Small, embeddable LISP-like language.
C M4 Makefile Shell
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


This is a small, embeddable LISP-like language. The intended use is for configuration files where it is desirable to be able to have complex configurations (JWM, for example).

This is still a work in progress: there are still more functions to be implemented and the functionality of existing functions may change.


Like other LISP languages, JL uses s-expressions. For example:

(list 1 2 3)

calls the "list" function, passing 1, 2, and 3 as arguments.

Data Types

There are 6 data types:

  1. Numbers (floating point numbers)
  2. Strings
  3. Variables
  4. Lambdas (functions defined within the language)
  5. Lists
  6. Special functions

For comparisons, 0 and nil (the empty list) are considered false and all other values are considered true.


The following built-in functions are available:

  • < Test if less than
  •    Test if greater than
  • <= Test if less than or equal to
  • = Test if greater than or equal to

  • = Test if equal
  • != Test if not equal
    •    Return the sum of a list
    •    Subtract
    •    Return the produce of a list
  • / Divide
  • % Modulus
  • and Logical AND.
  • concat Concatenate strings.
  • cons Prepend an item to a list.
  • begin Execute a sequence of functions, return the value of the last.
  • define Insert a binding into the current namespace.
  • head Return the first element of a list
  • if Test a condition and evaluate and return the second argument if true, otherwise evaluate and return the third argument.
  • lambda Declare a function.
  • list Create a list
  • list? Determine if a value is a list.
  • not Logical NOT.
  • null? Determine if a value is nil.
  • number? Determine if a value is a number.
  • or Logical OR.
  • rest Return all but the first element of a list
  • string? Determine if a value is a string.
  • substr Return a substring of a string.


Here are some example programs. See the "examples" directory for more.

Return the factorial of a number:

(define fact (lambda (n)
(if n
(* (fact (- n 1)) n)
(fact 5)

Find the nth item of a list:

(define nth (lambda (n lst)
(if (<= n 1)
(head lst)
(nth (- n 1) (rest lst)))))
(nth 2 (list 1 2 3))

Find nth Fibonacci number:

(define fib (lambda (n)
(if (> n 1)
(+ (fib (- n 1)) (fib (- n 2)))
(fib 10)

The map function:

(define map (lambda (f lst)
(if lst
(cons (f (head lst)) (map f (rest lst)))
(map (lambda (x) (+ x 1)) (list 1 2 3 4))

The foldl function:

(define foldl (lambda (f i lst)
(if lst
(foldl f (f i (head lst)) (rest lst))
(foldl (lambda (a b) (+ a b)) 0 (list 1 2 3 4))

The reverse function implemented in terms of foldl:

(define reverse (lambda (lst) (foldl (lambda (a b) (cons b a)) (list) lst)))
(reverse (list 1 2 3 4))


JL uses the BSD 2-clause license. See LICENSE for more information.