hidden flag

from Jargon File (4.4.4, 14 Aug 2003)
hidden flag
 n.

   [scientific computation] An extra option added to a routine without
   changing the calling sequence. For example, instead of adding an
   explicit input variable to instruct a routine to give extra diagnostic
   output, the programmer might just add a test for some otherwise
   meaningless feature of the existing inputs, such as a negative mass.
   The use of hidden flags can make a program very hard to debug and
   understand, but is all too common wherever programs are hacked on in a
   hurry.
    
from The Free On-line Dictionary of Computing (8 July 2008)
hidden flag

   (scientific computation) An extra option added to a routine
   without changing the calling sequence.  For example, instead
   of adding an explicit input variable to instruct a routine to
   give extra diagnostic output, the programmer might just add a
   test for some otherwise meaningless feature of the existing
   inputs, such as a negative mass.  The use of hidden flags can
   make a program very hard to debug and understand, but is all
   too common wherever programs are hacked in a hurry.

   [{Jargon File}]

   (1994-11-24)
    

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