Expreva NPM | npm.io

Expreva

Expreva is a language based on arithmetic and algebra expressions.

npm.io

Overview

This is a parser and interpreter for the Expreva language, written in TypeScript.

Expreva aims to be a modular, extensible language and cross-platform virtual machine.

It's an exploration to design a suitable medium for end-user programming, for example: in a spreadsheet formula; for data transport or query protocol; and creation of dynamic hypertext documents and interactive textbooks.

The source code is compiled to a compact JSON-serializable format, suitable for transfer over HTTP, WebSocket, or inter-process communication.

The goal is to stay beginner-friendly, and simple enough to implement on a range of platforms. Currently it runs in a web browser, Node.js server, and on PHP. An eventual plan is to compile to WebAssembly.

Parser

The grammar is based on infix notation, a superset of familiar arithmetics.

The parser produces a syntax tree of symbolic expressions in prefix notation, as in Lisp.

References

The original paper: Top Down Operator Precedence - Vaughan R. Pratt (1973)
How JSLint parses JavaScript: Top Down Operator Precedence - Douglas Crockford (2006)
Excellent overview of the algorithm, with examples in Python (2010)
Pratt Parsing and Precedence Climbing Are the Same Algorithm (2016)

Interpreter

The interpreter is based on a study of make-a-lisp, with lexical scope, lambda, macro, conditions, and tail-call optimization.

Compiled expressions are evaluated in an isolated runtime environment. It allows passing values, objects, and functions, to and from the host. More work is needed to ensure safe evaluation, such as limiting the number of operations and time to live.