为什么我无法使用在其他程序包中定义的结构？

I am unable to use a struct in package main which has been defined in a different package. Please note that I am importing the other package correctly

I named the struct and its fields starting with a capital letter because I read that in Golang that is how we indicate that it is an exported field. Although it is not required if the package is imported.

fsm.go

package fsm

import (
"fmt"
"strings"
 )
// EKey is a struct key used for storing the transition map.
type EKey struct {
// event is the name of the event that the keys refers to.
Event string

// src is the source from where the event can transition.
Src string
}

test.go

package main

import (
"encoding/json"
"fmt"

"github.com/looplab/fsm"
) 
func main(){
    Transitions := make(map[EKey]string) 
}

Error: undefined EKey

You have to first import the package whose identifiers you want to refer to:

import "path/to/fsm"

Once you do this, the package name fsm becomes a new identifier in your file block, and you can refer to its exported identifiers (identifiers that start with an uppercase letter) by using a qualified identifier, which is packagename.IdentifierName like this:

Transitions := make(map[fsm.EKey]string)

See related question: Getting a use of package without selector error

try this

package main

import (
"encoding/json"
"fmt"

"github.com/looplab/fsm"
) 
func main(){
    Transitions := make(map[fsm.EKey]string) 
}

You need to refer to your struct using fsm.EKey

If you want to import it to your local name space, you need a dot before the import path.

import (
   // ....
   . "github.com/looplab/fsm"
)

Now you can refer to your struct directly as EKey