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https://github.com/rui314/minilisp

A readable lisp in less than 1k lines of C
https://github.com/rui314/minilisp

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A readable lisp in less than 1k lines of C

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README 

        
MiniLisp

========



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.



Compile

-------



    $ 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.



Test

----



MiniLisp comes with a comprehensive test suite. In order to run the tests, give

"test" argument to make.



    $ make test



Language features

-----------------



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".



### Literals



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.

* `t` is 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)`.



### List operators



`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)



### Numeric operators



`+` 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)    ; -> ()



### Conditionals



`(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.



### Loops



`(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.



### Equivalence test operators



`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`.



### Output operators



`println` prints a given object to the standard output.



    (println 3)               ; prints "3"

    (println '(hello world))  ; prints "(hello world)"



### Definitions



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"



### Macros



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



### Comments



As in the traditional Lisp syntax, `;` (semicolon) starts a single line comment.

The comment continues to the end of line.



No GC Branch

------------



There is a MiniLisp branch from which the code for garbage collection has been

stripped. The accepted language is the same, but the code is simpler than the

master branch's one. The reader might want to read the nogc branch first, then

proceed to the master branch, to understand the code step by step.



The nogc branch is available at

[nogc](https://github.com/rui314/minilisp/tree/nogc). The original is available

at [master](https://github.com/rui314/minilisp).