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<body class='hmmessage'><div dir='ltr'>The Green Book's explanation of Rd/Wr might have finally given me the clue I needed.<br><br><br>Private <: ROOT;<br>Public = Private OBJECT ...<br><br><br>The private part is a prefix of the public part.<br>I still don't see why this is useful, vs. more derived more private types, unless it is to safe runtime casts.<br><br>I'm trying to put together a small sample to exercise opaque types..and I'm getting:<br><br>***<br>*** runtime error:<br>*** A compile-time type is missing.<br>***<br><br><br>I wish it could say which type. :(<br><br> - Jay<br><br><br><div><div id="SkyDrivePlaceholder"></div><hr id="stopSpelling">From: jay.krell@cornell.edu<br>To: m3devel@elegosoft.com<br>Date: Sun, 31 Mar 2013 05:10:00 +0000<br>Subject: [M3devel] opaque types again<br><br>
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<div dir="ltr">I don't understand opaque types.<br><br><br>I do understand the notion of fully opaque types. That are fully revealed in one step.<br>In C this is common:<br><br><br> window.h <br> struct Window_t; <br> typedef struct Window_t Window_t; <br> Window_t* Window_Create(); <br> void Window_Show(Window_*t); <br> void Window_Close(Window_*t); <br> long Window_GetHeight(Window_*t); <br> long Window_GetWidth(Window_*t); <br> etc.<br><br> window.c <br> struct Window_t { ... } /* reveal */ <br><br><br>Back to Modula-3... opaque types must be OBJECTs, right?<br><br><br>1. What do they provide vs. more derived types?<br><br><br>INTERFACE Animal;<br><br>TYPE T = OBJECT<br>METHODS<br> makeNoise();<br>END;<br><br><br>INTERFACE AnimalImpl;<br><br>TYPE T = Animal.T OBJECT<br>METHODS<br> private();<br>END;<br><br><br>If I had an Animal.T and wanted to call private(),<br>wouldn't I just NARROW it to AnimalImpl.T?<br><br><br>Is the point that it is more efficient and avoids<br>a runtime type check?<br><br><br><br>INTERFACE Animal;<br><br>TYPE Public = OBJECT<br>METHODS<br> makeNoise();<br>END;<br><br>TYPE T <: Public;<br><br>INTERFACE AnimalImpl;<br><br>REVEAL Animal.T = Animal.Public OBJECT<br>METHODS<br> private();<br>END;<br><br><br>?<br>This way I can import AnimalImpl and then just call private()<br>without a NARROW?<br><br><br><br><br><br>2. Given that the final revelation must be a linear<br>form of all the declared subtypes, I don't understand<br>how the offset used by any module w/o a full revelation<br>could be anything other than zero.<br><br><br>I'd like to fully understand this, so I can make the C backend<br>provide maximal type information. More pointers to structs, with members/fields,<br>fewer untyped void*/char*.<br><br><br>Thanks,<br> - Jay<br><br><br> </div></div> </div></body>
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