collect: Oops, forgot to export `collect-tail'.

[lisp] / mdw-base.lisp

;;; -*-lisp-*-
;;;
;;; $Id$
;;;
;;; Basic definitions
;;;
;;; (c) 2005 Mark Wooding
;;;

;;;----- Licensing notice ---------------------------------------------------
;;;
;;; This program is free software; you can redistribute it and/or modify
;;; it under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 2 of the License, or
;;; (at your option) any later version.
;;; 
;;; This program is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;; 
;;; You should have received a copy of the GNU General Public License
;;; along with this program; if not, write to the Free Software Foundation,
;;; Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

;;;--------------------------------------------------------------------------
;;; Package things.

(defpackage #:mdw.base
  (:use #:common-lisp)
  (:export #:compile-time-defun
	   #:show
	   #:stringify #:listify #:fix-pair #:pairify #:parse-body
	   #:whitespace-char-p
	   #:slot-uninitialized
	   #:nlet #:while #:case2 #:ecase2
	   #:with-gensyms #:let*/gensyms #:with-places
	   #:locp #:locf #:ref #:with-locatives
	   #:update-place #:update-place-after
	   #:incf-after #:decf-after))
(in-package #:mdw.base)

;;;--------------------------------------------------------------------------
;;; Some simple macros to get things going.

(defmacro compile-time-defun (name args &body body)
  "Define a function which can be used by macros during the compilation
process."
  `(eval-when (:compile-toplevel :load-toplevel)
     (defun ,name ,args ,@body)))

(defmacro show (x)
  "Debugging tool: print the expression X and its value."
  (let ((tmp (gensym)))
    `(let ((,tmp ,x))
       (format t "~&~S: ~S~%" ',x ,tmp)
       ,tmp)))

(defun stringify (str)
  "Return a string representation of STR.  Strings are returned unchanged;
symbols are converted to their names (unqualified!).  Other objects are
converted to their print representations."
  (typecase str
    (string str)
    (symbol (symbol-name str))
    (t (with-output-to-string (s)
	 (princ str s)))))

(compile-time-defun listify (x)
  "If X is a (possibly empty) list, return X; otherwise return (list X)."
  (if (listp x) x (list x)))

(compile-time-defun do-fix-pair (x y defaultp)
  "Helper function for fix-pair and pairify."
  (flet ((singleton (x) (values x (if defaultp y x))))
    (cond ((atom x) (singleton x))
	  ((null (cdr x)) (singleton (car x)))
	  ((atom (cdr x)) (values (car x) (cdr x)))
	  ((cddr x) (error "Too many elements for a pair."))
	  (t (values (car x) (cadr x))))))

(compile-time-defun fix-pair (x &optional (y nil defaultp))
  "Return two values extracted from X.  It works as follows:
  (A) -> A, Y
  (A B) -> A, B
  (A B . C) -> error
  (A . B) -> A, B
  A -> A, Y
where Y defaults to A if not specified."
  (do-fix-pair x y defaultp))

(compile-time-defun pairify (x &optional (y nil defaultp))
  "As for fix-pair, but returns a list instead of two values."
  (multiple-value-call #'list (do-fix-pair x y defaultp)))

(defun whitespace-char-p (ch)
  "Return whether CH is a whitespace character or not."
  (case ch
    ((#\space #\tab #\newline #\return #\vt #\formfeed) t)
    (t nil)))

(declaim (ftype (function nil ()) slot-unitialized))
(defun slot-uninitialized ()
  "A function which signals an error.  Can be used as an initializer form in
structure definitions without doom ensuing."
  (error "No initializer for slot."))

(compile-time-defun parse-body (body)
  "Given a BODY (a list of forms), parses it into three sections: a
docstring, a list of declarations (forms beginning with the symbol `declare')
and the body forms.  The result is returned as three lists (even the
docstring), suitable for interpolation into a backquoted list using `@,'."
  (multiple-value-bind
      (doc body)
      (if (and (consp body)
	       (stringp (car body)))
	  (values (list (car body)) (cdr body))
	  (values nil body))
    (loop for forms on body
	  for form = (car forms)
	  while (and (consp form)
		     (eq (car form) 'declare))
	  collect form into decls
	  finally (return (values doc decls forms)))))

;;;--------------------------------------------------------------------------
;;; Generating symbols.

