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POPorts and Cells in Oz
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copied!<p>I'm taking a teach-yourself course at the university at the moment and noticed I bitten off a bit more than I can chew. The course is about the Oz programming language and I'm reading an e-book about it to try to get to know it better and I'm trying to solve some exercises to test my understanding. I'm quite stuck at a certain exercise and I don't quite know how to solve it. The question goes:</p> <blockquote> <p>Implementing ports. In Chapter 5 we introduced the concept of port, which is a simple communication channel. Ports have the operations {NewPort S P}, which returns a port P with stream S, and {Send P X}, which sends message X on port P. From these operations, it is clear that ports are a stateful unbundled ADT. For this exercise, implement ports in terms of cells, using the techniques of Section 6.4.</p> </blockquote> <p>The key words are stateful unbundled and cells. I tried to implement the behavior of ports using cells with the following approach:</p> <pre><code>declare MyPort MyStream proc {NewPortEx ?S ?P} P = {NewCell nil} S = !!P %read only view on the cell end proc {SendEx P X} P:=X|@P %extend the cell's content, a list, with X end in {NewPortEx MyStream MyPort} {Browse @MyStream} {SendEx MyPort c} {Browse @MyStream} </code></pre> <p>My final step would be to add a wrapper/unwrapper pair to make the ADT secure but first I want the functionality to work properly. This seems to do the right behavior but it's not as I had wanted it. I would like to be able to call <code>{Browse MyStream}</code>, without the <code>@</code>, just once. I was hoping the browser would display something along the lines of <code>firstSent|secondSent|_<future></code> instead it displays <code><Cell></code> as is to be expected and I need to call <code>Browse</code> to it after each <code>Send</code>, and the output is shown as a list: <code>[firstSent secondSent]</code>.</p> <p>If I remember correctly from the theory I read, this has to do with eager vs lazy evaluation, respectively resulting in (finite) lists vs (infinite) streams.</p> <p>I have the feeling I'm not doing it quite right though and I have no experience whatsoever with functional languages, can anyone help me out with making an example of a port implemented with cells? (basically your own implementation of the existing <code>NewPort</code> and <code>Send</code>)</p> <p>Thanks in advance</p>

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