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grantm (164)

grantm
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http://www.mclean.net.nz/

Just a simple [cpan.org] guy, hacking Perl for fun and profit since way back in the last millenium. You may find me hanging around in the monestary [perlmonks.org].

What am I working on right now? Probably the Sprog project [sourceforge.net].

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Journal of grantm (164)

Sunday July 30, 2006
09:48 PM

Scary code

[ #30481 ]

From our production codebase:

unless (not condition1 xor (condition2 or condition3)) {

I think most of us would at least have to slow down to extract the essential meaning of that line.

The only saving grace of this piece of code was that condition1, condition2 and condition3 were all named boolean scalars.


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  • It was such a staple in my assembler days and now I can’t remember the last time I used it.

    Anyway, you can easily decomplexify that condition:

    1. We start here:

      unless ( not $condition1 xor ( $condition2 or $condition3 ) )

    2. unless should be reserved for trivial conditions:

      if ( not( not $condition1 xor ( $condition2 or $condition3 ) ) )

    3. The properties of xor mean that not(a xor b) trivially distributes as not(a) xor not(b):

      if ( not( not $condition1 ) xor not( $condition2 or $condition3 ) )

    • The properties of xor mean that not(a xor b) trivially distributes as not(a) xor not(b):

      You are wrong here. Take for instance the case where a and b are both false. Then a xor b is false, and hence, not (a xor b) is true. However, both not(a) and not(b) are true, and hence, not(a) xor not(b) is false.

      (a xor b) is equal to not (a) xor not (b). not (a xor b) is equal to not (a) xor b, or a xor not (b).

      Which means that the original case could have been written as:

        if ($condition1 xor ($condition2

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