Calling a function with Go Reflect - go

Question

I was wondering if it was possible to not know a function name but call it anyway and get values from it. This lead me to the reflection package and I got pretty close but I’m not sure about the last step - if there is one. Again please forgive me if I am missing something obvious, this is my first attempt at doing anything in Go other than getting it setup.
Of course being a compiled language there is no need to iterate through things to find function names, I know them all, but this is something I want to see if it is possible… I’m playing and learning.
Below is the code. What I would really like to do is in the main line extract the values set in ModuleBoot() <“1.0012”, 23> and SomethingBoot() <“1.0000”, 10> but so far all as I can get is structure information. Perhaps that’s just the way it is but perhaps there is a step or change that can make it go the next step.
Hopefully I copied all the relevant code over correctly so it compiles as is:
// Using: go version go1.9.7 linux/amd64
=======================================
FILE: main.go
=======================================
package main
import (
"fmt"
"reflect"
"playing/modules/core"
)
func main() {
miType := reflect.TypeOf(core.ModuleInfo{})
fmt.Println("")
for i := 0; i < miType.NumMethod(); i++ {
method := miType.Method(i)
fmt.Println(method.Name)
in := make([]reflect.Value, method.Type.NumIn())
in[0] = reflect.ValueOf(core.ModuleInfo{})
//fmt.Println("Params in:", method.Type.NumIn(), "Params out:", method.Type.NumOut())
mi := method.Func.Call(in)
fmt.Println("mi:", mi)
fmt.Println("")
}
}
=======================================
FILE: playing/modules/core/something.go
=======================================
package core
func (mi ModuleInfo) SomethingBoot() ModuleInfo {
mi.Version = "1.0000"
mi.Priority = 10
return mi
}
=======================================
FILE: playing/modules/core/modules.go
=======================================
package core
type ModuleInfo struct {
Version string
Priority int
}
func (mi ModuleInfo) ModuleBoot() ModuleInfo {
mi.Version = "1.0012"
mi.Priority = 23
return mi
}
The output I got from this was:
Started delve with config "Debug"
SomethingBoot
mi: [<core.ModuleInfo Value>]
ModuleBoot
mi: [<core.ModuleInfo Value>]
delve closed with code 0

Answer 1

To get the return value as a ModuleInfo, get the underlying value of the first return value and type assert that interface value to ModuleInfo:
// mi has type core.ModuleInfo
mi := method.Func.Call(in)[0].Interface().(core.ModuleInfo)
Run it on the Playground.
You can cut some of the reflect code by type asserting the method to a function with the correct signature and calling that function directly:
for i := 0; i < miType.NumMethod(); i++ {
method := miType.Method(i).Func.Interface().(func(core.ModuleInfo) core.ModuleInfo)
mi := method(core.ModuleInfo{})
fmt.Println("Version", mi.Version)
fmt.Println("Priority", mi.Priority)
fmt.Println("")
}
Run it on the Playground

Answer 2

Go natively supports functions as values; you don't need reflection to do this.
In particular, if you make your two functions top-level functions (not specifically tied to a struct):
package core
type ModuleInfo struct { ... }
func SomethingBoot() ModuleInfo
func ModuleBoot() ModuleInfo
Then you can write a function that takes a function as a parameter:
func PrintVersion(func booter() core.ModuleInfo) {
mi := booter()
fmt.Printf("version %s\n", mi.Version)
}
And you can just pass the pre-existing functions as parameters:
PrintVersion(core.SomethingBoot)
PrintVersion(core.ModuleBoot)
Notice that there aren't parentheses after the function name: you are passing the function itself as a parameter, not calling the function and passing its return value.