Print struct definition from an instance in Go

Question

I am looking for a lib or snippet that allows to (pretty) print not the content of a struct instance but its structure. Heres a some code and the expected output:

package main

import "fantastic/structpp"

type Foo struct {
    Bar string
    Other int
}

func main() {
    i := Foo{Bar: "This", Other: 1}
    str := structpp.Sprint{i}
    fmt.Println(str)
}

would print (this or similar):

Foo struct {
    Bar string
    Other int
}   

Note that I am aware of github.com/davecgh/go-spew/spew but I don't want to prettyprint the data, I only need the definition of the struct.

Answer 1

Would something like this work? Might need some adjusting depending on your specific struct and use case (whether you want to print interfaces{} where the value is actually a struct, etc.)

package main

import (
    "fmt"
    "reflect"
)

func printStruct(t interface{}, prefix string) {
    s := reflect.Indirect(reflect.ValueOf(t))
    typeOfT := s.Type()

    for i := 0; i < s.NumField(); i++ {
        f := s.Field(i)

        fmt.Printf("%s%s %s\n", prefix, typeOfT.Field(i).Name, typeOfT.Field(i).Type)
        switch f.Type().Kind() {
        case reflect.Struct, reflect.Ptr:
            fmt.Printf("%s{\n", prefix)
            printStruct(f.Interface(), prefix+"\t")
            fmt.Printf("%s}\n", prefix)

        }
    }
}

Then, for this struct:

type C struct {
    D string
}

type T struct {
    A int
    B string
    C *C
    E interface{}
    F map[string]int
}

t := T{
    A: 23,
    B: "hello_world",
    C: &C{
        D: "pointer",
    },
    E: &C{
        D: "interface",
    },
}

You get:

A int
B string
C *main.C
{
    D string
}
E interface {}
F map[string]int

Go Playground Link: https://play.golang.org/p/IN8-fCOe0OS

Answer 2

I can see no other option than using reflection

func Sprint(v interface{}) string {

    t := reflect.Indirect(reflect.ValueOf(v)).Type()

    fieldFmt := ""

    for i := 0; i < t.NumField(); i++ {
        field := t.Field(i)
        fieldFmt += "\t" + field.Name + " " + field.Type.Name() + "\n"
    }

    return "type " + t.Name() + " struct {\n" + fieldFmt + "}"
}

Note though that this function has no validation/checking and might panic for non struct input.

Edit: Go playground: https://play.golang.org/p/5RiAt86Wj9F

Which outputs:

type Foo struct {
    Bar string
    Other int
}