivalent in practice, they differ when you need to return a slice of the same type as the input argument.

When to Use ~[]E Type Parameter

Use ~[]E when you have to return a slice of the same (possibly named) type as the argument. For example, if you want to create a generic function that grows a slice of a specific named type:

<code class="go">func Grow[S ~[]E, E any](s S, n int) S {
    // ...
}</code>

When to Use []E Type Parameter

Use []E when you don't need to return a slice or when returning an unnamed slice is acceptable:

<code class="go">func Contains[E comparable](s []E, v E) bool {
    // ...
}</code>

Demonstration

Consider two implementations of Grow():

<code class="go">// Using ~[]E
func Grow[S ~[]E, E any](s S, n int) S {
    // ...
}

// Using []E
func Grow2[E any](s []E, n int) []E {
    // ...
}</code>

When passing a custom slice type (having a slice as its underlying type), Grow() can return a value of the same type, while Grow2() cannot and returns an unnamed slice type.

Output:

x2 []int len=1 cap=12
x3 []int len=1 cap=12
y2 main.ints len=1 cap=12
y3 []int len=1 cap=12

As seen above, Grow2(y, 10) returns a slice of an unexpected type, which may not be desirable.

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