在Go中的函数内定义递归函数

Question

I am trying to define a recursive function within another function in Go but I am struggling to get the right syntax. I am looking for something like this:

func Function1(n) int {
   a := 10
   Function2 := func(m int) int {
      if m <= a {
         return a
      }
      return Function2(m-1)
   }

   return Function2(n)
}

I'd like to keep Function2 inside the scope of Function1 as it is accessing some elements of its scope.

How can I do this in Go?

Many thanks

Answer 1

You can't access Function2 inside of it if it is in the line where you declare it. The reason is that you're not referring to a function but to a variable (whose type is a function) and it will be accessible only after the declaration.

Quoting from Spec: Declarations and scope:

The scope of a constant or variable identifier declared inside a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl for short variable declarations) and ends at the end of the innermost containing block.

In your example Function2 is a variable declaration, and the VarSpec is:

Function2 := func(m int) int {
    if m <= a {
        return a
    }
    return Function2(m-1)
}

And as the quote form the language spec describes, the variable identifier Function2 will only be in scope after the declaration, so you can't refer to it inside the declaration itself. For details, see Understanding variable scope in Go.

Declare the Function2 variable first, so you can refer to it from the function literal:

func Function1(n int) int {
    a := 10
    var Function2 func(m int) int

    Function2 = func(m int) int {
        if m <= a {
            return a
        }
        return Function2(m - 1)
    }

    return Function2(n)
}

Try it on Go Playground.