Importing local library and files in an application

Question

I'm new to Go (but not at programming), I love the language but I have a bit of trouble fully understanding the way I'm supposed to make internal libraries in an application through packages. For reference, getting external packages and then importing/using them is fine.

Let's say I'm making an application A.

/home/me/A/a.go (package main)

Then, I realize a.go start to be rather big, so I cut it into two parts

/home/me/A/a.go (package main)
/home/me/A/b.go (package main)

How am I supposed to import/include b.go from a.go to make its function available ?

As a continuation of the question, in the A I'm manipulation lots of objects O, so I figure it would be a lot better if I just give them their own package and encapsulate the functionalities in a public/exported api. How do I do that ?

I've tried creating ./lib/o.go (package o) and import lib/o but I keep getting error like

./a.go:6: imported and not used: "o"
./a.go:43: undefined: o

I have no GOPATH in my env but I tried export GOPATH=$GOPATH:/home/me/A and it didn't change the result.

I've tried to read the article on "go layout" but it felt a bit too overwhelming at once and I would really love a simpler explanation of that one "small" step I am trying to make

Thanks !

GOPATH/src/me/a/a.go:

package main

func main() {
        test()
}

GOPATH/src/me/a/test.go:

package main

import "fmt"

func test() {
        fmt.Println("test func !")
}

Exec:

$ go run a.go 
# command-line-arguments 
./a.go:4: undefined: test

EDIT: got my answer here: https://groups.google.com/forum/?fromgroups#!topic/golang-nuts/qysy2bM_o1I

Either list all files in go run (go run a.go test.go) or use go build and run the resulting executable.

Answer 1

You're trying to use the Go build system while not following the necessaary required directory layouts. You will benefit a lot from reading this document.

In short, these are, wrt the go tool, the show stoppers:

• You must have a valid, exported GOPATH

• Package files with import path "example/foo" must be located in the $GOPATH/src/example/foo directory.

For more details please see the above linked article.