Title
Handling rest list
Author
Joo ChurlSoo
Related SRFIs
A procedure and 4 macros proposed in this SRFI make a strong combination with
RECEIVE (SRFI 8) and LET-VALUES (SRFI 11).
Abstract
This SRFI introduces the REST-VALUES procedure which has two modes of
operation:
1. it processes a rest list after checking its elements with default values or
predicate procedures,
2. it processes a rest list with default values without checking its elements;
the ARG-AND, ARG-ANDS, ARG-OR, and ARG-ORS macros check the rest arguments
that are returned by REST-VAULES that functions as the latter.
Rationale
When defining a procedure with a variable number of arguments, REST-VALUES
with or without ARG-AND (or ARG-ANDS or ARG-OR or ARG-ORS) reduces the clutter
of various conditionals and error conditions.
Specification
(REST-VALUES [] [] ...)
* is a procedure that has
* should be an integer or #f or #t.
* 1. When the is missing or #t or a positive integer,
each should be a list that contains default value(s), or a
pair whose car is a default value and whose cdr is a predicate
procedure.
2. When the is #f or a negative integer,
each is any scheme expression.
1. (first mode of operation)
REST-VALUES checks whether each element of the is a member of
the corresponding list, or satisfies the predicate procedure of
the corresponding pair, and then returns the checked element(s).
If the element doesn't pass, REST-VALUES signals an error. When there are
no more elements in the , then REST-VALUES additionally returns
the car values of the remaining s. On the other hand, when the
number of elements of the are more than the number of the
s, the supernumerary elements are additionally returned if the
is missing or #t, or its value is not less than the
number of elements of the .
2. (second mode of operation)
This is the same as the first except that REST-VALUES does not check each
element of the , and it uses #f instead of #t, and an absolute
value instead of a simple value as the value of .
(ARG-AND [] ...)
(ARG-ANDS [] ( ...) ...)
* is a procedure that has each as an argument.
* Each should contain the corresponding .
These macros are the same as AND except that these signal an error in
case AND returns a false value.
(ARG-OR [] ...)
(ARG-ORS [] ( ...) ...)
* is a procedure that has each as an argument.
* Each should contain the corresponding .
These macros are the same as OR except that these signal an error in
case OR returns a true value.
Examples
caller =>
rest-list => (x 1)
(rest-values rest-list 2 '(x y z)) => x 1
(rest-values rest-list -2 'y) => x 1
(rest-values caller rest-list 1 '(x y z))
=> error: too many arguments (x 1)
(rest-values caller rest-list 2 (list 'x 'y 'z) (cons "string" string?))
=> error: incorrect argument 1
(rest-values rest-list '(y z) `(100 . ,number?))
=> error: unmatched argument x (y z)
(rest-values rest-list #f 'y 100 "string")
=> x 1 "string"
(rest-values rest-list #t `(x y z) `(100 . ,number?) `("string" . ,string?))
=> x 1 "string"
(rest-values rest-list `(x y z) `(100 . ,number?) `("string" . ,string?))
=> x 1 "string"
caller =>
str => "string"
num => 10
a => 20
b => 30
(arg-and num (number? num) (< a num b))
=> error: incorrect argument 10 num (< a num b)
(arg-and caller num (number? num) (< a num b))
=> error: incorrect argument 10 num (< a num b)
(arg-ands (str (string? str) (< (string-length str) 10))
(num (number? num) (< a num b)))
=> error: incorrect argument 10 num (< a num b)
(arg-ands caller
(str (string? str) (< (string-length str) 10))
(num (number? num) (< a num b)))
=> error: incorrect argument 10 num (< a num b)
(define (read-line . p-d)
(receive (p d) (rest-values p-d 2
(cons (current-input-port) input-port?)
(list 'trim 'concat 'split...))
...))
(define (read-line . p-d)
(receive (p d) (rest-values p-d -2 (current-input-port) 'trim)
(arg-ands (p (input-port? p))
(d (memq d '(trim concat split...))))
...))
(define (delete x ls . predicate)
(let ((pred (rest-values delete predicate 1 (list equal? eqv? eq?))))
...))
(define (delete x ls . predicate)
(let ((pred (rest-values delete predicate -1 equal?)))
(arg-ands delete
(ls (list? ls))
(pred (memq pred (list equal? eqv? eq?))))
