First glance in Go

Because I forgot to install the Chinese input in this OS, I have to use English input.

The first problem which I ran into is "how to import the local file in Golang"

First, it may be my fault but I really didn't see any documents say that if you want to import a local file in another Go file, you should create a folder to put the imported file. Below is my case:

I created a file said "example2.go", then typed some codes:

package math

func Add(a, b int) int {
    return a + b
}

I wanted to import it in another file said "example1.go"

package main

import (
        "fmt"
        "./example2"
       )

func main() {
    fmt.Println("Hello Go!")
    fmt.Println(math.Add(1,2))
}

when I tried to run the example1.go file, the go runtime threw an error said "couldn't find the example2". what's the problem? Is GOPATH not set? Is example2.go not compiled? Finally, I found it is the folder structure problem. My orignal folder structure likes this

Goworkspace
- pkg
- src
    - example1.go
    - example2.go
- bin

But Go 1.4 doesn't support import a single file as a package. So I have to put the "example2.go" in a folder said "package2"

Goworkspace
- pkg
- src
    - example1.go
    - package2
        - example2.go
- bin

then change the code

package main

import (
        "fmt"
        "package2"
       )

func main() {
    fmt.Println("Hello Go!")
    fmt.Println(math.Add(1,2))
}

it works. So it means a package always refers to a folder in the Golang. But you may notice that the package name isn't same as the folder name. In Golang, when you use import keyword to import a package. There is two things will happen

1. Golang to find the package file location

2. Parse the package structure for using

So, the first thing is you should use a folder to indicate the package file location. Then you can go to use package by name.

Another thing is a package can accross multiple files, it looks like below

Goworkspace
- pkg
- src
    - example1.go
    - package
        - example2.go
        - example3.go
-bin

The example2 and example3 files are under the same package. Below is the codes

//-- example3.go--
package math

func Sub(a, b int) int {
    return a - b
}

func AddASubB(a, b int) int {
    c := Add(a, b)
    return c - b
}

//-- example2.go ---
package math

func Add(a int, b int) int {
    return a+b
}

 You may notice AddASubB function invoked the example2.go's Add function, but I didn't import anything. Yes, if the function under the same package, you can invoke it directly no matter it's export or not. But the most import thing is the package must be imported in some where. If you write a file said example4.go under the main package, then you want to invoke the function in exampl1.go file. It will be failed. Because no place to trigger the import main package operation.  

 

这里有篇文章讲Go包的管理机制：http://io-meter.com/2014/07/30/go%27s-package-management/