ulisp

Micro lisp implementation for study.

How to build

$ make ulisp

How to execute

$ ./ulisp

Special forms

• quote ... quote symbol
• cons ... construct pair
• atom ... predicate to check if next s-expression is atom or not (i.e. pair)
• car ... return the 1st expression of pair
• cdr ... return the 2nd expression of pair
• cond ... conditional construct. syntax: (cond (pred1 conseq1) [(pred2 conseq2) ...])
• set ... set variable to current environment
• lambda ... construct anonymous function. symtax: (lambda (params) body1 [body2 ...])

There is no value, so you CAN NOT use number nor string.

Example

$ ./ulisp
> (quote ulisp)
ulisp
> t
True
> (set (quote hello) (quote ulisp))
ulisp
> hello
ulisp
> (set (quote r) (lambda (x y) (cond ((atom x) y) (t (r (cdr x) (cons (car x) y))))))
(*applicable* (x y) (cond ((atom x) y) (t (r (cdr x) (cons (car x) y)))))
> (r (quote (a b c d e f)) ())
(f e d c b a)
> 

You can quit REPL with Ctrl+D.

Verbose evaluation

If you set *verbose-eval* non-nil value, each eval prints its evaluation process.

$ ./ulisp
> (set (quote *verbose-eval*) t)
True
> (set (quote map) (lambda (f xs) (cond
    ((atom xs) xs)
    ((quote else) (cons (f (car xs)) (map f (cdr xs)))))))
EVALUATE: (set (quote map) (lambda (f xs) (cond ((atom xs) xs) ((quote else) (cons (f (car xs)) (map f (cdr xs)))))))
|  env.*verbose-eval*=True
|  env.map=*applicable*
|  env.t=True
| EVALUATE: (quote map)
| |  env.*verbose-eval*=True
| |  env.map=*applicable*
| |  env.t=True
| \___ map
| EVALUATE: (lambda (f xs) (cond ((atom xs) xs) ((quote else) (cons (f (car xs)) (map f (cdr xs))))))
| |  env.*verbose-eval*=True
| |  env.map=*applicable*
| |  env.t=True
| \___ *applicable*
\___ *applicable*
*applicable*
> 

You can trun off this verbose print to set nil to *verbose-eval*.

> (set (quote *verbose-eval*) ()))

Acknowledgement

This work inspired heavily 小さな Lisp インタープリタ.