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.

Goals

The goal of the project was to learn about the internals of programming languages and to build a simple language suitable for scripting and similar use cases.

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 core project does not have any dependencies to external Composer libraries, but it does currently 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 start 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

You can evaluate a file by giving it as argument:

$ php run.php file.mad

With no arguments the script will read input from stdin:

$ echo "(+ 1 2 3)" | php run.php
6

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 wrapped in a class, and the only difference between the two is how they are evaluated. Another reason for adding vectors is the familiarity of the square bracket syntax for PHP developers. They can be thought of as PHP arrays for most intents and purposes.

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 wrapped in a class.

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)

Environments

Environments are hash-maps which store key-value pairs and use symbols as keys. Symbols are evaluated by looking up the corresponding value from the current environment. If the key is not defined in current environment the lookup proceeds to the parent environment and so forth. The initial environment is called root and contains all the built-in functions listed here. Then another environment called user is created for anything the user wants to define. The let and fn special forms create new local environments. Note that def always uses the current environment, so anything defined with def is not visible in the parent environment.

Special forms

Name	Example	Example result	Description
and	(and 1 0 2)	0	Return the first value that evaluates to 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 evaluates to true.
(case (= 1 0) 0 "no match")	"no match"	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	(if (< 1 2) "yes" "no")	"yes"	If the first argument evaluates to true, evaluate and return the second argument, otherwise the third argument. If the third argument is omitted return null in its place.
let	(let (a (+ 1 2)) a)	3	Create a new local environment using the first argument (list) to define values. Odd arguments are treated as keys and even arguments are treated as values. The last argument is the body of the let-expression which is evaluated using this new environment.
load	(load "file.mad")		Read and evaluate a file. The contents are implicitly wrapped in a do expression.
or	(or false 0 1)	1	Return the first value that evaluates to true, or the last value.
quote	(quote (1 2 3))	(1 2 3)	Return the argument without evaluation. This is same as the ' shortcut described above.

Functions

Core functions

Name	Example	Example result	Description
doc	(doc +)	"Return the sum of all arguments."	Show description of a built-in function.
read	(read "(+ 1 2 3)")	(+ 1 2 3)	Read a string as code and return the expression.
print	(print "hello world")	"hello world"null	Print expression on the screen. Print returns null (which is shown due to the extra print in repl). Give optional second argument as true to show strings in readable format.
error	(error "invalid value")	error: invalid value	Throw an exception with message as argument.

Collection functions

Name	Example	Example result	Description
hash	(hash "a" 1 "b" 2)	{"a":1 "b":2}	Create a new hash-map.
list	(list 1 2 3)	(1 2 3)	Create a new list.
vector	(vector 1 2 3)	[1 2 3]	Create a new vector.
range	(range 2 5)	[2 3 4]	Create a vector with integer values from first to argument (inclusive) to second argument (exclusive).
range	(range 5)	[0 1 2 3 4]	Range can also be used with one argument in which case it is used as length for a vector of integers starting from 0.
empty?	(empty? [])	true	Return true if collection is empty, otherwise false.
get	(get [1 2 3] 0)	1	Return the nth element from a sequence, or the corresponding value for the given key from a hash-map.
len	(len [1 2 3])	3	Return the number of elements in a collection.
first	(first [1 2 3 4])	1	Return the first element of a sequence.
second	(second [1 2 3 4])	2	Return the second element of a sequence.
penult	(penult [1 2 3 4])	3	Return the second-last element of a sequence.
last	(last [1 2 3 4])	4	Return the last element of a sequence.
head	(head [1 2 3 4])	[1 2 3]	Return new sequence which contains all elements except the last.
tail	(tail [1 2 3 4])	[2 3 4]	Return new sequence which contains all elements except the first.
apply	(apply + 1 2 [3 4])	10	Call the first argument using a sequence as argument list. Intervening arguments are prepended to the list.
chunk	(chunk [1 2 3 4 5] 2)	[[1 2] [3 4] [5]]	Divide a sequence to multiple sequences with specified length using array_chunk.
push	(push [1 2] 3 4)	[1 2 3 4]	Create new sequence by inserting arguments at the end.
pull	(pull 1 2 [3 4])	[1 2 3 4]	Create new sequence by inserting arguments at the beginning.
map	(map (fn (a) (* a 2)) [1 2 3])	[2 4 6]	Create new sequence by calling a function for each item. Uses array_map internally.
map2	(map2 + [1 2 3] [4 5 6])	[5 7 9]	Create new sequence by calling a function for each item from both sequences.
reduce	(reduce + [2 3 4] 1)	10	Reduce a sequence to a single value by calling a function sequentially of all arguments. Optional third argument is used to give the initial value for first iteration. Uses array_reduce internally.
filter	(filter odd? [1 2 3 4 5])	[1 3 5]	Create a new sequence by using the given function as a filter. Uses array_filter internally.
reverse	(reverse [1 2 3])	[3 2 1]	Reverse the order of a sequence. Uses array_reverse internally.
key?	(key? {"a" "b"} "a")	true	Return true if the hash-map contains the given key.
set	(set {"a" 1} "b" 2)	{"a":1 "b":2}	Create new hash-map which contains the given key-value pair.
set!	(set! {"a" 1} "b" 2)	2	Modify the given hash-map by setting the given key-value pair and return the set value. This function is mutable!
keys	(keys {"a" 1 "b" 2})	("a" "b")	Return a list of the keys for a hash-map.
values	(values {"a" 1 "b" 2})	(1 2)	Return a list of the values for a hash-map.
zip	(zip ["a" "b"] [1 2])	{"a":1 "b":2}	Create a hash-map using the first sequence as keys and the second as values. Uses array_combine internally.
sort	(sort [6 4 8 1])	[1 4 6 8]	Sort the sequence using sort.

