Hey everyone,
I was working on a procedure to prompt the user for confirmation and found
something a bit strange - it did not appear to read for input when usingt
"racket -i" or in the Emacs Racket REPL buffer. Here is the code:
(define (yn #:read-one-char? [read-one-char? #f])
(display "y/n: ")
(flush-output (current-output-port))
(if read-one-char?
(match (read-char)
[(or #\y #\Y) #t]
[m #f])
(match (read-line)
[(or "y" "Y") #t]
[m #f])))
Regardless if I use read-char or read-line and type y or Y, the behavior seeims
similar, each of the match's second clause is followed because (read-char)
returns #\newline whereas read-line returns "" (empty string).
I mentioned this strange behavior on the Libera chat and got an enlightening
response from tonyg outlining what is happening:
> at the repl, if you type "(read-char)" and press enter, the reader sees ( r e
> a d - c h a r ) NEWLINE. When it gets to the close-parenthesis, it executes
> the expression, which reads the NEWLINE character and returns. Similar for
> read-line, where it sees NEWLINE and returns an empty line
>
> try typing (list (read-line) (read-line))
I can confirm tonyg's solution works. The (list (read-char) (read-char)) also
appears to work, though one always has to type a final newline to send the
line-buffered input.
The behavior feels very surprising and took me a bit of time to figure out,
even then I didn't really understand it so I had to ask for help. Can this
behavior be changed in a future release? Is a reasonable request or could this
break a lot of code? If this could break stuff, is it worth doing changing it
anyway, so the behavior is less surprising? I hope to understand if it's
agreeable to make a PR for this change before investing the time.
Keep on Racketing!
Winston Weinert
winny.tech
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