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To run tests for Lambda Calculus programs, Codewars uses a custom compiling and testing framework, published and available in this GitHub repository.

The framework is hosted in Javascript, and as such, both the sample and submission tests of Kata must be written in Javascript, while the preloaded section, example solution and user solution are Lambda Calculus.

Please refer to specific documentation for further information on the modules which lc-test utilizes.

Basic Usage#

Example tests#

import { assert, config, LC, getSolution } from "./lc-test.js";

LC.configure({purity: "Let", numEncoding: "Church"});
const { multiply } = LC.compile(getSolution());

describe("Fixed tests", () => {
  it("can multiply church numbers", () => {
    assert.numEql(multiply(5)(0), 0);
    assert.numEql(multiply(1)(8), 8);
    assert.numEql(multiply(7)(9), 63);


lc-test provides the following utilities:

  • chai.assert as assert
  • chai.config as config
  • LC = { compile, configure, fromInt, toInt }
  • getSolution, getPreloaded and getSolutionWithPreloaded


assert is the standard chai.assert, extended with the following additional assertion:

assert.numEql(actual, expected [ , message ] )

actual and expected are automatically decoded according to the current numEncoding, if necessary, before making an equality assertion between them. A custom failure message can optionally be set with message.


config is merely a re-export of chai.config. A notable use case is allowing arrays to be displayed fully in assertion messages, using config.truncateThreshold = 0.

getSolution / getPreloaded / getSolutionWithPreloaded#

getSolution() returns the user Solution code, getPreloaded returns the author Preloaded code, getSolutionWithPreloaded() returns a safe version of Preloaded and Solution code, concatenated.


LC.compile(getSolution()) returns a JavaScript Object with all top level definitions from the compiled Lambda Calculus source code. Each definition is a Function which can be directly called.


Use destructuring to conveniently extract the desired definitions, renaming them to valid Javascript identifiers.

const solution = LC.compile(getSolution());
const { "is-empty": isEmpty } = solution;

Note that definitions in Lambda Calculus source code can be overwritten; for each name defined multiple times, only the last definition will be exported.


Default configuration is

{ verbosity: "Calm"    //  Calm | Concise | Loquacious | Verbose
, purity: "PureLC"     //  Let | LetRec | PureLC
, numEncoding: "None"  //  None | Church | Scott | BinaryScott

LC.configure can be used to change the settings, by passing an object with key-values of the settings to be changed. For example LC.configure({purity: "Let", numEncoding: "Scott"});

verbosity controls the amount of debug and error information displayed.

purity controls the strictness/purity of the Lambda Calculus syntax allowed.

  • PureLC ensures that all terms must be self-contained, using no external definitions, including itself.
  • Let allows defined terms to be used in later terms.
  • LetRec allows defined terms to be used in later terms, and additionally allows a term to reference itself (direct recursion).

numEncoding selects the specific number encoding to be used when compiling number literals, or conversion functions such as toInt. The encoding may be one of the predefined encodings, a custom encoding, or "None".

  • Custom encodings can be set by assigning to numEncoding an object with both toInt and fromInt attribute functions to perform conversion.
  • "None" will cause number literals in code to throw an error.

toInt / fromInt#

These functions use the current numEncoding for converting between functions (i.e. Lambda Calculus terms), and Javascript numbers.

toInt converts a function to a Javascript number.

fromInt converts a Javascript number to a function.


lc-test and lambda-calculus was authored by @JohanWiltink and @Kacarott.