Chaining functions in Go?

Question

I tried doing this:

package main

import (
    "fmt"
    "strings"
)

type String string


func (s *String) tolower() String {
    *s = String(strings.ToLower(string(*s)))
    return *s
}

func (s *String) toupper() String {
    *s = String(strings.ToUpper(string(*s)))
    return *s
}

func main() {
    var s String = "ASDF"
    (s.tolower()).toupper() // this fails
    // s.toupper();s.tolower(); // this works
    // s.tolower().toupper() // this fails too
    fmt.Println(s)
}

But I got these errors:

prog.go:30: cannot call pointer method on s.tolower()
prog.go:30: cannot take the address of s.tolower()

Program exited.

Why can't I make this chain work?

Answer 1

This works:

package main

import (
    "fmt"
    "strings"
)

type String string


func (s *String) tolower() *String {
    *s = String(strings.ToLower(string(*s)))
    return s
}

func (s *String) toupper() *String {
    *s = String(strings.ToUpper(string(*s)))
    return s
}

func main() {
    var s String = "ASDF"
    (s.tolower()).toupper()
    s.toupper();
    s.tolower();
    s.tolower().toupper()
    fmt.Println(s)
}

Your return type is of String, for functions defined on pointers to String. It wouldn't make sense to be able to chain them.

Answer 2

tolower() and toupper() have pointer-to-String as the receivers, but they are returning String (not pointer-to-String).

You can fix this by changing one or the other.

e.g. change the signature of the function to either:

func (s *String) toupper() *String

or

func (s String) toupper() String

(see: http://play.golang.org/p/FaCD8AQtIX)