Comparison functions

Name	Example	Example result	Description
=	(= 1 "1")	true	Compare arguments for equality using the == operator in PHP.
==	(== 1 "1")	false	Compare arguments for strict equality using the === operator in PHP.
!=	(!= 1 "1")	false	Compare arguments for not-equality using the != operator in PHP.
!==	(!== 1 "1")	true	Compare arguments for strict not-equality using the !== operator in PHP.
<	(< 1 2)	true	Return true if first argument is less than second.
<=	(<= 1 2)	true	Return true if first argument is less or equal to second.
>	(> 1 2)	false	Return true if first argument is greater than second.
>=	(>= 1 2)	false	Return true if first argument is greater or equal to second.

IO functions

Name	Example	Example result	Description
wd	(wd)	"/home/pekka/code/madlisp/"	Get the current working directory.
chdir	(chdir "/tmp")	true	Change the current working directory.
file?	(file? "test.txt")	true	Return true if the file exists.
fread	(fread "test.txt")	"content"	Read the contents of a file.
fwrite	(fwrite "test.txt" "content")	true	Write string to file. Give optional third parameter as true to append.

Json functions

Name	Example	Example result	Description
to-json	(to-json { "a" [1 2 3] "b" [4 5 6] })	"{\"a\":[1,2,3],\"b\":[4,5,6]}"	Encode the argument as a JSON string.
from-json	(from-json "{\"a\":[1,2,3],\"b\":[4,5,6]}")	{"a":[1 2 3] "b":[4 5 6]}	Decode the JSON string given as argument.

Math functions

Name	Example	Example result	Description
+	(+ 1 2 3)	6	Return the sum of the arguments.
-	(- 4 2 1)	1	Subtract the other arguments from the first.
*	(* 2 3 4)	24	Multiply the arguments.
/	(/ 7 2)	3.5	Divide the arguments.
//	(// 7 2)	3	Divide the arguments using integer division.
%	(% 6 4)	2	Calculate the modulo.
inc	(inc 1)	2	Increment the argument by one.
dec	(dec 2)	1	Decrement the argument by one.
sin	(sin 1)	0.84	Calculate the sine.
cos	(cos 1)	0.54	Calculate the cosine.
tan	(tan 1)	1.55	Calculate the tangent.
abs	(abs -2)	2	Get the absolute value.
floor	(floor 2.5)	2	Get the next lowest integer.
ceil	(ceil 2.5)	3	Get the next highest integer.
pow	(pow 2 4)	16	Raise the first argument to the power of the second argument.
sqrt	(sqrt 2)	1.41	Calculate the square root.

String functions

Name	Example	Example result	Description
len	(len "hello world")	11	Return the length of a string using strlen.
trim	(trim " abc ")	"abc"	Trim the string using trim.
upcase	(upcase "abc")	"ABC"	Make the string upper case using strtoupper.
lowcase	(lowcase "Abc")	"abc"	Make the string lower case using strtolower.
substr	(substr "hello world" 3 5)	"lo wo"	Get a substring using substr.
replace	(replace "hello world" "hello" "bye")	"bye world"	Replace substrings using str_replace.
split	(split "-" "a-b-c")	("a" "b" "c")	Split string using explode.
join	(join "-" "a" "b" "c")	"a-b-c"	Join string together using implode.
format	(format "%d %.2f" 56 4.5)	"56 4.50"	Format string using sprintf.

Note that support for multibyte characters in strings is limited because the provided functions do not use the mbstring extension.

Time functions

Name	Example	Example result	Description
time	(time)	1592011969	Return the current unix timestamp using time.
date	(date "Y-m-d H:i:s")	"2020-06-13 08:33:29"	Format the current time and date using date.
strtotime	(strtotime "2020-06-13 08:34:47")	1592012087	Parse datetime string into unix timestamp using strtotime.
sleep	(sleep 2000)	null	Sleep for the given period given in milliseconds using usleep.

Type functions

Skipped examples here as these are pretty self-explanatory.

Name	Description
bool	Convert the argument to boolean.
float	Convert the argument to floating-point value.
int	Convert the argument to integer.
str	Convert the argument to string. Also concatenate multiple strings together.
symbol	Convert the argument to symbol.
not	Turns true to false and vice versa.
type	Return the type of the argument as a string.
fn?	Return true if the argument is a function.
list?	Return true if the argument is a list.
vector?	Return true if the argument is a vector.
seq?	Return true if the argument is a sequence (list or vector).
hash?	Return true if the argument is a hash-map.
symbol?	Return true if the argument is a symbol.
object?	Return true if the argument is an object.
resource?	Return true if the argument is a resource.
bool?	Return true if the argument is a boolean value (strict comparison).
true?	Return true if the argument evaluates to true (non-strict comparison).
false?	Return true if the argument evaluates to false (non-strict comparison).
null?	Return true if the argument is null (strict comparison).
int?	Return true if the argument is an integer.
float?	Return true if the argument is a floating-point value.
str?	Return true if the argument is a string.
zero?	Return true if the argument is integer 0 (strict comparison).
one?	Return true if the argument is integer 1 (strict comparison).
even?	Return true if the argument is even number (0, 2, 4, ...).
odd?	Return true if the argument is odd number (1, 3, 5, ...).

Constants

The following constants are defined by default:

Name	PHP constant
dirsep	DIRECTORY_SEPARATOR
homedir	$_SERVER['HOME']
ln	PHP_EOL
pi	M_PI

Extending

The project is easy to extend because it is trivial to add new functions whether the implementation is defined on the PHP or Lisp side.

License

MIT