λGPT — Natural Language REPL

A read-eval-print loop in Haskell that answers questions written in plain English. Built with parser combinators rather than a language model, so every answer is derived from the grammar rather than predicted.

What it does

Hello. Hi there!

What day is it tomorrow? Tomorrow is Sunday.

How long ago was 2026-02-01? 2026-02-01 was 13 days ago.

What is two plus three times four? The answer is twenty.

Remember that the sky is blue. Okay.

Tell me about the sky. Sure - the sky is blue.

What is that plus five? The answer is twenty-five.

How it works

Input is parsed into a Request algebraic data type, and respondTo pattern matches on that type to produce a reply. Parsing and responding are separated, so adding a feature means adding a constructor, a parser, and a case.

Parser combinators. Small parsers compose into larger ones. The arithmetic grammar is built from a number parser and three operator parsers rather than written as one function.

Arithmetic follows natural language, not maths. two plus three times four evaluates left to right as (2 + 3) × 4 = 20, because that is how the sentence reads aloud. Numbers are parsed from and printed back to English words.

State threads through the loop, so the chatbot remembers stored facts and the result of the previous expression, which that refers back to.

Structure

src/LGPT/TUI.hs the REPL, Request type, parsing and responses src/LGPT/Numbers.hs longhand number parser and printer src/LGPT/Helpers.hs convenience functions test/ test suite

Building

stack build
stack run
stack test

Notes

Written for CS141 Functional Programming at the University of Warwick, March 2026. The skeleton (TUI.hs scaffolding, Numbers.hs, Helpers.hs) was provided; the parsers, request algebra, evaluation and state handling are mine.