Tutorial: Arithmetic Parser

Tutorial: arithmetic expression parser with `rdp-gen`

This tutorial walks through the complete rdp-gen workflow: write an EBNF grammar, generate a TypeScript parser class, and use it to parse input.

Step 1 — write the grammar

Create arithmetic.ebnf:

(* Arithmetic expression grammar — ISO 14977 EBNF *)

Expr   = wsp, Term, {wsp, ('+' | '-'), wsp, Term}, wsp;
Term   = Factor, {wsp, ('*' | '/'), wsp, Factor};
Factor = '(', Expr, ')' | Number;
Number = Digit, {Digit};
Digit  = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9';
wsp    = {' '};

Expr

Term

Factor

Operator precedence is encoded in the production hierarchy: Expr → Term → Factor → Number. Each alternative is distinguished by its first character, so the grammar is LL(1).

Valid inputs: 42, 3 + 4, 3 + 4 * 2, (3 + 4) * 2, 10 / (2 + 3) - 1

Step 2 — generate the parser

rdp-gen arithmetic.ebnf --parser-name ArithmeticParser --output ArithmeticParser.ts

rdp-gen emits a fully typed TypeScript class with a static parse(input: string) entry point that encodes the input, constructs the parser, and returns a typed parse tree on success or throws RDParserException on failure.

Step 3 — use the generated parser

import { ArithmeticParser } from './ArithmeticParser.js'
import { RDParserException } from '@configuredthings/rdp.js'

function tryParse(input: string): boolean {
  try {
    ArithmeticParser.parse(input)
    return true
  } catch (e) {
    if (e instanceof RDParserException) return false
    throw e
  }
}

tryParse('3 + 4 * 2')   // → true
tryParse('??')           // → false

With tracing support

Pass --observable to generate a parser that extends ObservableRDParser:

rdp-gen arithmetic.ebnf \
  --parser-name ArithmeticParser \
  --observable \
  --output ArithmeticParser.ts

Pass a DebugObserver as the second argument to static parse to print an indented call tree:

import { ArithmeticParser } from './ArithmeticParser.js'
import { DebugObserver } from '@configuredthings/rdp.js/observable'

ArithmeticParser.parse('3 + 4', new DebugObserver())
// → expr   pos:0
//   → term  pos:0
//   ← term  matched  pos:1
// ← expr   matched  pos:5

See Debugging for full ObservableRDParser documentation.

See it live