Build and reference my own local package in Go

Question

I'm playing with Google Go and I'm having fun (!), but I'm having some problems with package subsystem.

I'm running Go 1.0.1 on Mac OS X Lion. I've build also various single file programs without problems (I've also build a small webapp using html/templates without problems and it compiles and runs without any error).

I've defined a "reusable" package (even.go):

package even

func Even(i int) bool {
    return i % 2 == 0
}

func Odd(i int) bool {
    return i % 2 == 1
}

and a consumer program (useeven.go):

package main

import (
    "./even"
    "fmt"
)

func main() {
    a := 5
    b := 6

    fmt.Printf("%d is even %v?\n", a, even.Even(a))
    fmt.Printf("%d is odd %v?\n", b, even.Odd(b))
}

But when I compile the "library" using

go build even.go

I got nothing... No errors, no message... What happens?

How should I do this?

Answer 1

The answer to your question, "How should I do this?" is explained in How to Write Go Code. It's really pretty important stuff and worth taking a look at.

The behavior of go build might seem puzzling, but is actually conventional for command line programs--no output means that the program ran successfully. So what did it do? For that your answer is in go help build

... Otherwise build compiles the packages but discards the results, serving only as a check that the packages can be built.

What, you wanted more? Of course. "How to Write Go Code" explains good ways of doing this. For a quick fix to your program, I'll explain that the go command expects each package and each executable program to be in a separate directory. If you just make a directory called even, immediately under the location of useeven.go, and move even.go to it, then go run useeven.go should run just as you have it.

Answer 2

go build only generates an output file when it builds a "single main package". If you want to output a file for your other package you should use go install. That will build and install that package to your pkgs directory.

Answer 3

As noted in a comment by the OP, go build and go run useeven.go work fine, once you put even.go in its own folder: ./even/even.go. There is a bit of Go magic in the ./ (in the import) that makes the "local package" build automatically without requiring it to be installed anywhere.

useeven/
├── even
│   └── even.go
└── useeven.go

---

If you wanted to make this work without the magic ./, then the library need to be installed. A quick-and-dirty way (at the risk of polluting your library namespace) of making this work for your project is to simply register the library's source location by using a symbolic link like so (in this example GOPATH is ~/go):

[ useeven ]
$ ln -s $(pwd)/even ~/go/src

Now when you build the program, it automatically performs a go get even to install an up-to-date version of your library.

Note that this doesn't make go install* work for the library, it just makes doing that unnecessary.

---

There are more idiomatic ways of doing this, if you're not in a hurry: How to import local packages in go?

Answer 4

The reason that output is not being generated here is because you're running the

go build even.go

command on the package and not the main go file. Doing so on the package will check for errors on the package, but because it's not a main with an output, no success messages will be generated. If there are issues with the package's contents, errors will be displayed, otherwise if it compiles fine there will be no output

Instead, you should run:

go build useeven.go

To create the binary, then to execute the binary and get output from the program:

./useeven