(defmacro with-gensyms (syms &body body)
  "Everyone's favourite macro helper."
  `(let (,@(mapcar (lambda (sym) `(,sym (gensym ,(symbol-name sym))))
                   (listify syms)))
     ,@body))

(defmacro let*/gensyms (binds &body body)
  "A macro helper.  BINDS is a list of binding pairs (VAR VALUE), where VALUE
defaults to VAR.  The result is that BODY is evaluated in a context where
each VAR is bound to a gensym, and in the final expansion, each of those
gensyms will be bound to the corresponding VALUE."
  (labels ((more (binds)
             (let ((tmp (gensym "TMP")) (bind (car binds)))
               `((let ((,tmp ,(cadr bind))
                       (,(car bind) (gensym ,(symbol-name (car bind)))))
                   `(let ((,,(car bind) ,,tmp))
                      ,,@(if (cdr binds)
                             (more (cdr binds))
                             body)))))))
    (if (null binds)
        `(progn ,@body)
        (car (more (mapcar #'pairify (listify binds)))))))

;;;--------------------------------------------------------------------------
;;; Some simple yet useful control structures.

(defmacro nlet (name binds &body body)
  "Scheme's named let."
  (multiple-value-bind (vars vals)
      (loop for bind in binds
	    for (var val) = (pairify bind nil)
	    collect var into vars
	    collect val into vals
	    finally (return (values vars vals)))
    `(labels ((,name ,vars
	        ,@body))
       (,name ,@vals))))

(defmacro while (cond &body body)
  "If COND is false, evaluate to nil; otherwise evaluate BODY and try again."
  `(loop
     (unless ,cond (return))
     ,@body))

(compile-time-defun do-case2-like (kind vform clauses)
  "Helper function for `case2' and `ecase2'."
  (with-gensyms (scrutinee argument)
    `(multiple-value-bind (,scrutinee ,argument) ,vform
       (declare (ignorable ,argument))
       (,kind ,scrutinee
	 ,@(mapcar (lambda (clause)
		     (destructuring-bind
			 (cases (&optional var) &rest forms)
			 clause
		       `(,cases
			 ,@(if var
			       (list `(let ((,var ,argument)) ,@forms))
			       forms))))
		   clauses)))))

(defmacro case2 (vform &body clauses)
  "VFORM is a form which evaluates to two values, SCRUTINEE and ARGUMENT.
The CLAUSES have the form (CASES ([VAR]) FORMS...), where a standard `case'
clause has the form (CASES FORMS...).  The `case2' form evaluates the VFORM,
and compares the SCRUTINEE to the various CASES, in order, just like `case'.
If there is a match, then the corresponding FORMs are evaluated with VAR (if
specified) bound to the value of ARGUMENT."
  (do-case2-like 'case vform clauses))

(defmacro ecase2 (vform &body clauses)
  "Like `case2', but signals an error if no clause matches the SCRUTINEE."
  (do-case2-like 'ecase vform clauses))

;;;--------------------------------------------------------------------------
;;; with-places

(defmacro %place-ref (getform setform newtmp)
  "Grim helper macro for with-places."
  (declare (ignore setform newtmp))
  getform)

(define-setf-expander %place-ref (getform setform newtmp)
  "Grim helper macro for with-places."
  (values nil nil newtmp setform getform))

