guile-paip/common-lisp-to-scheme.org

Common Lisp to Scheme translation

Straight forward translations

• defun -> define
• defvar -> define or let
• setf -> define or let
• function or #' -> nothing, no difference is made in referring to values or procedures, Scheme is a Lisp 1.
• mapcar -> map
• funcall -> simply call the procedure, s-expr with procedure in the first place

Others

NIL

In Common Lisp NIL is a defined symbol:

(define NIL '())

car

car of an empty list in Common Lisp returns NIL, the empty list again. car in Scheme does not return the empty list, but raises an error. This means, that, for compatibility reasons, we have to write a little procedure, which acts like car from Common Lisp:

<<def-nil>>

(define car/nil
  (lambda (possibly-empty-list)
    "In Common Lisp car of an empty list returns the empty list called NIL. In
Scheme it would be an error to call car on an empty list. We write a wrapper, to
avoid an error."
    (cond
     [(null? possibly-empty-list) NIL]
     [else (car possibly-empty-list)])))

rest

rest in Common Lisp also returns NIL, when called with the empty list as argument, so a wrapper is needed:

<<def-nil>>

(define rest
  (lambda (lst)
    (cond
     [(null? lst) NIL]
     [else (cdr lst)])))

defparameter

A parameter is explained in the book as a binding, that usually does not chance, a constant. A change to it is explained as a change to the program, not a change by the program, in contrast to using defvar.

Since in Scheme we usually try to avoid using assignment or set! anyway, we can get away with following a convention of naming constants with enclosing asterisks: *constant* and using define.

assoc

In Common Lisp assoc returns NIL, when there is no matching entry in the association list. In Scheme #f is returned instead. We write a wrapper:

(use-modules
 ((srfi srfi-1) #:prefix srfi-1:))

<<def-nil>>

(define assoc
  (lambda (key alist)
    (let ([res (srfi-1:assoc key alist)])
      (cond
       [res res]
       [else NIL]))))

case

???