When Should You Use S ~[]E vs. []E for Slice Argument Types in Go?

Understanding the Difference between Slice Argument Types

In Go, the slices package provides two distinct type parameters for slice argument types. The first type, []E, constrains the argument to be a slice of elements of type E, while the second type, S ~[]E, constrains the argument to have an underlying type of a slice of E.

Practical Implications

While both type parameters allow for similar operations within functions, a crucial difference arises when returning a slice.

If returning a slice of the same (possibly named) type as the argument, S ~[]E must be used. For instance, if a function receives a slice of a custom type and needs to return a slice of the same type, it must use S ~[]E. A type parameter that only constrains the element type, such as E, can't handle this.

If returning a slice without a specific type requirement (e.g., a bool value for containment checks), E can be used. This type parameter allows for greater flexibility in returning named or unnamed slice types.

Example: Grow Function

To demonstrate the practical difference, consider two implementations of a Grow function:

func Grow[S ~[]E, E any](s S, n int) S {
    // ...
}

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

Grow can return a value of the same type as the argument, while Grow2 can only return an unnamed slice type. If a custom slice type is passed, Grow can handle it, while Grow2 will return an unnamed slice.

Conclusion

Choosing the appropriate slice argument type depends on the specific requirements of the function. If returning a slice of the same (possibly named) type as the argument, S ~[]E should be used. Otherwise, E provides greater flexibility.

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