precendences.go

// iota enumerator gives an effective way in go language
// to use constants in enum-like constructs
const (
  none = iota
  plus
  mul
)
// enum-like map of operator token keys and iota values defined above
var precendences = map[string]int{
  token.PLUS:     plus,
  token.ASTERISK: mul,
}
// calculate binding power
func bp(tok *token.Token) int {
  if precendence, ok := precendences[string(tok.Type)]; ok {
    return precendence
  }
  return none
}