typeparams

Documentation

Overview

Package typeparams contains common utilities for writing tools that interact with generic Go code, as introduced with Go 1.18.

Many of the types and functions in this package are proxies for the new APIs introduced in the standard library with Go 1.18. For example, the typeparams.Union type is an alias for go/types.Union, and the ForTypeSpec function returns the value of the go/ast.TypeSpec.TypeParams field. At Go versions older than 1.18 these helpers are implemented as stubs, allowing users of this package to write code that handles generic constructs inline, even if the Go version being used to compile does not support generics.

Additionally, this package contains common utilities for working with the new generic constructs, to supplement the standard library APIs. Notably, the StructuralTerms API computes a minimal representation of the structural restrictions on a type parameter.

An external version of these APIs is available in the golang.org/x/exp/typeparams module.

Index

• Variables
• func CoreType(T types.Type) types.Type
• func GenericAssignableTo(ctxt *types.Context, V, T types.Type) bool
• func InterfaceTermSet(iface *types.Interface) ([]*types.Term, error)
• func IsTypeParam(t types.Type) bool
• func OriginMethod(fn *types.Func) *types.Func
• func PackIndexExpr(x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) ast.Expr
• func StructuralTerms(tparam *types.TypeParam) ([]*types.Term, error)
• func UnionTermSet(union *types.Union) ([]*types.Term, error)
• func UnpackIndexExpr(n ast.Node) (x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos)

Variables

var ErrEmptyTypeSet = errors.New("empty type set")

Functions

func CoreType

func CoreType(T types.Type) types.Type

CoreType returns the core type of T or nil if T does not have a core type.

See https://go.dev/ref/spec#Core_types for the definition of a core type.

func GenericAssignableTo

func GenericAssignableTo(ctxt *types.Context, V, T types.Type) bool

GenericAssignableTo is a generalization of types.AssignableTo that implements the following rule for uninstantiated generic types:

If V and T are generic named types, then V is considered assignable to T if, for every possible instantation of V[A_1, ..., A_N], the instantiation T[A_1, ..., A_N] is valid and V[A_1, ..., A_N] implements T[A_1, ..., A_N].

If T has structural constraints, they must be satisfied by V.

For example, consider the following type declarations:

type Interface[T any] interface {
	Accept(T)
}

type Container[T any] struct {
	Element T
}

func (c Container[T]) Accept(t T) { c.Element = t }

In this case, GenericAssignableTo reports that instantiations of Container are assignable to the corresponding instantiation of Interface.

func InterfaceTermSet

func InterfaceTermSet(iface *types.Interface) ([]*types.Term, error)

InterfaceTermSet computes the normalized terms for a constraint interface, returning an error if the term set cannot be computed or is empty. In the latter case, the error will be ErrEmptyTypeSet.

See the documentation of StructuralTerms for more information on normalization.

func IsTypeParam

func IsTypeParam(t types.Type) bool

IsTypeParam reports whether t is a type parameter.

func OriginMethod

func OriginMethod(fn *types.Func) *types.Func

OriginMethod returns the origin method associated with the method fn. For methods on a non-generic receiver base type, this is just fn. However, for methods with a generic receiver, OriginMethod returns the corresponding method in the method set of the origin type.

As a special case, if fn is not a method (has no receiver), OriginMethod returns fn.

func PackIndexExpr

func PackIndexExpr(x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos) ast.Expr

PackIndexExpr returns an *ast.IndexExpr or *ast.IndexListExpr, depending on the cardinality of indices. Calling PackIndexExpr with len(indices) == 0 will panic.

func StructuralTerms

func StructuralTerms(tparam *types.TypeParam) ([]*types.Term, error)

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

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

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.

StructuralTerms 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, StructuralTerms returns a slice of tilde terms corresponding to the terms of the union in the normalized structural restriction. An error is returned if the constraint interface is invalid, exceeds complexity bounds, or has an empty type set. In the latter case, StructuralTerms returns ErrEmptyTypeSet.

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

func UnionTermSet

func UnionTermSet(union *types.Union) ([]*types.Term, error)

UnionTermSet computes the normalized terms for a union, returning an error if the term set cannot be computed or is empty. In the latter case, the error will be ErrEmptyTypeSet.

See the documentation of StructuralTerms for more information on normalization.

func UnpackIndexExpr

func UnpackIndexExpr(n ast.Node) (x ast.Expr, lbrack token.Pos, indices []ast.Expr, rbrack token.Pos)

UnpackIndexExpr extracts data from AST nodes that represent index expressions.

For an ast.IndexExpr, the resulting indices slice will contain exactly one index expression. For an ast.IndexListExpr (go1.18+), it may have a variable number of index expressions.

For nodes that don't represent index expressions, the first return value of UnpackIndexExpr will be nil.