How to terminate early in Golang Walk?

You should return an error from your walkfunc. To make sure no real error has been returned, you can just use a known error, like io.EOF.

func Find(needle string, haystack string) (result string, err error) {

    err = filepath.Walk(haystack, 
      filepath.WalkFunc(func(path string, fi os.FileInfo, errIn error) error {
        fmt.Println(path)

        if fi.Name() == needle {
            fmt.Println("Found " + path)
            result = path
            return io.EOF
        }

        return nil
    }))

    if err == io.EOF {
        err = nil
    }

    return
}