A top-down parser begins by hypothesizing a sentence and successively predicting lower level constituents until individual pre-terminal symbols are written.
A top-down parser is a parsing technique used in computer science and artificial intelligence to analyze the structure of a text or code based on a formal grammar. In a top-down parser, the parsing process starts from the root of the parse tree (typically the start symbol of the grammar) and works its way down towards the leaves, attempting to match the input against the production rules of the grammar. It begins with the start symbol and repeatedly expands non-terminal symbols using production rules until the entire input is recognized or an error is encountered.
A typical example of a top-down parser is a recursive descent parser, which employs recursive procedures to match the input against grammar rules. This approach is commonly used in languages with LL(k) grammars, where LL stands for left-to-right, leftmost derivation, and k denotes the number of tokens of lookahead. Top-down parsers are often favored for their simplicity and ability to closely mirror the grammar rules, making them easier to implement and debug. However, they may suffer from inefficiency or limitations in handling certain types of grammars, such as those with left recursion or ambiguous productions.