Participle solution to https://www.reddit.com/r/golang/comments/9e58oe/how_to_develop_a_simple_language_parser_like_yaml/

output.go

&main.Script{
  Command: []*main.Command{
    &main.Command{
      Name: "open",
      Arg: &main.Value{
        Var: &"loginPage",
      },
    },
    &main.Command{
      Name: "validate",
      Arg: &main.Value{
        List: []string{
          "emailBox",
          "passBox",
          "loginButton",
        },
      },
    },
    &main.Command{
      Name: "type",
      Arg: &main.Value{
        Var: &"emailCred",
      },
      On: "emailBox",
    },
    &main.Command{
      Name: "type",
      Arg: &main.Value{
        Var: &"passCred",
      },
      On: "passwordBox",
    },
    &main.Command{
      Name: "click",
      Arg: &main.Value{
        Var: &"loginButton",
      },
    },
    &main.Command{
      Name: "letLoad",
      Arg: &main.Value{
        Var: &"validate",
      },
    },
    &main.Command{
      Name: "leftNav",
      Arg: &main.Value{
        Var: &"validate",
      },
    },
    &main.Command{
      Name: "topNav",
      Arg: &main.Value{
        Var: &"end",
      },
    },
  },
}

parser.go

package main

import (
	"github.com/alecthomas/participle"
	"github.com/alecthomas/repr"
)

const source = `
open loginPage
validate [emailBox, passBox, loginButton]
type emailCred on emailBox
type passCred on passwordBox
click loginButton
letLoad
validate leftNav
validate topNav
end
`

type Script struct {
	Command []*Command `{ @@ }`
}

type Command struct {
	Name string `@Ident`
	Arg  *Value `[ @@`
	On   string `  [ "on" @Ident ] ]`
}

type Value struct {
	Var  *string  `  @Ident`
	List []string `| "[" [ @Ident { "," @Ident } ] "]"`
}

func main() {
	parser := participle.MustBuild(&Script{})
	ast := &Script{}
	err := parser.ParseString(source, ast)
	if err != nil {
		panic(err)
	}
	repr.Println(ast)
}