1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218
|
// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package typeparams
import (
"errors"
"fmt"
"go/types"
"os"
"strings"
)
//go:generate go run copytermlist.go
const debug = false
var ErrEmptyTypeSet = errors.New("empty type set")
// 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 StructuralTerms(tparam *types.TypeParam) ([]*types.Term, error) {
constraint := tparam.Constraint()
if constraint == nil {
return nil, fmt.Errorf("%s has nil constraint", tparam)
}
iface, _ := constraint.Underlying().(*types.Interface)
if iface == nil {
return nil, fmt.Errorf("constraint is %T, not *types.Interface", constraint.Underlying())
}
return InterfaceTermSet(iface)
}
// 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 InterfaceTermSet(iface *types.Interface) ([]*types.Term, error) {
return computeTermSet(iface)
}
// 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 UnionTermSet(union *types.Union) ([]*types.Term, error) {
return computeTermSet(union)
}
func computeTermSet(typ types.Type) ([]*types.Term, error) {
tset, err := computeTermSetInternal(typ, make(map[types.Type]*termSet), 0)
if err != nil {
return nil, err
}
if tset.terms.isEmpty() {
return nil, ErrEmptyTypeSet
}
if tset.terms.isAll() {
return nil, nil
}
var terms []*types.Term
for _, term := range tset.terms {
terms = append(terms, types.NewTerm(term.tilde, term.typ))
}
return terms, nil
}
// A termSet holds the normalized set of terms for a given type.
//
// The name termSet is intentionally distinct from 'type set': a type set is
// all types that implement a type (and includes method restrictions), whereas
// a term set just represents the structural restrictions on a type.
type termSet struct {
complete bool
terms termlist
}
func indentf(depth int, format string, args ...interface{}) {
fmt.Fprintf(os.Stderr, strings.Repeat(".", depth)+format+"\n", args...)
}
func computeTermSetInternal(t types.Type, seen map[types.Type]*termSet, depth int) (res *termSet, err error) {
if t == nil {
panic("nil type")
}
if debug {
indentf(depth, "%s", t.String())
defer func() {
if err != nil {
indentf(depth, "=> %s", err)
} else {
indentf(depth, "=> %s", res.terms.String())
}
}()
}
const maxTermCount = 100
if tset, ok := seen[t]; ok {
if !tset.complete {
return nil, fmt.Errorf("cycle detected in the declaration of %s", t)
}
return tset, nil
}
// Mark the current type as seen to avoid infinite recursion.
tset := new(termSet)
defer func() {
tset.complete = true
}()
seen[t] = tset
switch u := t.Underlying().(type) {
case *types.Interface:
// The term set of an interface is the intersection of the term sets of its
// embedded types.
tset.terms = allTermlist
for i := 0; i < u.NumEmbeddeds(); i++ {
embedded := u.EmbeddedType(i)
if _, ok := embedded.Underlying().(*types.TypeParam); ok {
return nil, fmt.Errorf("invalid embedded type %T", embedded)
}
tset2, err := computeTermSetInternal(embedded, seen, depth+1)
if err != nil {
return nil, err
}
tset.terms = tset.terms.intersect(tset2.terms)
}
case *types.Union:
// The term set of a union is the union of term sets of its terms.
tset.terms = nil
for i := 0; i < u.Len(); i++ {
t := u.Term(i)
var terms termlist
switch t.Type().Underlying().(type) {
case *types.Interface:
tset2, err := computeTermSetInternal(t.Type(), seen, depth+1)
if err != nil {
return nil, err
}
terms = tset2.terms
case *types.TypeParam, *types.Union:
// A stand-alone type parameter or union is not permitted as union
// term.
return nil, fmt.Errorf("invalid union term %T", t)
default:
if t.Type() == types.Typ[types.Invalid] {
continue
}
terms = termlist{{t.Tilde(), t.Type()}}
}
tset.terms = tset.terms.union(terms)
if len(tset.terms) > maxTermCount {
return nil, fmt.Errorf("exceeded max term count %d", maxTermCount)
}
}
case *types.TypeParam:
panic("unreachable")
default:
// For all other types, the term set is just a single non-tilde term
// holding the type itself.
if u != types.Typ[types.Invalid] {
tset.terms = termlist{{false, t}}
}
}
return tset, nil
}
// under is a facade for the go/types internal function of the same name. It is
// used by typeterm.go.
func under(t types.Type) types.Type {
return t.Underlying()
}
|