Golang: How to use vgo? error: "cannot find package"

Question

1. I have a project which is currently under my GOPATH /usr/local/go/src/

2. Then I went inside my project root and ran: go get -u golang.org/x/vgo

3. In my main.go I want to use lib httprouter so I added in import statement:

   import ( "github.com/julienschmidt/httprouter" )

4. I ran vgo build

5. Then when I start to run my server as usual I get the following error:

main.go:8:2: cannot find package "github.com/julienschmidt/httprouter" in any of: /usr/local/go/src/github.com/julienschmidt/httprouter (from $GOROOT) /Users/myuser/go/src/github.com/julienschmidt/httprouter (from $GOPATH)

I understand the error, it seems to be searching for httprouter under GOPATH or GOROOT. So do I have to do anything else to just let vgo do its thing and I can run my server successfully?

Answer 1

Using vgo build

Here is a "Hello, World" walk-through of using vgo, including common errors and how to respond to them.

1. $ go get -u golang.org/x/vgo
2. $ cd /tmp/hello # Outside of GOPATH.

3. $ cat main.go

   package main
   
   import "fmt"
   
   func main() {
       fmt.Println("Hello, World")
   }
   

4. $ vgo build

   cannot determine module root; please create a go.mod file there
   

5. This error tells us we need to specify what the root of our vgo module is. A module is a collection of packages (possibly one) versioned together. Create an empty go.mod file to signify this is the module root:

   $ touch go.mod
   

6. Try building again: $ vgo build.

   vgo: cannot determine module path for source directory
   /tmp/hello (outside GOPATH, no import comments)
   

7. This error tells us vgo doesn't know the import path of our package. There are two ways to tell it:

   1. Add a module statement to go.mod.
   2. Add an import comment to our package.

   We'll go with option 2 right now and come back to option 1. Change the first line of main.go to be:

   package main // import "my/import/path/hello"
   

8. $ vgo build one last time.

9. Run the binary: $ ./hello

   Hello, World
   

Note: go.mod now contains a module line (option 1 above):

$ cat go.mod 
module my/import/path/hello

If you are inside your GOPATH, the error in step 5 should not occur - vgo build should automatically figure out you're inside your GOPATH and add the module line accordingly.

---

Using vgo run (without using vgo build)

For this example, lets use the hello.go from the A Tour of Versioned Go (vgo):

1. $ go get -u golang.org/x/vgo
2. $ mkdir /tmp/hello2
3. $ curl -sS https://swtch.com/hello.go >hello.go

4. $ vgo run hello.go

   cannot determine module root; please create a go.mod file there
   

5. $ touch go.mod

6. $ vgo run hello.go

   vgo: resolving import "rsc.io/quote"
   vgo: finding rsc.io/quote (latest)
   vgo: adding rsc.io/quote v1.5.2
   Hello, world.