[plt-dev] Racket web page

From: Carl Eastlund (cce at ccs.neu.edu)
Date: Wed May 26 12:32:31 EDT 2010

On Wed, May 26, 2010 at 10:27 AM, Matthias Felleisen
<matthias at ccs.neu.edu> wrote:
> On May 25, 2010, at 8:48 PM, Matthew Flatt wrote:
>> #lang racket
>> ;; Report each unique line from stdin:
>> (let ([saw (make-hash)])
>>   (for ([line (in-lines)])
>>     (unless (hash-ref saw line #f)
>>       (displayln line))
>>     (hash-set! saw line #t)))
> I would prefer to sacrifice one line to avoid the repetition of the test (leakage). I doubt Perl and friends do that:
> #lang racket
> ;; Report each unique line from stdin:
> (define saw (make-hash))
> (for ([line (in-lines)])
>  (hash-ref saw line
>    (lambda ()
>      (displayln line)
>      (hash-set! saw line #t))))

To anyone who isn't intimately familiar with hash-ref, both versions
are kind of obscure; the second one more so.  If you insist on the
second one, though, use a lambda character and put the displayln on
the same line and you'll be back to 7.  Personally, I'd recommend a
variant of the first that uses hash-has-key? instead of hash-ref.
It'll be much more readable.


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