golang中可比的接口是什么？

Question

I'm working on a simple linked list implementation in golang for learning purposes. The definition of an element is below:

type Element struct {
    next, prev *Element
    Value      interface{}
}

As you can see, the Value can be anything that satisfies the empty interface. Now, as a new feature, I would like to make it so that when you insert a new element into the list, it inserts it in a sorted manner - each element will be <= the next.

In order to do this, I wrote the following method:

func (l *LinkedList) Add(val interface{}) *Element {
    this := &l.Root
    e := Element{Value: val}
    for {
        if this.next.Value != nil && this.next.Value < val {  // <-comparison here
            this = this.next
        } else {
            return l.insert(&e, this)
        }
    }
}

The compiler complains operator < not defined on interface which is fair. So I understand that in my Element typedef, I should restrict Value to types that can be compared using the < operator. I learned this while researching this problem that golang does not support operator overloading - I am not trying to do that. Instead, I am just trying to make sure that Element.Value is a type that can be compared using the < operator. How do I do this?

Update:

It occurs to me that it might not be too difficult to simple define a new type, based on a built-in, that can be compared through some function. so I wrote this mess (as well as a bunch of other ways of trying to do the same thing):

type Comparable interface {
    LessThan(j interface{}) bool // tried (j Comparable), (j MyInt), etc
    EqualTo(j interface{}) bool  // tried (j Comparable), (j MyInt), etc
}

type MyInt int

func (i MyInt) LessThan(j MyInt) bool {
    return i < j
}

func (i MyInt) EqualTo(j MyInt) bool {
    return i == j
}

type Element struct {
    next, prev *Element
    Value      Comparable
}

What I would really like is to define an interface, which, if implemented for a type, provides functions LessThan and EqualTo that operate on two instances of that type and provides a bool - something like LessThan(i, j WhatEvers) bool which can be used in place of <. I realize below it's implemented as an instance method - I've tried both ways but no success. With the above, I would use it something like: this.next.Value.LessThan(val) in the Add function. I get either:

linkedlist.MyInt does not implement linkedlist.Comparable (wrong type for EqualTo method)
    have EqualTo(linkedlist.MyInt) bool
    want EqualTo(interface {}) bool

or

linkedlist.MyInt does not implement linkedlist.Comparable (wrong type for EqualTo method)
    have EqualTo(linkedlist.MyInt) bool
    want EqualTo(linkedlist.Comparable) bool

Is this possible to use an interface to require that a certain function must exist that operates on two instances of a custom type, or is it only for Methods?

Answer 1

Edit:
consider this user type:

type userType struct {
    frequency int
    value     rune
}

and suppose you want to add this types to your Linked List:
and It should be sorted by frequency first, then if the frequencies are the same, look at the char value. so the Compare function will be:

func (a userType) Compare(b userType) int {
    if a.frequency > b.frequency {
        return 1
    }
    if a.frequency < b.frequency {
        return -1
    }
    if a.value > b.value {
        return 1
    }
    if a.value < b.value {
        return -1
    }
    return 0
}

which satisfies this interface:

type Comparer interface {
    Compare(b userType) int
}

now add these {1,'d'} {2,'b'} {3,'c'} {4,'a'} {4,'b'} {4,'c'} types to LinkeList:
sample code:

package main

import (
    "container/list"
    "fmt"
)

type Comparer interface {
    Compare(b userType) int
}

type userType struct {
    frequency int
    value     rune
}

// it should sort by frequency first, then if the frequencies are the same, look at the char value.
func (a userType) Compare(b userType) int {
    if a.frequency > b.frequency {
        return 1
    }
    if a.frequency < b.frequency {
        return -1
    }
    if a.value > b.value {
        return 1
    }
    if a.value < b.value {
        return -1
    }
    return 0
}

func Insert(val userType, l *list.List) {
    e := l.Front()
    if e == nil {
        l.PushFront(val)
        return
    }
    for ; e != nil; e = e.Next() {
        var ut userType = e.Value.(userType)
        if val.Compare(ut) < 0 {
            l.InsertBefore(val, e)
            return
        }
    }
    l.PushBack(val)
}

func main() {
    l := list.New()
    Insert(userType{4, 'c'}, l)
    Insert(userType{4, 'a'}, l)
    Insert(userType{4, 'b'}, l)
    Insert(userType{2, 'b'}, l)
    Insert(userType{3, 'c'}, l)
    Insert(userType{1, 'd'}, l)
    for e := l.Front(); e != nil; e = e.Next() {
        ut := e.Value.(userType)
        fmt.Printf("{%d,%q} ", ut.frequency, ut.value)
    }
    fmt.Println()

    var t interface{} = userType{4, 'c'}
    i, ok := t.(Comparer)
    fmt.Println(i, ok)
}

and output:

{1,'d'} {2,'b'} {3,'c'} {4,'a'} {4,'b'} {4,'c'} 
{4 99} true

---

so if you ready to use known type (e.g. int), see this sample:

package main

import (
    "container/list"
    "fmt"
)

func Insert(val int, l *list.List) {
    e := l.Front()
    if e == nil {
        l.PushFront(val)
        return
    }
    for ; e != nil; e = e.Next() {
        v := e.Value.(int)
        if val < v {
            l.InsertBefore(val, e)
            return
        }
    }
    l.PushBack(val)
}

func main() {
    l := list.New()
    Insert(4, l)
    Insert(2, l)
    Insert(3, l)
    Insert(1, l)
    for e := l.Front(); e != nil; e = e.Next() {
        fmt.Print(e.Value, " ") // 1 2 3 4
    }
    fmt.Println()
}

---

old:

there is no such interface in Golang.
you may write this Less function to compare your types:

func Less(a, b interface{}) bool {
    switch a.(type) {
    case int:
        if ai, ok := a.(int); ok {
            if bi, ok := b.(int); ok {
                return ai < bi
            }
        }
    case string:
        if ai, ok := a.(string); ok {
            if bi, ok := b.(string); ok {
                return ai < bi
            }
        }
    // ...
    default:
        panic("Unknown")
    }
    return false
}

test sample code:

package main

import (
    "container/list"
    "fmt"
)

func Less(a, b interface{}) bool {
    switch a.(type) {
    case int:
        if ai, ok := a.(int); ok {
            if bi, ok := b.(int); ok {
                return ai < bi
            }
        }
    case string:
        if ai, ok := a.(string); ok {
            if bi, ok := b.(string); ok {
                return ai < bi
            }
        }
    default:
        panic("Unknown")
    }
    return false
}

func Insert(val interface{}, l *list.List) *list.Element {
    e := l.Front()
    if e == nil {
        return l.PushFront(val)
    }
    for ; e != nil; e = e.Next() {
        if Less(val, e.Value) {
            return l.InsertBefore(val, e)
        }
    }
    return l.PushBack(val)
}

func main() {
    l := list.New()

    Insert(4, l)
    Insert(2, l)
    Insert(3, l)
    Insert(1, l)
    for e := l.Front(); e != nil; e = e.Next() {
        fmt.Print(e.Value, " ")
    }
    fmt.Println()

    Insert("C", l)
    Insert("A", l)
    Insert("AB", l)
    Insert("C", l)
    Insert("C2", l)
    Insert("C1", l)

    for e := l.Front(); e != nil; e = e.Next() {
        fmt.Print(e.Value, " ")
    }
    fmt.Println()
}

output:

1 2 3 4 
1 2 3 4 A AB C C C1 C2