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UML Point/Counterpoint: Mellor vs. Selic

Stephen J. Mellor
  Modeling Complex Behavior Simply
  described what he thinks are the problems with UML. 
Bran Selic 
  "How to Simplify Complexity" is by  at
  fights back, and Mellor replies again.

In "Misuse of hierarchy", Mellor argues that UML has "non-
transparent" higher levels in the state machine hierarchy. 
Mellor says "In a well-formed hierarchy, the larger unit
is transparent: it simply groups state machines together
with no additional semantics." In his Last Word Mellor 
says (after reading Selic's convincing arguments)
that "I began to dislike using flat state machines myself, 
until I remembered that my argument involved partitioning 
into multiple smaller state machines." Selic argues 
that UML state machines are possible to understand, and in
fact needed to handle the inherent complexity of a normal

If I have a PAR in occam, the father process freezes, and
I cannot communicate with it. Neither can it communicate
with anybody else. It only connects the lower level state
machines (processes). It is "transparent". However, I could 
have been given a channel "down there" to communicate 
three levels up and two down. I haven't missed anything
on that point in occam.

Q1. Does this make occam more like Mellor (Structured) or
    Selic (UML).
Q2. How does clean CSP handle this. Like occam?

)       Oyvind Teig )          oyvind.teig@xxxxxxxxxxxx ) Tel: +47 )
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