Top-down Parsing with Simultaneous Evaluation of Noncircular Attribute Grammars

This paper introduces a machinery called attributed top-down parsing automaton which performs top{down parsing of strings and, simultaneously, the evaluation of arbitrary noncircular attribute grammars. The strategy of the machinery is based on a single depth{ rst left{to{right traversal over the syntax tree. There is no need to traverse parts of the syntax tree more than once, and hence, the syntax tree itself does not have to be maintained. Attribute values are stored in a graph component, and values of attributes which are needed but not yet computed are represented by particular nodes. Values of attributes which refer to such uncomputed attributes are represented by trees over operation symbols in which pointers to the particular nodes at their leaves are maintained. Whenever eventually the needed attribute value is computed, it is glued into the graph at the appropriate nodes.