[plt-dev] Racket web page

From: Jay McCarthy (jay.mccarthy at gmail.com)
Date: Wed May 26 12:34:34 EDT 2010

On Wed, May 26, 2010 at 10:32 AM, Carl Eastlund <cce at ccs.neu.edu> wrote:
> 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.

And I would recommend using hash-ref! to save a line.


Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University

"The glory of God is Intelligence" - D&C 93

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