Go map of functions

Question

I have Go program that has a function defined. I also have a map that should have a key for each function. How can I do that?

I have tried this, but this doesn't work.

func a(param string) {

}

m := map[string] func {
    'a_func': a,
}

for key, value := range m {
   if key == 'a_func' {
    value(param) 
   }
}

Wehner

Answer 1

Are you trying to do something like this? I've revised the example to use varying types and numbers of function parameters.

package main

import "fmt"

func f(p string) {
    fmt.Println("function f parameter:", p)
}

func g(p string, q int) {
    fmt.Println("function g parameters:", p, q)
}

func main() {
    m := map[string]interface{}{
        "f": f,
        "g": g,
    }
    for k, v := range m {
        switch k {
        case "f":
            v.(func(string))("astring")
        case "g":
            v.(func(string, int))("astring", 42)
        }
    }
}

Actuate

that's almost exactly what I need, but let's say g needed 2 string parameters. Is that possible? – Wehner

Could you elaborate on "v.(func(string))"; that seems like it would almost need the reflect package. Do the parens here do any casting, or is it all dealing with the basic interface? – Pasco

And instead of using f and g as the map values, could you instead have string values there, and instead call f(mapValue) under case "f":? – Pasco

what if your functions are from a struct type? – Tenon

Answer 2

m := map[string]func(string, string)

Works if you know the signature (and all the funcs have the same signature) I think this is cleaner/safer than using interface{}

Pumpernickel

All the functions don't have the same signature. – Actuate

Actually I think this answers my above question; my functions have the same signature. But... here I see the map value is a function with arguments. Interesting, I'll try that as well as passing just the arguments to compare. – Pasco

Also, say the function returns a string variable, it would look like this: m := map[string]func(string, string)string – Masera

Answer 3

You can define a type if functions are same interface.

package main

import "log"

type fn func (string)

func foo(msg string) {
  log.Printf("foo! Message is %s", msg)
}

func bar(msg string) {
  log.Printf("bar! Message is %s", msg)
}

func main() {
  m := map[string] fn {
    "f": foo,
    "b": bar,
  }
  log.Printf("map is %v", m)
  m["f"]("Hello")
  m["b"]("World")
}

Am

Answer 4

@Seth Hoenig's answer helped me best, but I just wanted to add that Go accepts functions with defined return value as well:

package main

func main() {
    m := map[string]func(string) string{
        "foo": func(s string) string { return s + "nurf" },
    }

    m["foo"]("baz") // "baznurf"
}

If you think it's ugly, you could always use a type (see @smagch's answer).

Miracidium

How to use a type: play.golang.org/p/CtVoOzkT3hw. Good example +1. – Armillary

Answer 5

Hope this works for you(you can use interface{} instead any)

package main

import (

    "fmt"

)


func toon(v any) {

    fmt.Println(v)

}

func main() {

    names := map[string]any{

        "Function": toon,

    }

    names["Function"].(func(any))("a")

}

Miter

Answer 6

Here is the way I made it work in my case:

package main

import (
    "fmt"
)

var routes map[string]func() string

func main() {
    routes = map[string]func() string{
        "GET /":      homePage,
        "GET /about": aboutPage,
    }

    fmt.Println("GET /", pageContent("GET /"))
    fmt.Println("GET /about", pageContent("GET /about"))
    fmt.Println("GET /unknown", pageContent("GET /unknown"))
    // Output:
    // GET / Home page
    // GET /about About page
    // GET /unknown 404: Page Not Found
}

func pageContent(route string) string {
    page, ok := routes[route]
    if ok {
        return page()
    } else {
        return notFoundPage()
    }
}

func homePage() string {
    return "Home page"
}

func aboutPage() string {
    return "About page"
}

func notFoundPage() string {
    return "404: Page Not Found"
}

https://play.golang.org/p/8_g6Di1OKZS

Shoestring

Answer 7

I used a map[string]func (a type, b *type) I passed a string to search the map and a pointer to modify the slice.

Hope that helps!

var Exceptions map[string]func(step string, item *structs.Item)

func SetExceptions() {
    Exceptions = map[string]func(a string, i *structs.Item){
        "step1": step1,
    }
}

func RunExceptions(state string, item *structs.Item) {
    method, methBool := Exceptions[state]
    if methBool {
        method(state, item)
    }
}

func step1(step string, item *structs.Item) {
    item.Title = "Modified"
}

Lynnet