...))
(define (substring str . start-end)
(let ((str-len (arg-and substring str (string? str) (string-length str))))
(receive (start end) (rest-values substring start-end -2 0 str-len)
(arg-ands substring
(start (integer? start) (<= 0 start str-len))
(end (integer? end) (<= start end str-len)))
...)))
Implementation
ERROR (SRFI 23) and TAKE (SRFI 1) are used in implementation of this SRFI.
(define (rest-values rest-list . default-list)
(let* ((caller (if (procedure? rest-list) (list rest-list) '()))
(rest (if (null? caller)
rest-list
(if (null? default-list)
(error "too few arguments"
(cons rest-list default-list)
`(rest-values ,@(cons rest-list default-list)))
(car default-list))))
(rest-length (if (list? rest)
(length rest)
(error "incorrect rest list"
rest `(rest-values
,@(cons rest-list default-list)))))
(default (if (null? caller) default-list (cdr default-list)))
(number
(and (not (null? default))
(let ((d (car default)))
(or (and (number? d)
(or (and (> rest-length (abs d))
(apply error "too many arguments"
rest caller))
(and (> (length (cdr default)) (abs d))
(error "too many defaults"
(cdr default)
`(rest-values
,@(cons rest-list default-list))))
d))
(and (eq? d #t) #t)
(and (eq? d #f) 'false)))))
(default (if number (cdr default) default))
(default-length (length default)))
(if (or (and (number? number) (< number 0))
(eq? number 'false))
(apply values (if (> default-length rest-length)
(append rest (list-tail default rest-length))
rest))
(begin
(set! number (min rest-length default-length))
(for-each (lambda (r d)
(cond
((list? d)
(if (not (member r d))
(apply error "unmatched argument" r d caller)))
((pair? d)
(let ((p (cdr d)))
(if (procedure? p)
(if (not (p r))
(apply error "incorrect argument"
r p caller))
(error "incorrect predicate"
p `(rest-values ,@(cons rest-list
default-list))))))
(else
(error "incorrect default"
d `(rest-values ,@(cons rest-list
default-list))))))
(take rest number) (take default number))
(apply values
(if (> default-length rest-length)
(append rest
(map (lambda (x)
(if (pair? x)
(car x)
(error "incorrect default"
x `(rest-values
,@(cons rest-list
default-list)))))
(list-tail default rest-length)))
rest))))))
(define-syntax arg-and
(syntax-rules()
((arg-and arg (a1 a2 ...) ...)
(and (or (a1 a2 ...)
(error "incorrect argument" arg `arg `(a1 a2 ...)))
...))
((arg-and caller arg (a1 a2 ...) ...)
(and (or (a1 a2 ...)
(error "incorrect argument" arg `arg `(a1 a2 ...) caller))
...))))
(define-syntax arg-ands
(syntax-rules ()
((arg-ands (arg expr ...) ...)
(and (and (or expr (error "incorrect argument" arg `arg `expr))
...)
...))
((arg-ands caller (arg expr ...) ...)
(and (and (or expr (error "incorrect argument" arg `arg `expr caller))
...)
...))))
(define-syntax arg-or
(syntax-rules ()
((arg-or arg (a1 a2 ...) ...)
(or (and (a1 a2 ...)
(error "incorrect argument" arg `arg `(a1 a2 ...)))
...))
((arg-or caller arg (a1 a2 ...) ...)
(or (and (a1 a2 ...)
(error "incorrect argument" arg `arg `(a1 a2 ...) caller))
...))))
(define-syntax arg-ors
(syntax-rules ()
((arg-ors (arg expr ...) ...)
(or (or (and expr (error "incorrect argument" arg `arg `expr))
...)
...))
((arg-ors caller (arg expr ...) ...)
(or (or (and expr (error "incorrect argument" arg `arg `expr caller))
...)
...))))
Copyright
Copyright (C) Joo ChurlSoo (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to others, and
derivative works that comment on or otherwise explain it or assist in its
implementation may be prepared, copied, published and distributed, in whole or
in part, without restriction of any kind, provided that the above copyright
notice and this paragraph are included on all such copies and derivative works.
However, this document itself may not be modified in any way, such as by
removing the copyright notice or references to the Scheme Request For
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other than English.
The limited permissions granted above are perpetual and will not be revoked by
the authors or their successors or assigns.
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