The Hack of bitmask used as Predicate Parameters

Discussion in 'Perl Misc' started by Xah Lee, Apr 23, 2007.

  1. Xah Lee

    Xah Lee Guest

    In this article, i explain how the use of bit masks is a hack in many
    imperative languages.

    Often, a function will need to take many True/False parameters. For
    example, suppose i have a function that can draw a rainbow, and each
    color of the rainbow can be turned on or off individually. My function
    specification can be of this form: “rainbow(red, orange, yellow,
    green, blue, violet, purple)â€. Each parameter is a true or false
    value. So, to draw a rainbow with only red and yellow stripes on, one
    would code, for example “rainbow(t,f,t,f,f,f,f)â€, where “t†stands for
    true and “f†stands for false. (or, similar values for the true/false
    of the language's boolean system)

    The problem with this simple approach is that when a function has too
    many parameters, “which position means what†becomes difficult to
    remember and manage. Alternatively, a high-level language may provide
    a system for named parameters. So, for example, the function may be
    called like this with 2 arguments “rainbow(red:t, yellow:t)â€, meaning,
    give the true values to the parameters named “red†and “yellowâ€.
    Parameters not given may automatically assumed to have false values by
    default. Similarly, the language can simply have the function call
    look like this: “rainbow(red, yellow)â€, where omitted parameter names
    simply means false.

    LSL deals with this issue by using a concept of bit-mask that came
    from low-level languages. From the programer's point of view, the way
    to call this rainbow function would look like this: “rainbow(red|
    yellow)â€. On the surface, it seems just a syntax variation. But
    actually, the “red†and “yellow†here are global constants of type
    integer, defined by the language, and the “|†is actually abit-wise
    binary operator. To explain this to a educated person (e.g. a
    mathematician) but who are not a professional programer, it gets a bit
    complex as one has to drag in binary notation, boolean operation on
    binary notation realized as a sequence of slots, and the compiler ease
    in processing numbers as binary digits, and the compiler writer and
    language designer's laziness in resorting to these instead of a high-
    level interface of named parameters.

    The hack of using the so-called bit-mask as a interface for functions
    that need named parameters, is similar to languages using “1†and “0â€
    as the true/false symbols in its boolean system, and languages using
    the “or†operator “||†as a method of nested “if else†program flow
    constructs. The problem with these hacks, is that they jam logically
    disparate semantics into the same construct. Their effects is making
    the source code more difficult to read, and thus increased programer
    error.

    ----

    It may seem like nickpicking to say that it is a hack. However, when
    many such seemingly trivially improper designs appear in a language,
    adds up to the language's illness, and overall making the language
    difficult to learn, difficult to read, difficult to extend, increase
    programing errors, and most importantly, reduce a clear understanding
    of key concepts.

    Unix and C, are the primary perpetrator of this sin. Due to their
    “$free$†and “speedy†and “simplistic†nature as cigarettes given to
    children, have passed these designs to many imperative languages and
    left programers not understanding the basic issues of a function's
    parameters and named parameters.

    Examples of using bitmask as a hack:

    • Many functions in C. (e.g. fcntl)

    • Unix's function/“command line toolâ€'s error values. (as bits)

    • Perl's “sysopenâ€, “selectâ€, and others. (if you know a perl example
    that isn't related to unix, pls let me known)

    • Second Life's Linden Scripting Language.
    (see http://xahlee.org/sl/ls-prob.html )

    A example of confusion about function's parameters is exhibited in
    unix's bewildering, inconsist syntaxes in its command line tools's
    ways of taking arguments. (some parameter influence other parameters.
    Argument order sometimes matter, sometimes doesn't, sometimes causing
    unintented output and sometimes causing syntax error. Named parameters
    sometimes have the names optional(!). Named parameters that are
    predicates sometimes act by their presence along, sometimes by their
    value, sometimes accept both, sometimes causes syntax error. Some
    defaults are supplied to unnamed parameters, and some are to named
    parameters. Some parameters has synonyms. ...)

    For another example in a more modern language, is Python's
    “re.search()†function for text pattern matching. Its optional third
    parameter is a bitmask. ( see “Regular Expressions in Pythonâ€
    http://xahlee.org/perl-python/python_re-write/lib/node111.html )

    As a example of not clearly understanding a function's parameters and
    the need and role of named parameters in computing languages, Python's
    “sorted†function as well as its “lambda†construct are both victims.

    (Further reading:
    • “Sorting in Python and Perlâ€
    http://xahlee.org/perl-python/python_doc_sort.html

    • “Python Doc Problem Example: sort()â€
    http://xahlee.org/perl-python/sort_list.html

    • “Lambda in Python 3000â€
    http://xahlee.org//perl-python/python_3000.html
    )

    Bitmask used as a function's parameter but not considered as a hack,
    would be if it actually deals with bits necessarily (e.g. in protocols
    that employ bits. (e.g. in networking), byte processing, binary digit
    computation, ...)

    I would appreciate other examples you know in the above languages, and
    or in particular if Java, Lisp, Haskell.

    ----
    This article is a modified excerpt from
    “Linden Scripting Language Problems†at
    http://xahlee.org/sl/ls-prob.html

    Xah

    ∑ http://xahlee.org/
    Xah Lee, Apr 23, 2007
    #1
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