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CO538 Anonymous Questions and Answers Keyword Index

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Keyword reference for free-variable


Question 85 (2000):

I have expanded my dining philosophers animation network like this:

  [5]CHAN OF FORK b:
    secure.college (a, b)
    display (a, b, screen)

but the compiler is telling me:

    Aliasing error, 'screen' is a free variable.

Any thoughts?

Answer 85:

Yes. It must be the case that your display process does not name its third parameter screen, but nevertheless outputs to a channel called screen. Something like:

  PROC display ([5]CHAN OF PHILOSOPHER from.phil,
                [5]CHAN OF FORK b,
                CHAN OF BYTE out)
    ...  and in here outputs to the 'screen' channel instead of 'out'

Now, this PROC compiles because screen is a globally visibly channel carrying the right protocol (BYTE). It must also be the case that that channel exists because you have declared display inside the body of your main q7 process, where screen is declared as a parameter. [If it had been declared on its own before your main q7 process, it would not have compiled - complaining instead about the unknown channel called screen, which would have been nice.]

Assuming the above, when the compiler came to the attempted instance:

    display (a, b, screen)

it would see an attempt to associate the real channel screen with the formal parameter out. But the compiler knows that your display outputs to this screen directly - so that the process would have two names for the same channel: out and screen. occam does not allow such aliases to happen - hence the error message.

Keywords: q7 , free-variable

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Last modified Mon May 20 13:50:27 2013
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