How To Break Build With Go Vendor Experiment

The GO15VENDOREXPERIMENT provides a powerful way to place packages your project needs within the project. Yet, there is an easy way for this to break things.

Take a project with this structure:

- $GOPATH/src/github.com/mattfarina/golang-broken-vendor
  - foo.go
  - vendor/
    - a/
    - b/
        - vendor/a/

In this example both a packages are exactly the same. The package b stores the a package within its codebase. The top level application also includes the a package.

The file foo.go is fairly simple in that it does...

func main() {
	var it a.A
	it = "foo"

	b.Do(it)
}

The problem is this doesn't build. Trying to build this returns the error:

$ GO15VENDOREXPERIMENT=1 go build
./foo.go:12: cannot use it (type "github.com/mattfarina/golang-broken-vendor/vendor/a".A) as type "github.com/mattfarina/golang-broken-vendor/vendor/b/vendor/a".A in argument to b.Do

You can clone this repo and try this for yourself.

With Glide (a Go package manager) we've found the best solution is to flatten the included dependencies to the top level vendor/ directory. If a package is meant to be included by another application it may be better to not store your external dependencies. If you use Glide you can specify them in a glide.yaml file and Glide will take care to install the right versions.

In any case, be careful how and where you include dependencies.