XPost: comp.lang.scheme

Frode Vatvedt Fjeld wrote:

(defun read-to-char (c stream)
(let ((x (read-char stream nil)))
(cond ((null x) nil)
((char= x c) t)
(t (read-to-char c stream)))))

This isn't much worse looking then the obvious iterative solutions.

..only it has Scheme written all over it ;-) In Common Lisp this is
spelled as

(defun read-to-char (c stream)
(loop for x = (read-char stream nil)
while x do (when (char= x c) (return t))))

It's shorter in Gauche Scheme:

(use srfi-42) ;; first-ec

(define (read-to-char c port)
(first-ec #f
(:port x port read-char)
(if (char=? x c))
#t))

Testing.

(call-with-input-string "foo-Frodo is foolish."
(lambda (in) (read-to-char #\- in)
(read-line in)))
===>
"Frodo is foolish."

Shorter yet:

(use srfi-121) ; generators

(define (read-to-char c port)
(generator-find (is c) (cut read-char port)))

Given:

(define-syntax is
(syntax-rules ()
[(is x)
(lambda (y) (equal? y x))]
[(is compare x)
(lambda (y) (compare y x))]
[(is key compare x)
(lambda (y) (compare (key y) x))]))

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XPost: comp.lang.scheme

On 2024-09-13, B. Pym <[email protected]> wrote:

Frode Vatvedt Fjeld wrote:

(defun read-to-char (c stream)
(let ((x (read-char stream nil)))
(cond ((null x) nil)
((char= x c) t)
(t (read-to-char c stream)))))

This isn't much worse looking then the obvious iterative solutions.

..only it has Scheme written all over it ;-) In Common Lisp this is
spelled as

(defun read-to-char (c stream)
(loop for x = (read-char stream nil)
while x do (when (char= x c) (return t))))

It's shorter in Gauche Scheme:

(use srfi-42) ;; first-ec

(define (read-to-char c port)
(first-ec #f
(:port x port read-char)
(if (char=? x c))
#t))

Testing.

(call-with-input-string "foo-Frodo is foolish."
(lambda (in) (read-to-char #\- in)
(read-line in)))

"Frodo is foolish."

Shorter yet:

What, that is longer?

(use srfi-121) ; generators

(define (read-to-char c port)
(generator-find (is c) (cut read-char port)))

Given:

(define-syntax is
(syntax-rules ()
[(is x)
(lambda (y) (equal? y x))]
[(is compare x)
(lambda (y) (compare y x))]
[(is key compare x)
(lambda (y) (compare (key y) x))]))

The "Given:" has to count as part of the length of the solution.

Otherwiwse, we can just say:

" Shorter yet:

;; nothing

Given:

(define (read-to-char c port) ...) "

If you can factor out any aspect of your solution into macros
and functions which don't count toward its size, then you just
do that with the function you are supposed to write, and
the solution has zero size.

--
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @[email protected]

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

(defun read-to-char (c stream)
(loop for x = (read-char stream nil)
while x do (when (char= x c) (return t))))

How verbose!! The function can be expressed much more concisely as:

(defun read-to-char (c stream)
(loop for x = (read-char stream nil)
while x when (char= x c) return t))

or if one doesn't mind the possibility of an extra return value, as:

(defun read-to-char (c stream)
(ignore-errors (loop when (eql (read-char stream) c) return it))

Why is LOOP needed?

Scheme:

(define (read-to-char c port)
(let ((r (read-char port)))
(unless (or (eof-object? r) (eq? c r))
(read-to-char c port))))

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

B. Pym wrote:

(defun read-to-char (c stream)
(loop for x = (read-char stream nil)
while x do (when (char= x c) (return t))))

How verbose!! The function can be expressed much more concisely as:

(defun read-to-char (c stream)
(loop for x = (read-char stream nil)
while x when (char= x c) return t))

or if one doesn't mind the possibility of an extra return value, as:

(defun read-to-char (c stream)
(ignore-errors (loop when (eql (read-char stream) c) return it))

Why is LOOP needed?

Scheme:

(define (read-to-char c port)
(let ((r (read-char port)))
(unless (or (eof-object? r) (eq? c r))
(read-to-char c port))))

Gauche Scheme

(use srfi-121) ; generators

(define (read-to-char c port)
(generator-find (is c) (cut read-char port)))


Given:

(define is
(case-lambda
[(x) (lambda(y) (equal? y x))]
[(pred x) (lambda(y) (pred y x))]
[(pred key x) (lambda(y) (pred (key y) x))]))

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