This project demonstrates the fundamentals of a Pratt Parser. It is based on this paper by Vaughan Pratt, and also learns from this article and this article.
Additionally, this README file attempts to simplify the concepts so that, when I come back to try to implement this again (at some undetermined point in the future), I will be able to remember how this works.
This README assumes that you already have read the previous two articles as this text merely attempts to simplify some of the concepts, hopefully adding intuition to them.
In general, the Pratt Parser solves the following problem: given the string "1 + 2 * 3", does the "2" associate with the "+" or the "*". It also solves "-" being both a prefix and infix operator, as well as elegantly handling right associativity.
The Pratt Parser is based on three computational units:
parser.expr(rbp) // the expression parser
token.nud() // "Null Denotation" (operates on no "left" context)
token.led(left, bp) // "Left Denotation" (operates with "left" context)The parser.expr(rbp) function looks like:
function expr(rbp) {
let left = lexer.next().nud() // (1)
while (rbp < lexer.peek().bp) { // (2)
const operator = lexer.next() // (3)
left = operator.led(left, operator.bp) // (4)
}
return left
}Of course, nud and led may recursively call expr.
The expr method can be summarized in english as "The loop (while) builds out the tree to the left, while recursion (led -> expr) builds the tree out to the right; nud handles prefix operators":
function expr(rbp) {
// (1) handle prefix operator
// (2) continue until I encounter an operator of lesser precedence than myself
// (3) "eat" the operator
// (4) give the operator the left side of the tree, and let it build the right side; this new tree is our new "left"
}The Pratt Parser is a new concept to me, and thus this README would probably benefit from clarification, and the code would probably also benefit from cleanup. Submit a PR if you feel so inclined!
ISC License (ISC) Copyright (c) 2016, Jonathan Apodaca jrapodaca@gmail.com
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