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Here's how to organize your source code into packages. We'll also show you how to download and install new packages automatically, using the "go get" tool.
Your OS's $GOPATH environment variable should be set to a directory where you will keep all your Go packages. Once that's done, from your console/terminal, run this command:
go get github.com/treehouse-projects/go-intro/welcome
That will visit this GitHub project and download a file with the below contents at $GOPATH/src/github.com/treehouse-projects/go-intro/welcome/welcome.go:
package welcome
const English = "Welcome"
const Japanese = "ようこそ"
// Below languages not yet approved by committee
const klingon = "yI'el"
Once you've set up the welcome package, you can use the package by creating another file named print_welcome.go in the current directory with the following contents:
package main
import (
"fmt"
"github.com/treehouse-projects/go-intro/welcome"
)
func main() {
fmt.Println(welcome.English)
fmt.Println(welcome.Japanese)
}
Downloading a package from the Go team
The Go team has made this example package available on GitHub. You can download and install it by running:
go get github.com/golang/example/hello/reverse
Once you've done so, you can use the package via the following code:
package main
import (
"fmt"
"github.com/golang/example/hello/reverse"
)
func main() {
fmt.Println(stringutil.Reverse("hello"))
}
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In addition to the package name, Go
programs that want to import your package
0:01
will need to know its package path.
0:04
The package path is decided by what
directory you store your package code in.
0:06
The Go workspace directory is a folder
on your computer where all of
0:10
your Go code should be stored.
0:13
The Go workspace is not the same
thing as Treehouse workspaces.
0:15
Sorry if that's a little confusing.
0:19
By default, the Go workspace will be
a go sub directory in your user's
0:21
home directory, but you can change this
using the GOPATH environment variable.
0:25
So let's take a look what GOPATH
is set to here in this workspace.
0:32
Here in treehouse/workspaces,
0:35
GOPATH is setup to point to
the workspace subdirectory.
0:37
Go code should go in a source
subdirectory within the workspace, and
0:41
then you should create a subdirectory for
the each segment of the package path.
0:44
We've set this package under
github.com/treehouse-projects/go-intro and
0:47
then the welcome package.
0:54
We'll explain why we use such
a detailed package path shortly.
0:56
Let's create a Go program
that uses this package.
1:00
To import a package into a go file,
1:03
you'll use the package path,
followed by the package name.
1:05
So let's go here into our temp.go file,
and as always, this is going
1:08
to run from the command line, so we're
going to put this in the main package.
1:13
And then we'll do an import statement.
1:17
We're gonna import a couple
different packages here.
1:18
So we'll use the multi-package
form of the import command.
1:23
First we'll import the format packet so
that we can use the print line function.
1:29
And then we'll import our welcome package.
1:34
To do that, we're going to
leave the source directory off,
1:38
that's always assumed by default.
1:41
But we'll start with
1:43
github.com/treehouse-projects/go-intro/
1:44
and that's the import path, and now we
need the name of the actual package.
1:55
That'll be the last segment
of our import string.
2:01
So we'll use the welcome package name.
2:07
I never actually used the contents
that we imported from that package, so
2:11
we'll create a main function here
which will run when our program does.
2:15
And we're gonna call the Println function,
and
2:20
we'll just pass it some of
the constants from the welcome package.
2:24
So since these constants
are coming from another package,
2:31
we need to qualify it using the package
name, so we'll say welcome.,
2:35
and then we'll say the name
of our English constant.
2:40
Remember, that starts with a capital
letter because it's exported.
2:44
And then we'll just copy this and,
Print out the Japanese welcome as well.
2:48
Save that, now let's try running it.
2:59
go run temp.go,
3:02
and it prints out our two welcomes,
the first in English, second in Japanese.
3:05
Now, what if we were to try to use
the unexported constant though?
3:11
So, let's try adding a third Println call.
3:16
And this time, we're going to try
to access the welcome packages
3:21
klingon, Constant.
3:26
But remember,
this is an unexported constant,
3:30
because it begins with a lowercase letter.
3:33
Let's try running this.
You see, this time it refuses to compile.
3:36
It can't refer to an unexported name,
welcome.klingon.
3:41
That means that the klingon constant can
only be used within the welcome package.
3:45
It can't be used from outside it.
3:49
We can also download packages from the web
and install them on our system for
3:52
our go programs to use.
3:56
So for example,
the golang team here has an example
3:57
repository which includes a stringutil
package that our programs can use.
4:02
So here within the stringutil package I
can look in the reverse.go source file and
4:07
see that there's a reverse function
that I'd like to try using in a package.
4:14
So let's try writing
a program that uses that.
4:18
So I'm gonna import that package,
4:21
which has a package path of
4:26
github.com/golang/example/stringutil.
4:30
stringutil is the package name, and
4:38
everything preceding
that is the package path.
4:39
Once I've imported that package, then I no
longer need to use the full package path,
4:43
I can just say string, I can just use
the package name, stringutil.Reverse.
4:48
And I pass it the string that
I'd like it to reverse for me.
4:58
So the return value of this would be
the string that I passed to it backwards.
5:03
Let's just print that result out.
5:08
fmt.Println.
5:10
Hello reversed.
5:14
Now if we try to go run temp.go right now,
we'll get an error.
5:16
Cannot find packet,
github.com/golang/example/stringutil.
5:21
So first, we'll use the go command's
get subcommand to download and
5:26
install the package we need.
5:30
So I'll just copy and
paste the package path and
5:32
package name right out of my
code down here into the console.
5:35
And I'll run the get subcommand of the go
command and paste that right here.
5:39
The go get command sees that the library
package path starts with github.com.
5:49
That has no meaning to the go language,
but go get knows that GitHub is a site
5:55
that hosts git repositories that it
can clone the package code from.
6:00
So go get downloads
the package from GitHub for us.
6:04
Now let me refresh our file tree so
6:07
that I can show you what
changes go get made over here.
6:10
go get will create a source subdirectory
in your Go workspace directory,
6:13
if it doesn't already exist.
6:17
And then it'll create subdirectories for
each segment of the package path.
6:19
So github.com/golang/example/ and
6:22
then the stringutil package.
6:28
The github.com/go/example repo also
contains several other packages.
6:32
And go get just clones
the whole repo at once.
6:37
Now that the stringutil package is
installed, we can try rerunning temp.go.
6:39
So we'll go back to our console.
6:44
And type go run temp.go.
6:45
And this time the package will be found.
6:52
The program will run successfully, and
we'll see our reverse string as output.
6:54
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