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// run
// 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"
// Overriding the predeclare "any", so it can be used as a type constraint or a type
// argument
type any interface{}
type Function[a, b any] interface {
Apply(x a) b
}
type incr struct{ n int }
func (this incr) Apply(x int) int {
return x + this.n
}
type pos struct{}
func (this pos) Apply(x int) bool {
return x > 0
}
type compose[a, b, c any] struct {
f Function[a, b]
g Function[b, c]
}
func (this compose[a, b, c]) Apply(x a) c {
return this.g.Apply(this.f.Apply(x))
}
type _Eq[a any] interface {
Equal(a) bool
}
type Int int
func (this Int) Equal(that int) bool {
return int(this) == that
}
type List[a any] interface {
Match(casenil Function[Nil[a], any], casecons Function[Cons[a], any]) any
}
type Nil[a any] struct {
}
func (xs Nil[a]) Match(casenil Function[Nil[a], any], casecons Function[Cons[a], any]) any {
return casenil.Apply(xs)
}
type Cons[a any] struct {
Head a
Tail List[a]
}
func (xs Cons[a]) Match(casenil Function[Nil[a], any], casecons Function[Cons[a], any]) any {
return casecons.Apply(xs)
}
type mapNil[a, b any] struct {
}
func (m mapNil[a, b]) Apply(_ Nil[a]) any {
return Nil[b]{}
}
type mapCons[a, b any] struct {
f Function[a, b]
}
func (m mapCons[a, b]) Apply(xs Cons[a]) any {
return Cons[b]{m.f.Apply(xs.Head), Map[a, b](m.f, xs.Tail)}
}
func Map[a, b any](f Function[a, b], xs List[a]) List[b] {
return xs.Match(mapNil[a, b]{}, mapCons[a, b]{f}).(List[b])
}
func main() {
var xs List[int] = Cons[int]{3, Cons[int]{6, Nil[int]{}}}
var ys List[int] = Map[int, int](incr{-5}, xs)
var xz List[bool] = Map[int, bool](pos{}, ys)
cs1 := xz.(Cons[bool])
cs2 := cs1.Tail.(Cons[bool])
_, ok := cs2.Tail.(Nil[bool])
if cs1.Head != false || cs2.Head != true || !ok {
panic(fmt.Sprintf("got %v, %v, %v, expected false, true, true",
cs1.Head, cs2.Head, ok))
}
}