(defmacro with-places ((&key environment) places &body body)
  "A hairy helper, for writing setf-like macros.  PLACES is a list of binding
pairs (VAR PLACE), where PLACE defaults to VAR.  The result is that BODY is
evaluated in a context where each VAR is bound to a gensym, and in the final
expansion, each of those gensyms will be bound to a symbol-macro capable of
reading or setting the value of the corresponding PLACE."
  (if (null places)
      `(progn ,@body)
      (let*/gensyms (environment)
        (labels
            ((more (places)
               (let ((place (car places)))
                 (with-gensyms (tmp valtmps valforms
                                    newtmps setform getform)
                   `((let ((,tmp ,(cadr place))
                           (,(car place)
                            (gensym ,(symbol-name (car place)))))
                       (multiple-value-bind
                           (,valtmps ,valforms
                            ,newtmps ,setform ,getform)
                           (get-setf-expansion ,tmp
                                               ,environment)
                         (list 'let*
                               (mapcar #'list ,valtmps ,valforms)
                               `(symbol-macrolet ((,,(car place)
                                                   (%place-ref ,,getform
                                                               ,,setform
                                                               ,,newtmps)))
                                  ,,@(if (cdr places)
                                         (more (cdr places))
                                         body))))))))))
          (car (more (mapcar #'pairify (listify places))))))))

;;;--------------------------------------------------------------------------
;;; Update-in-place macros built using with-places.

(defmacro update-place (op place arg &environment env)
  "Update PLACE with the value of OP PLACE ARG, returning the new value."
  (with-places (:environment env) (place)
    `(setf ,place (,op ,place ,arg))))

(defmacro update-place-after (op place arg &environment env)
  "Update PLACE with the value of OP PLACE ARG, returning the old value."
  (with-places (:environment env) (place)
    (with-gensyms (x)
      `(let ((,x ,place))
	 (setf ,place (,op ,x ,arg))
	 ,x))))

(defmacro incf-after (place &optional (by 1))
  "Increment PLACE by BY, returning the old value."
  `(update-place-after + ,place ,by))

(defmacro decf-after (place &optional (by 1))
  "Decrement PLACE by BY, returning the old value."
  `(update-place-after - ,place ,by))

;;;--------------------------------------------------------------------------
;;; Locatives.

(defstruct (loc (:predicate locp) (:constructor make-loc (reader writer)))
  "Locative data type.  See `locf' and `ref'."
  (reader (slot-uninitialized) :type function)
  (writer (slot-uninitialized) :type function))

(defmacro locf (place &environment env)
  "Slightly cheesy locatives.  (locf PLACE) returns an object which, using
the `ref' function, can be used to read or set the value of PLACE.  It's
cheesy because it uses closures rather than actually taking the address of
something.  Also, unlike Zetalisp, we don't overload `car' to do our dirty
work."
  (multiple-value-bind
      (valtmps valforms newtmps setform getform)
      (get-setf-expansion place env)
    `(let* (,@(mapcar #'list valtmps valforms))
       (make-loc (lambda () ,getform)
                 (lambda (,@newtmps) ,setform)))))

(declaim (inline loc (setf loc)))

(defun ref (loc)
  "Fetch the value referred to by a locative."
  (funcall (loc-reader loc)))

(defun (setf ref) (new loc)
  "Store a new value in the place referred to by a locative."
  (funcall (loc-writer loc) new))

(defmacro with-locatives (locs &body body)
  "LOCS is a list of items of the form (SYM [LOC-EXPR]), where SYM is a
symbol and LOC-EXPR evaluates to a locative.  If LOC-EXPR is omitted, it
defaults to SYM.  As an abbreviation for a common case, LOCS may be a symbol
instead of a list.  The BODY is evaluated in an environment where each SYM is
a symbol macro which expands to (ref LOC-EXPR) -- or, in fact, something
similar which doesn't break if LOC-EXPR has side-effects.  Thus, references,
including `setf' forms, fetch or modify the thing referred to by the
LOC-EXPR.  Useful for covering over where something uses a locative."
  (setf locs (mapcar #'pairify (listify locs)))
  (let ((tt (mapcar (lambda (l) (declare (ignore l)) (gensym)) locs))
	(ll (mapcar #'cadr locs))
	(ss (mapcar #'car locs)))
    `(let (,@(mapcar (lambda (tmp loc) `(,tmp ,loc)) tt ll))
       (symbol-macrolet (,@(mapcar (lambda (sym tmp)
				     `(,sym (ref ,tmp))) ss tt))
	 ,@body))))

;;;----- That's all, folks --------------------------------------------------