XPost: comp.lang.scheme

On 2024-09-13, B. Pym <Nobody447095@here-nor-there.org> 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
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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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