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// run -gcflags=-G=3
// 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 main
import (
"fmt"
"strconv"
)
// Various implementations of fromStrings().
type Setter[B any] interface {
Set(string)
type *B
}
// Takes two type parameters where PT = *T
func fromStrings1[T any, PT Setter[T]](s []string) []T {
result := make([]T, len(s))
for i, v := range s {
// The type of &result[i] is *T which is in the type list
// of Setter, so we can convert it to PT.
p := PT(&result[i])
// PT has a Set method.
p.Set(v)
}
return result
}
func fromStrings1a[T any, PT Setter[T]](s []string) []PT {
result := make([]PT, len(s))
for i, v := range s {
// The type new(T) is *T which is in the type list
// of Setter, so we can convert it to PT.
result[i] = PT(new(T))
p := result[i]
// PT has a Set method.
p.Set(v)
}
return result
}
// Takes one type parameter and a set function
func fromStrings2[T any](s []string, set func(*T, string)) []T {
results := make([]T, len(s))
for i, v := range s {
set(&results[i], v)
}
return results
}
type Setter2 interface {
Set(string)
}
// Takes only one type parameter, but causes a panic (see below)
func fromStrings3[T Setter2](s []string) []T {
results := make([]T, len(s))
for i, v := range s {
// Panics if T is a pointer type because receiver is T(nil).
results[i].Set(v)
}
return results
}
// Two concrete types with the appropriate Set method.
type SettableInt int
func (p *SettableInt) Set(s string) {
i, err := strconv.Atoi(s)
if err != nil {
panic(err)
}
*p = SettableInt(i)
}
type SettableString struct {
s string
}
func (x *SettableString) Set(s string) {
x.s = s
}
func main() {
s := fromStrings1[SettableInt, *SettableInt]([]string{"1"})
if len(s) != 1 || s[0] != 1 {
panic(fmt.Sprintf("got %v, want %v", s, []int{1}))
}
s2 := fromStrings1a[SettableInt, *SettableInt]([]string{"1"})
if len(s2) != 1 || *s2[0] != 1 {
x := 1
panic(fmt.Sprintf("got %v, want %v", s2, []*int{&x}))
}
// Test out constraint type inference, which should determine that the second
// type param is *SettableString.
ps := fromStrings1[SettableString]([]string{"x", "y"})
if len(ps) != 2 || ps[0] != (SettableString{"x"}) || ps[1] != (SettableString{"y"}) {
panic(s)
}
s = fromStrings2([]string{"1"}, func(p *SettableInt, s string) { p.Set(s) })
if len(s) != 1 || s[0] != 1 {
panic(fmt.Sprintf("got %v, want %v", s, []int{1}))
}
defer func() {
if recover() == nil {
panic("did not panic as expected")
}
}()
// This should type check but should panic at run time,
// because it will make a slice of *SettableInt and then call
// Set on a nil value.
fromStrings3[*SettableInt]([]string{"1"})
}
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