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This makes sure that the expression parser stops when it reaches the end of the program. To use it, we need a tokenizer that can generate the right kind of token objects for a given source program.Here’s a simple regular expression-based version that handles the minimal language we’ve used this far: Not counting the calls to the tokenizer, the parser algorithm will make a total of four calls to parse this expression; one for each token, and one extra for the recursive call to the expression parser in the “led” method.However, recursive-descent is less efficient for expression syntaxes, especially for languages with lots of operators at different precedence levels.
Pratt’s algorithm associates semantics with tokens instead of grammar rules, and uses a simple “binding power” mechanism to handle precedence levels.
The method then adds the left value (from the literal, in this case) to the return value from the expression parser, and returns the result.
The end of the program is indicated by a special marker token, with binding power zero (lower than any other token).
Given this brief introduction, we’re ready to look at the core of Pratt’s algorithm, the expression parser:(Pratt calls this function “parse”, but we’ll use the name from Crockford’s article instead.)Here, “token” is a global variable that contains the current token, and “next” is a global helper that fetches the next token.
The “nud” and “led” functions are represented as methods, and the “lbp” is an attribute.