tree.go

tree := p.lexer.Tokens().
  Scan(func(_ context.Context, acc interface{}, elem interface{}) (interface{}, error) {
    it, isIterator := acc.(iterator)
    next := elem.(token.Token)
    if !isIterator || it.left == nil {
      // null denotation
      return nud(acc, next), nil
    }
    if it.operator != nil {
      var prevIt *iterator
      if len(it.stack) > 0 {
        prevIt = &it.stack[len(it.stack)-1]
      }
      // binding power comparison
      for len(it.stack) > 0 && prevIt != nil && bp(it.operator) < bp(prevIt.operator) {
        prevIt = &it.stack[len(it.stack)-1]
        // left denotation
        it.left = led(*prevIt.left, *prevIt.operator, *it.left)
        // in golang slices are immutable
        it.stack = it.stack[:len(it.stack)-1]
      }
      var newIt iterator
      newIt.stack = append(it.stack, it)
      // next iteration
      return nud(newIt, next), nil
    } 
    it.operator = &next
    return it, nil
  })