typeparams

func NormalTerms ¶

func NormalTerms(typ types.Type) ([]*Term, error)

NormalTerms returns a slice of terms representing the normalized structural type restrictions of a type, if any.

For all types whose underlying type is not *types.TypeParam, *types.Interface, or *types.Union, this is just a single term with Tilde() == false and Type() == typ. For types whose underlying type is *types.TypeParam, *types.Interface, and *types.Union, see below.

Structural type restrictions of a type parameter are created via non-interface types embedded in its constraint interface (directly, or via a chain of interface embeddings). For example, in the declaration type T[P interface{~int; m()}] int is the structural restriction of the type parameter P is ~int.

With interface embedding and unions, the specification of structural type restrictions may be arbitrarily complex. For example, consider the following:

type A interface{ ~string|~[]byte }

type B interface{ int|string }

type C interface { ~string|~int }

type T[P interface{ A|B; C }] int

In this example, the structural type restriction of P is ~string|int: A|B expands to ~string|~[]byte|int|string, which reduces to ~string|~[]byte|int, which when intersected with C (~string|~int) yields ~string|int.

NormalTerms computes these expansions and reductions, producing a "normalized" form of the embeddings. A structural restriction is normalized if it is a single union containing no interface terms, and is minimal in the sense that removing any term changes the set of types satisfying the constraint. It is left as a proof for the reader that, modulo sorting, there is exactly one such normalized form.

Because the minimal representation always takes this form, NormalTerms returns a slice of tilde terms corresponding to the terms of the union in the normalized structural restriction. An error is returned if the type is invalid, exceeds complexity bounds, or has an empty type set. In the latter case, NormalTerms returns ErrEmptyTypeSet.

NormalTerms makes no guarantees about the order of terms, except that it is deterministic.