When Returning Slices from Functions, How Do Slice Constraints Affect Type Preservation?

Generic Slice Arguments vs. Slice Type Constraints

In Go's experimental slices package, two functions, Contains and Grow, take arguments with different type constraints. Contains takes the generic []E type, while Grow constrains its argument type to be a slice (~[]E).

Type Constraints

A type constraint, denoted by ~, ensures that a type conforms to a specific interface or set of rules. In Grow, the ~[]E constraint enforces that the argument type must ultimately be a slice with element type E.

Equivalent Functionality?

Despite the different type constraints, the operations available within both functions appear nearly identical. However, a key practical difference emerges when functions must return a slice of the same type as the input argument.

Returning Slices

When returning a slice of the same type as the input, it is crucial to use a type constraint like ~[]E. This allows the function to preserve the named type of the input slice.

Consider two versions of Grow():

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

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

If Grow() receives a custom slice type, it can return a slice of the same type, while Grow2() will always return an unnamed slice.

Demonstration

type MyInts []int
x := MyInts{1} // Custom slice type
x2 := Grow(x, 10) // Type preserved
x3 := Grow2(x, 10) // Unnamed type

In this example, x2 will be a slice of type MyInts, while x3 will be an unnamed slice.

Conclusion

When you need to preserve the named type of an input slice in the return value, it is necessary to use a type constraint like ~[]E. Otherwise, the returned slice will always be an unnamed type.

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