assignable.md

Coherent interface constraints for assignability and convertibility?

This document is a sketch of two interface types that could be added to the draft Type Parameters design to enable conversion and assignment in generic code.

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The interface type convertible.To(T) is implemented by any dynamic type that can be converted to T, including T itself, any type assignable to T, and any type that has the same underlying type as T. A variable of type convertible.To(T) may be converted to T itself. If the variable is the nil interface value, the result of the conversion is the zero-value of T.

If T is itself an interface type, convertible.To(T) has the same method set as T.

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The interface type assignable.To(T) is implemented by any dynamic type that is assignable to T. A variable of the interface type assignable.To(T) is assignable to T and, if the underlying type of T is a not a defined type, also to that underlying type. If the variable is nil, the value assigned is the zero-value of T.

If T is itself an interface type, assignable.To(T) has the same method set as T.

(If T is not an interface type, a variable of type assignable.To(T) must not be assignable to any other defined type whose underlying type is T — even if T itself is not a defined type — because assignable.To(T) may store values of other defined types.)

For example, given:

type MyChan <-chan int
type OtherChan <-chan int

var (
    a chan int
    b <-chan int
    c assignable.To(MyChan)
    d MyChan
    e OtherChan
    f assignable.To(<-chan int)
    g chan<- int
)

• a is assignable to b, c, d, e,f, and g.
• b is assignable to c, d, e, and f.
• c is assignable to b, d, and f (but not e, because c could contain a variable of type MyChan — which is not assignable to OtherChan).
• d is assignable to b, c, and f.
• e is assignable to b and f.
• f is assignable to b (but not d, d, or e, because f could contain a variable of either type OtherChan or MyChan).
• g is assignable to nothing.