Foreword by N. Janin: This is my attempt at making Rui Ueyama (rui314)'s MiniLisp slightly more user friendly and powerful. Not being limited by the 1000 lines challenge, I've added a number of basic primitives to the original program, while trying to keep the goal of simplicity and conciseness.
The whole program compiles to less than 100 kb without debugging symbols and should be able to run on low powered devices.
The added primitives:
- strings and conversion
- predicates >, >=, <=, not,
- functions list, atom, length, reverse, progn, load.
This has the side effect of being much faster as well, since all these primitives are compiled instead of being interpreted.
Among the bells and whistles, I've added a Read-Eval-Print-Loop (REPL) based on Justine Tunney (jart)'s bestline.
In this version, instead of passing a file using pipes, you simply pass the files as command parameters : ./minilisp f1 f2 etc
The files all share the same environment, so all the symbols, functions and macros defined in f1 can be reused in the following files. The REPL is summonned after execution, unless we pass the option -r to the command line, in which case the interpreted quits immediately at the end.
You can also pass a simple Lisp command as parameter: ./minilisp -x "(+ 1 1)"
The Lisp command will be evaluated and then the REPL is summonned. However we can quit immediately after execution with -r (or --no-repl).
CTRL-Enter CONTINUE ON NEXT LINE
CTRL-E END
CTRL-A START
CTRL-B BACK
CTRL-F FORWARD
CTRL-L CLEAR
CTRL-H BACKSPACE
CTRL-D DELETE
CTRL-Y YANK
CTRL-D EOF (IF EMPTY)
CTRL-N NEXT HISTORY
CTRL-P PREVIOUS HISTORY
CTRL-R SEARCH HISTORY
CTRL-G CANCEL SEARCH
ALT-< BEGINNING OF HISTORY
ALT-> END OF HISTORY
ALT-F FORWARD WORD
ALT-B BACKWARD WORD
CTRL-ALT-F FORWARD EXPR
CTRL-ALT-B BACKWARD EXPR
ALT-RIGHT FORWARD EXPR
ALT-LEFT BACKWARD EXPR
CTRL-K KILL LINE FORWARDS
CTRL-U KILL LINE BACKWARDS
ALT-H KILL WORD BACKWARDS
CTRL-W KILL WORD BACKWARDS
CTRL-ALT-H KILL WORD BACKWARDS
ALT-D KILL WORD FORWARDS
ALT-Y ROTATE KILL RING AND YANK AGAIN
ALT-\ SQUEEZE ADJACENT WHITESPACE
CTRL-T TRANSPOSE
ALT-T TRANSPOSE WORD
ALT-U UPPERCASE WORD
ALT-L LOWERCASE WORD
ALT-C CAPITALIZE WORD
CTRL-C INTERRUPT PROCESS
CTRL-Z SUSPEND PROCESS
CTRL-\ QUIT PROCESS
CTRL-S PAUSE OUTPUT
CTRL-Q UNPAUSE OUTPUT (IF PAUSED)
CTRL-Q ESCAPED INSERT
CTRL-SPACE SET MARK
CTRL-X CTRL-X GOTO MARK
CTRL-Z SUSPEND PROCESS
The REPL also saves the history of commands in the file history.txt This file is loaded at startup, so one can recall previous commands.
Known bugs:
- recall of multiline commands does not work as expected.
- this doesn't have tail call optimization, so expect crashes with sometimes with surprisingly short lists.
One day I wanted to see what I can do with 1k lines of C and decided to write a Lisp interpreter. That turned to be a fun weekend project, and the outcome is a mini lisp implementation that supports
- integers, symbols, cons cells
- global variables,
- lexically-scoped local variables,
- closures,
- if conditional,
- primitive functions, such as +, =, <, or list,
- user-defined functions,
- a macro system,
- and a copying garbage collector.
All those in 1000 lines of C. I didn't sacrifice readability for size. The code is in my opinion heavily commented to help the reader understand how all these features work.
$ make
MiniLisp has been tested on Linux x86/x86-64 and 64 bit Mac OS. The code is not very architecture dependent, so you should be able to compile and run on other Unix-like operating systems.
MiniLisp comes with a comprehensive test suite. In order to run the tests, give "test" argument to make.
$ make test
MiniLisp is a traditional Lisp interpreter. It reads one expression at a time from the standard input, evaluates it, and then prints out the return value of the expression. Here is an example of a valid input.
(+ 1 2)
The above expression prints "3".
MiniLisp supports integer literals, (), t, symbols, and list literals.
- Integer literals are positive or negative integers.
()is the only false value. It also represents the empty list.tis a predefined variable evaluated to itself. It's a preferred way to represent a true value, while any non-()value is considered to be true.- Symbols are objects with unique name. They are used to represent identifiers. Because MiniLisp does not have string type, symbols are sometimes used as a substitute for strings too.
- List literals are cons cells. It's either a regular list whose last element's
cdr is
()or an dotted list ending with any non-()value. A dotted list is written as(a . b).
cons takes two arguments and returns a new cons cell, making the first
argument the car, and the second the cdr.
(cons 'a 'b) ; -> (a . b)
(cons 'a '(b)) ; -> (a b)
car and cdr are accessors for cons cells. car returns the car, and cdr
returns the cdr.
(car '(a . b)) ; -> a
(cdr '(a . b)) ; -> b
setcar mutates a cons cell. setcar takes two arguments, assuming the first
argument is a cons cell. It sets the second argument's value to the cons cell's
car.
(define cell (cons 'a 'b))
cell ; -> (a . b)
(setcar cell 'x)
cell ; -> (x . b)
length and reverse operate either on their arguments, or a single list or a string.
