jeremiahlangner / interpreter

Rather

Rather is a simple expression evaluator.

Syntax documentation and reference can be found in the file rather.md.

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Installation

npm i rather

Usage

Import Rather from the library.

import Rather from 'rather';

Instantiate a new instance.

const rather = new Rather();

Evaluate an expression.

const result = rather.eval('4 <= 5');
console.log(result); // logs 'true'

Data and Rule Sets

Add user specified data to the evaluation context. Reference the data in expressions using dot notation or bracket syntax.

const rather = new Rather({ some: 'foo', data: 'bar', here: 'baz', a: { thing: 2 } });
const result = rather.eval('some = "foo"'); // returns true
const result1 = rather.eval('a["thing"] = 2'); // returns true

Use rules to allow for and store more complex expressions.

const rather = new Rather(
  { some: 'foo', data: 'bar', here: 'baz' },
  { rule1: 'not (some = data)', rule2: ':rule1 and (here = "baz")' }
);
const result = rather.eval(':rule1 and :rule2'); // returns true

Rules and data are public properties. Set them to your desired objects without creating a new instance.

rather.rules = { rule1: 'not (some = data)', rule2: ':rule1 and (here = "baz")' };
rather.data = { here: "is", some: "data" };

Repl

Rather comes packaged with a basic repl to test and evaluate expressions.

To run the repl, navigate to the lib's directory and run the following command:

npm start

To exit the repl type exit or press ^C.

To add a rule set to the repl compile the rules in a JSON object and type the following command:

>> repl.data = <JSON string of rules>

To assign data to the repl execution context, compile the data as a JSON string and type the following command at the repl prompt:

>> repl.data = <JSON string of data>

Unit Tests

Unit tests use Node.js's native test runner from Node.js 22 or later to report coverage.

Run the test suite using the following command:

npm test

Acknowledgements

Much of the code here is produced with reference to Robert Nystrom's Crafting Interpreters and Thorsten Ball's Writing an Interpreter in Go.