此函数初始化语法是什么意思？

Question

While looking at the google plus sign in in go, I found a very interesting pattern. Here is a trivial example (live).

package main

import(
    "fmt"
)

type FuncType func(i int) int

func (fn FuncType) MultiplyByTwo(i int) int{
    return fn(i) * 2
}

func MultiplyByThree(i int) int{
    return i * 3
}


func main(){
    fn := FuncType(MultiplyByThree)
    fmt.Println("returns 2 * 3 * 5: ",fn.MultiplyByTwo(5))
}

My question is quite simple, how come can we initiate the FuncType with parentheses? I do not understant!

Thanks.

Answer 1

Go spec: Conversions:

Conversions are expressions of the form T(x) where T is a type and x is an expression that can be converted to type T.

So,

fn := FuncType(MultiplyByThree)

FuncType is a type. And MultiplyByThree is a pointer to function (which is an expression) with the same signature as FuncType. Therefore, it can be converted to this type.

BTW, the output is slightly wrong. Should be

returns 5 * 3 * 2: 30

This is the correct sequence of multiplications. :)