@tabnas/expr

An expression-syntax plugin for the Tabnas parser, available in both TypeScript and Go.

Adds Pratt-parser expressions to Tabnas: infix, prefix, suffix, ternary, and paren operators with a configurable binding-power (precedence) scale, so values can include arithmetic/logical expressions. Expressions parse into LISP-style S-expressions (arrays whose first element is the operator Op), which a user-supplied evaluator can reduce to values.

Install

npm install @tabnas/expr @tabnas/parser @tabnas/jsonic

Tiny example

const { Tabnas } = require('@tabnas/parser')
const { jsonic } = require('@tabnas/jsonic')
const { Expr } = require('@tabnas/expr')

// Show parse trees with each Op replaced by its source string.
const S = (x) => Array.isArray(x) && x.length ? [x[0].src || x[0].osrc || S(x[0]), ...x.slice(1).map(S)] : x

const j = new Tabnas().use(jsonic).use(Expr)
const show = (s) => JSON.parse(JSON.stringify(S(j.parse(s))))

show('1+2*3')   // => ['+', 1, ['*', 2, 3]]
show('-1+2')    // => ['+', ['-', 1], 2]

Add an evaluate callback to compute values instead:

const { Tabnas } = require('@tabnas/parser')
const { jsonic } = require('@tabnas/jsonic')
const { Expr } = require('@tabnas/expr')

const math = (rule, ctx, op, terms) => {
  switch (op.src) {
    case '+': return op.prefix ? +terms[0] : terms[0] + terms[1]
    case '*': return terms[0] * terms[1]
    case '(': return terms[0]
    default:  return NaN
  }
}

const j = new Tabnas().use(jsonic).use(Expr, { evaluate: math })

j.parse('1+2*3')     // => 7
j.parse('(1+2)*3')   // => 9

Documentation

Docs follow the Diátaxis framework:

• Tutorial — parse and evaluate your first expression, step by step.
• Guide — recipes: add a custom operator, build function-call syntax with paren-preval, add a ternary, restrict to strict math, walk the tree yourself.
• Reference — exports, OpDef options, the Op / Evaluate types, and the default operator table.
• Concepts — how Pratt parsing works, the binding-power scale, and the design trade-offs.

Go port docs: tutorial · guide · reference · concepts.

Grammar diagram

The installed grammar as a railroad/syntax diagram, generated from the live grammar with @tabnas/railroad:

A vertical ASCII version is in doc/grammar.txt.

License

MIT. See LICENSE.