golang多重作业评估

Question

I'm confused on the concept of multiple assignment. Given the following code:

func fibonacci() func() int {
    current, next := 0, 1
    return func() int {
        current, next = next, current+next
        return current
    }
}

How is the assignment evaluated, given the fact that both variables appear on both the left and right side of the assignment?

Answer 1

The Go Programming Language Specification

Assignments

The assignment proceeds in two phases. First, the operands of index expressions and pointer indirections (including implicit pointer indirections in selectors) on the left and the expressions on the right are all evaluated in the usual order. Second, the assignments are carried out in left-to-right order.

---

The usual example to illustrate multiple assignments is a swap. For example,

package main

import "fmt"

func main() {
    {
        i, j := 7, 42
        fmt.Println(i, j)
        // swap i and j - implicit temporaries
        i, j = j, i
        fmt.Println(i, j)
    }

    fmt.Println()

    {
        i, j := 7, 42
        fmt.Println(i, j)
        // swap i and j - explicit temporaries
        ti, tj := i, j
        i, j = tj, ti
        fmt.Println(i, j)
    }
}

Playground: https://play.golang.org/p/HcD9zq_7tqQ

Output:

7 42
42 7

7 42
42 7

The one statement multiple assignment, which uses implicit temporary variables, is equivalent to (a shorthand for) the two multiple assignment statements, which use explicit temporary variables.

---

Your fibonacci example translates, with explicit order and temporary variables, to:

package main

import "fmt"

func fibonacciMultiple() func() int {
    current, next := 0, 1
    return func() int {
        current, next = next, current+next
        return current
    }
}

func fibonacciSingle() func() int {
    current, next := 0, 1
    return func() int {

        // current, next = next, current+next
        // first phase, evaluation, left-to-right
        t1 := next
        t2 := current + next
        // second phase, assignmemt, left-to-right
        current = t1
        next = t2

        return current
    }
}

func main() {
    m := fibonacciMultiple()
    fmt.Println(m(), m(), m(), m(), m(), m())
    s := fibonacciSingle()
    fmt.Println(s(), s(), s(), s(), s(), s())
}

Playground: https://play.golang.org/p/XFq-0wyNke9

Output:

1 1 2 3 5 8
1 1 2 3 5 8