bwiedermann / pratt-parser-blog-code

The code to illustrate the pratt parser blog post for the desmos engineering blog.

Maybe-Undefined or Definitely Type

This project started as an implementation of a lexer and Pratt parser for a simple language.

We turned this into the proof of concept for a Maybe-Undefined or Definitely (MUD) type to be added to Desmos' Computation Layer.

So, in addition to the lexer and Pratt parser, we added capabilities for functions, variables, conditional expressions, and many more types. We also added type checking, constant checking and evaluation, and MUD checking for the proof of concept.

Setup

Clone the repo, then run

npm install
node fuse.js

Then open http://localhost:4444/

Notes

If you wish to add negative numbers, make sure to use parentheses around the negative number, or it will not parse correctly.

The keywords WHEN and OTHERWISE can only be used inside conditional expression and will be parsed into Choose Nodes.

Functions will only parse if they are included in the list of built-in functions, which are as follows:

• Sink(any type): the argument can be of any type but must either defined or error checked in a conditional expression
• InputN(): represents a maybe-undefined source of input in number form
• InputB(): represents a maybe-undefined source of input in boolean form
• Inverse(number): computes the inverse a number as long as the argument is not 0
• Sqrt(number): computes the square root of a number as long as the argument is not negative
• IsDefined(any type): the argument can be of any type, returns a boolean

Variables must be lowercase.

Each call to InputN() or InputB() represents a new source of possibly undefined input.

pratt-parser-blog-code's Issues

Parse Error instead of Type Error

Parser throws an "unexpected end of tokens" error when presented with an incorrect 3 operand expression. This expression
1 + True + 2 should produce a type error instead.