如何找到函数类型的实现？

Question

In Go, it's possible to create function types (https://golang.org/ref/spec#Function_types) like this

type Printer func(s string)

How can I find all of the functions which satisfy this type? For example, if I had the file below, how could I find out that consolePrinter is a Printer?

package main

import "fmt"

func main() {
    printToScreen(consolePrinter)
}

// Printer defines a function that prints a string.
type Printer func(s string)

func printToScreen(p Printer) {
    p("Hello")
}

// consolePrinter is a Printer (the function signature is identical).
func consolePrinter(s string) {
    fmt.Println(s)
}

I tried out guru, but the implements feature doesn't seem to support function types.

guru implements ./main.go:#134
/Users/adrian/go/gurutest/main.go:10.6-10.12: signature type Printer implements only interface{}

Answer 1

You are looking for functions that are assignable to Printer.

The guru tool does not support an "assignable to" query.

You can write a program to find functions that are assignable to Printer using the go/types package. Follow the tutorial to type check code and use AssignableTo to find the functions.

If you are only interested in finding functions declared at package level in gofmted code, then searching your code with a regexp might be good enough. This approach is not accurate, but is simple to do in many editors or the command line.