Add new LFE exercises

One of the easiest ways to add new exercises to the LFE track is to translate them from other language tracks on Exercism. This page lists all of the problem specifications in the common pool of exercises which have not yet been implemented in LFE.

The icon links to the language-independent description of the problem.

The icon links to the canonical test data.

We also link to all the implementations of each exercise in other languages, which you can use for inspiration.

Once you've picked an exercise, follow the instructions in this guide and submit your pull request to the LFE track repository on GitHub.





Convert a number to a string, the contents of which depend on the number's factors.

Roman Numerals

Write a function to convert from normal numbers to Roman Numerals.

Scrabble Score

Given a word, compute the scrabble score for that word.

Collatz Conjecture

Calculate the number of steps to reach 1 using the Collatz conjecture

Pascals Triangle

Compute Pascal's triangle up to a given number of rows.

Nth Prime

Given a number n, determine what the nth prime is.


Use the Sieve of Eratosthenes to find all the primes from 2 up to a given number.

All Your Base

Convert a number, represented as a sequence of digits in one base, to any other base.

Perfect Numbers

Determine if a number is perfect, abundant, or deficient based on Nicomachus' (60 - 120 CE) classification scheme for natural numbers.

Prime Factors

Compute the prime factors of a given natural number.

Bracket Push

Make sure the brackets and braces all match.

Crypto Square

Implement the classic method for composing secret messages called a square code.

Pig Latin

Implement a program that translates from English to Pig Latin

Run Length Encoding

Implement run-length encoding and decoding.


Given a number from 0 to 999,999,999,999, spell out that number in English.

Secret Handshake

Given a decimal number, convert it to the appropriate sequence of events for a secret handshake.

Queen Attack

Given the position of two queens on a chess board, indicate whether or not they are positioned so that they can attack each other.

Flatten Array

Take a nested list and return a single list with all values except nil/null


Add the numbers to a minesweeper board

Two Fer

Create a sentence of the form "One for X, one for me."


Parse and evaluate simple math word problems returning the answer as an integer.


Given a letter, print a diamond starting with 'A' with the supplied letter at the widest point.


Write a function to determine if a list is a sublist of another list.

Binary Search Tree

Insert and search for numbers in a binary tree.

Rotational Cipher

Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.

List Ops

Implement basic list operations

Saddle Points

Detect saddle points in a matrix.

Ocr Numbers

Given a 3 x 4 grid of pipes, underscores, and spaces, determine which number is represented, or whether it is garbled.


Correctly determine change to be given using the least number of coins

Isbn Verifier

Check if a given string is a valid ISBN-10 number.

Linked List

Implement a doubly linked list

Palindrome Products

Detect palindrome products in a given range.

Protein Translation

Translate RNA sequences into proteins.

Pythagorean Triplet

There exists exactly one Pythagorean triplet for which a + b + c = 1000. Find the product a * b * c.


Take input text and output it transposed.

Food Chain

Generate the lyrics of the song 'I Know an Old Lady Who Swallowed a Fly'

Kindergarten Garden

Given a diagram, determine which plants each child in the kindergarten class is responsible for.


Given a string representing a matrix of numbers, return the rows and columns of that matrix.

Simple Cipher

Implement a simple shift cipher like Caesar and a more secure substitution cipher


Implement a basic reactive system.

Simple Linked List

Write a simple linked list implementation that uses Elements and a List

Twelve Days

Output the lyrics to 'The Twelve Days of Christmas'


Write a function to solve alphametics puzzles.

Complex Numbers

Implement complex numbers.


Implement an evaluator for a very simple subset of Forth


Compute the result for a game of Hex / Polygon


Output the nursery rhyme 'This is the House that Jack Built'.

Reverse String

Reverse a string

Spiral Matrix

Given the size, return a square matrix of numbers in spiral order.

Armstrong Numbers

Determine if a number is an Armstrong number


Make a chain of dominoes.


Pick the best hand(s) from a list of poker hands.

Rail Fence Cipher

Implement encoding and decoding for the rail fence cipher.


Tally the results of a small football competition.

Variable Length Quantity

Implement variable length quantity encoding and decoding.

Book Store

To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts of multiple-book purchases.


For want of a horseshoe nail, a kingdom was lost, or so the saying goes. Output the full text of this proverbial rhyme.


Count the rectangles in an ASCII diagram.

Two Bucket

Given two buckets of different size, demonstrate how to measure an exact number of liters.


Creating a zipper for a binary tree.

Diffie Hellman

Diffie-Hellman key exchange.


Refactor a Markdown parser


Reparent a graph on a selected node

Scale Generator

Generate musical scales, given a starting note and a set of intervals.

Word Search

Create a program to solve a word search puzzle.

Error Handling

Implement various kinds of error handling and resource management


Search a file for lines matching a regular expression pattern. Return the line number and contents of each matching line.

Zebra Puzzle

Solve the zebra puzzle.

Go Counting

Count the scored points on a Go board.

Rational Numbers

Implement rational numbers.

Dot Dsl

Write a Domain Specific Language similar to the Graphviz dot language

Sgf Parsing

Parsing a Smart Game Format string.

Tree Building

Refactor a tree building algorithm.


Implement the logic of the hangman game using functional reactive programming.


Refactor a ledger printer.

Lens Person

Use lenses to update nested records (specific to languages with immutable data).


Report network IO statistics


Score a single throw of dice in the game Yacht