(length '(1 2 3)) ; -> 3
(length 1 2 t) ; -> 3
(length "1 2 3") ; -> 5
(reverse '(a b c)) ; -> (c b a)
(reverse "1234") ; -> "4321"
(reverse '((a) b "c")) ; -> (c b (a))
+ returns the sum of the arguments.
(+ 1) ; -> 1
(+ 1 2) ; -> 3
(+ 1 2 3) ; -> 6
- negates the value of the argument if only one argument is given.
(- 3) ; -> -3
(- -5) ; -> 5
If multiple arguments are given, - subtracts each argument from the first one.
(- 5 2) ; -> 3
(- 5 2 7) ; -> -4
= takes two arguments and returns t if the two are the same integer.
(= 11 11) ; -> t
(= 11 6) ; -> ()
< takes two arguments and returns t if the first argument is smaller than
the second.
(< 2 3) ; -> t
(< 3 3) ; -> ()
(< 4 3) ; -> ()
The other numerical predicates >, <=, >= work in a similar fashion.
(if cond then else) is the only conditional in the language. It first
evaluates cond. If the result is a true value, then is evaluated. Otherwise
else is evaluated.
(while cond expr ...) executes expr ... until cond is evaluated to
(). This is the only loop supported by MiniLisp.
If you are familiar with Scheme, you might be wondering if you could write a loop by tail recursion in MiniLisp. The answer is no. Tail calls consume stack space in MiniLisp, so a loop written as recursion will fail with the memory exhaustion error.
(progn expr expr ...) executes several expressions in sequence.
( progn (print "I own ")
(defun add(x y)(+ x y))
(print (add 3 7)
(println " cents") ) ; -> prints "I own 10 cents"
eq takes two arguments and returns t if the objects are the same. What eq
really does is a pointer comparison, so two objects happened to have the same
contents but actually different are considered to not be the same by eq.
eq can also compare two strings.
eq can also compare two strings.
(eq "Hello" "Hello") ; -> t
(eq "Hello" "hello") ; -> ()
string-concat concatenates strings.
(string-concat) ; -> ""
(string-concat "A" "B" "C" "D") ; -> "ABCD"
symbol->string turns a symbol into a string.
(define sym 'hello) ; -> hello
(symbol->string sym) ; -> "hello"
string->symbol turns a string into a symbol of the same name.
(string->symbol "hello") ; -> hello
print prints a given object to the standard output.
(print 3) ; -> "3"
(print '(hello world)) ; -> "(hello world)"
(print "hello" "world") ; -> "hello world"
println does the same, adding a return at the end.
MiniLisp supports variables and functions. They can be defined using define.
(define a (+ 1 2))
(+ a a) ; -> 6
There are two ways to define a function. One way is to use a special form
lambda. (lambda (args ...) expr ...) returns a function object which
you can assign to a variable using define.
(define double (lambda (x) (+ x x)))
(double 6) ; -> 12
((lambda (x) (+ x x)) 6) ; do the same thing without assignment
The other way is defun. (defun fn (args ...) expr ...) is short for
(define fn (lambda (args ...) expr ...).
;; Define "double" using defun
(defun double (x) (+ x x))
You can write a function that takes variable number of arguments. If the parameter list is a dotted list, the remaining arguments are bound to the last parameter as a list.
(defun fn (expr . rest) rest)
(fn 1) ; -> ()
(fn 1 2 3) ; -> (2 3)
Variables are lexically scoped and have indefinite extent. References to "outer" variables remain valid even after the function that created the variables returns.
;; A countup function. We use lambda to introduce local variables because we
;; do not have "let" and the like.
(define counter
((lambda (count)
(lambda ()
(setq count (+ count 1))
count))
0))
(counter) ; -> 1
(counter) ; -> 2
;; This will not return 12345 but 3. Variable "count" in counter function
;; is resolved based on its lexical context rather than dynamic context.
((lambda (count) (counter)) 12345) ; -> 3
setq sets a new value to an existing variable. It's an error if the variable
is not defined.
(define val (+ 3 5))
(setq val (+ val 1)) ; increment "val"
atom returns () if the argument is a cell, t otherwise.
(atom '(a b)) ; -> ()
(atom "") ; -> t
(atom ()) ; -> t
load loads a Lisp file and evaluates all its content, adding it to the environment.
(load "example/nqueens.lisp") -> run the file and store its evaluated functions
and macros
exit quits the interpreter and returns the integer passed as parameter.
(exit 0) -> quit with success
Macros look similar to functions, but they are different that macros take an
expression as input and returns a new expression as output. (defmacro macro-name (args ...) body ...) defines a macro. Here is an example.
(defmacro unless (condition expr)
(list 'if condition () expr))
The above defmacro defines a new macro unless. unless is a new conditional
which evaluates expr unless condition is a true value. You cannot do the
same thing with a function because all the arguments would be evaluated before
the control is passed to the function.
(define x 0)
(unless (= x 0) '(x is not 0)) ; -> ()
(unless (= x 1) '(x is not 1)) ; -> (x is not 1)
macroexpand is a convenient special form to see the expanded form of a macro.
(macroexpand (unless (= x 1) '(x is not 1)))
;; -> (if (= x 1) () (quote (x is not 1)))
gensym creates a new symbol which will never be eq to any other symbol other
than itself. Useful for writing a macro that introduces new identifiers.
(gensym) ; -> a new symbol
As in the traditional Lisp syntax, ; (semicolon) starts a single line comment.
The comment continues to the end of line.