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Sun Dec 6 17:56:43 EST 2009
GOTO
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It occurs to me why the GOTO statement is so bad, as in Dijk-
stra's eyes. Most other commentaries on GOTO indicate that it is
bad because it creates unreadable code; which is true - unread-
able for humans. It is not all that, however, when you want to
do well defined things very fast - and of course, it helps that
the source code is automatically generated.
During the activity of "programming", or enumerating the steps
that a computer is to follow, it is necessary to strike a balance
between implicit and explicit expression of methods and steps.
Pure functional programming is nearly fully implicit, being
declarative (i.e., "what" rather than "how"). Shared memory par-
allelism achieve much of its gains over explicit message based
parallelism due to the fact that much of what happens is implic-
it, and is therefore encoded in the nature of the method or pro-
cedure. The other extreme, i.e., of being maximally explicit, can
be illustrated through assembly lanuage programming - though
there is still room for implicit methods to be used (especially
through automation, via a compiler).
The GOTO statement is bad if wielded unwisely because it provides
the programmer with too much access to the explicit flow of the
program or change in state. These statements are not even possi-
ble in purely functional languages, but are in procedural lan-
guages - therefore it is not a problem there. In terms of maxi-
mizing efficient execution, the GOTO statement is indeed a valu-
able tool to have. One clear example would be in the programming
of Finite State Machines in a language. The Ragel State Machine
Compiler makes use of GOTOs with extreme prejudice; and it is
okay here because the source code creation is automated (i.e.,
one describes the FSM in the Ragel description language, and
picks a target language for which the FSM is written for source
compilation and execution). FSM are one example where GOTOs are
okay because FSM (i.e., non-deterministic ones) have defined for
each input, an action - and in that case, automating code (using
GOTOs or not) is safe (though not necessarily "easy").
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