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Author:halw

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Date:2011-03-01T22:52:38.000000Z


git-svn-id: https://svn.eiffel.com/eiffel-org/trunk@794 abb3cda0-5349-4a8f-a601-0c33ac3a8c38
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halw
2011-03-01 23:00:08 +00:00
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documentation/current/solutions/concurrent-computing/concurrent-eiffel-scoop/scoop-examples/barbershop.wiki
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@@ -26,6 +26,23 @@ The <code>SHOP</code> includes features <code>enter</code> and <code>leave</code
A typical customer lives in this way: As long as he still needs haircuts, he repeatedly does the following steps: He tries to enter the shop. If he's unsuccessful because all the chairs are occupied, he goes off for a while (in the implementation, his processor sleeps and then comes to the end of this step). If he is able to enter the shop, then he puts himself in the queue for a haircut. Once his haircut is complete, he reduces his number of haircuts needed by one, and leaves the shop.
One thing that you might find curious about this example is the feature <code>do_hair_cut</code> in class <code>BARBER</code>. This feature is called by a customer when the barber becomes available to cut the customer's hair. <code>do_hair_cut</code> is a function that returns a <code>BOOLEAN</code>. If you look at the source code for <code>do_hair_cut</code>, you'll see that the routine cannot complete with any result other than <code>True</code> ... and this may seem odd.
<code>
do_hair_cut (an_id: INTEGER): BOOLEAN
-- Called from a customer who wants to get hair cut
require
an_id >= 0
do
(create {EXECUTION_ENVIRONMENT}).sleep (hair_cut_time * 1000000)
result := true
end
</code>
Couldn't a function that always returns the exact same predictable result be safely made a procedure? Certainly it could. However, there is a SCOOP oriented reason for this routine being a function rather than a procedure.
<code>{BARBER}.do_hair_cut</code> is a function in order to become synchronous. Remember that a [[Concurrent Eiffel with SCOOP#Separate types and separate calls|separate call]] which is a query is always a [[Concurrent Eiffel with SCOOP#Synchronous and asynchronous feature calls|synchronous call. In the case of the customer, he needs to leave the shop only after his haircut is complete. Therefore, the query <code>do_hair_cut</code> will be synchronous and ensure that the haircut is complete before the customer leaves the shop.