Carlos León

If you have two files in Go that look like this:

a.go

package main

func main() {
  println(foo())
}

b.go

func foo() string {
  return "Hello, foo!"
}

If you go build . the project, it completes without errors and the binary file is generated:

$ ls -l
a.go
b.go
foo

And it works:

$ ./foo
Hello, foo!

But when you’re developing you want to run the project right from Go instead of compiling and then running the binary. The tool for doing this is run and your gut will dictate you something like:

$ go run a.go

But the output rather than being what you expected is somewhat more like:

# command-line-arguments
./a.go:4: undefined: foo

:(

go build is smart enough to figure out by itself that the function foo is defined in one of the .go files in our directory/package but go run is not as smart.

THE FIX

Simply specify all the files involved in your “run” and it will just work:

$ go run a.go b.go
Hello, foo!