MadLisp

MadLisp is a Lisp interpreter written in PHP. It is inspired by the Make a Lisp project, but does not follow that convention or syntax strictly.

Features

The implementation is pretty minimalistic, but there is a good collection of built-in functions. Also tail call optimization is included.

Requirements

The project requires PHP 7.4 or newer.

The project does not have any dependencies to external Composer libraries, but it does use Composer for the autoloader so you need to run composer install for that.

Usage

Use the run.php file to invoke the interpreter. You can invoke the Repl with the -r switch:

$ php run.php -r

You can also evaluate code directly with the -e switch:

$ php run.php -e "(+ 1 2 3)"
6

Or you can evaluate a file with the -f switch:

$ php run.php -f file.mad

Types

Numbers

Numeric literals are interpreted as integer or floating point values. For example 1 or 1.0.

Strings

Strings are limited by double quotes, for example "this is a string".

Comments

Comments start with semicolon ; and end on a newline character.

Keywords

Special keywords are true, false and null which correspond to same PHP values.

Sequences

Lists are limited by parenthesis. When they are evaluated, the first item of a list is called as a function with the remaining items as arguments. They can be defined using the built-in list function:

> (list 1 2 3)
(1 2 3)

Vectors are defined using square brackets or the built-in vector function:

> [1 2 3]
[1 2 3]

(vector 4 5 6)
[4 5 6]

Internally lists and vectors are just PHP arrays, and the only difference is how they are evaluated.

Hash maps

Hash maps are collections of key-value pairs. Keys are normal strings, not "keywords" starting with colon characters as in many Lisp languages.

Hash maps are defined using curly brackets or using the built-in hash function. Odd arguments are treated as keys and even arguments are treated as values. The key-value pair can optionally include colon as a separator to make it more readable, but it is ignored internally.

> (hash "a" 1 "b" 2)
{"a":1 "b":2}

> {"key":"value"}
{"key":"value"}

Internally hash maps are just regular associative PHP arrays.

Symbols

Symbols are words which do not match any other type and are separated by whitespace. They can contain special characters. Examples of symbols are a, name or +.

Quoting

The special single quote character can be used to quote an expression (skip evaluation).

> '(1 2 3)
(1 2 3)

Special forms

Name	Example	Example result	Description
and	(and 1 0 2)	0	Return the first value that is false, or the last value.
case	(case (= 1 0) 0 (= 1 1) 1)	1	Treat odd arguments as tests and even arguments as values. Evaluate and return the value after the first test that evalutes to true.
(case (= 1 0) 0 1)	1	You can also give optional last argument to case which is returned if none of the tests evaluated to true.
def	(def addOne (fn (a) (+ a 1)))	<function>	Define a value in the current environment.
do	(do (print 1) 2)	12	Evaluate multiple expressions and return the value of the last.
env	(env +)	<function>	Return a definition from the current environment represented by argument. Without arguments return the current environment as a hash-map.
eval	(eval (quote (+ 1 2)))	3	Evaluate the argument.
fn	(fn (a b) (+ a b))	<function>	Create a function.
if
let
load
or
quote

Core functions

Name	Example	Description
doc	(doc +)	Show description of a built-in function.
read	(read "(+ 1 2 3)")	Read a string as code and return the expression.
print	(print "hello world")	Print expression on the screen.
error	(error "invalid value")	Throw an exception with message as argument.

License

MIT