如何定义const * DataType

Question

I'm writing a Lisp variant in Go and want to define constants for Nil and EmptyList. These will be referenced throughout the codebase, but I want to prevent them from being accidentally re-defined.

// Representation of the empty list
var EmptyList = (*List)(nil)

I can't use a const here for two reasons:

1. const definitions cannot be nil
2. const definitions cannot be pointers

What options do I have to ensure EmptyList is always the nil pointer?

Answer 1

In Go, use a function. For example,

package main

import "fmt"

type List struct{}

func IsEmptyList(list *List) bool {
    // Representation of the empty list
    return list == (*List)(nil)
}

func main() {
    fmt.Println(IsEmptyList((*List)(nil)))
}

Output:

true

The function will be inlined.

$ go tool compile -m emptylist.go
emptylist.go:7: can inline IsEmptyList
emptylist.go:13: inlining call to IsEmptyList
emptylist.go:7: IsEmptyList list does not escape
emptylist.go:13: IsEmptyList((*List)(nil)) escapes to heap
emptylist.go:13: main ... argument does not escape
$