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| 7 | <title>Design of lisp streams in ABCL</title> |
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| 9 | |
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| 10 | /* |
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| 11 | :Author: David Goodger (goodger@python.org) |
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| 12 | :Id: $Id: html4css1.css 5196 2007-06-03 20:25:28Z wiemann $ |
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| 17 | See http://docutils.sf.net/docs/howto/html-stylesheets.html for how to |
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| 283 | |
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| 284 | </style> |
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| 285 | </head> |
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| 286 | <body> |
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| 287 | <div class="document" id="design-of-lisp-streams-in-abcl"> |
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| 288 | <h1 class="title">Design of lisp streams in ABCL</h1> |
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| 289 | |
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| 290 | <div class="section" id="the-previous-design"> |
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| 291 | <h1>The previous design</h1> |
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| 292 | <p>Previously, ABCL streams were built-in classes. This presented some problems for Gray streams, |
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| 293 | because ABCL CLOS can't use a built-in class as a base class, and Gray streams derive from |
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| 294 | a system-stream class. This was corrected by converting ABCL streams to be structure-objects |
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| 295 | instead of built-in classes, allowing CLOS to derive from the streams. There was, however, another |
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| 296 | problem that revealed a need to change the design in more drastic ways.</p> |
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| 297 | <div class="section" id="the-problem-with-the-previous-design"> |
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| 298 | <h2>The problem with the previous design</h2> |
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| 299 | <p>While converting the streams from built-in classes to structure-objects allowed derivation, |
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| 300 | the pretty printer still didn't work with Gray streams. Gray streams replace the system stream |
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| 301 | functions, saving the old function symbols so that they can be later invoked. The pretty printer, |
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| 302 | however, just replaces the stream functions, and calls the low-level primitives directly, thus |
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| 303 | bypassing Gray streams completely. The attached image portrays the problem, where pprint will, |
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| 304 | for example, invoke %stream-write-char, thus bypassing any methods that there may be for |
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| 305 | stream-write-char using Gray streams.</p> |
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| 306 | <img alt="pprint-problem.png" src="pprint-problem.png" /> |
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| 307 | </div> |
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| 308 | </div> |
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| 309 | <div class="section" id="the-planned-future-design-and-solution-to-the-problem"> |
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| 310 | <h1>The planned future design and solution to the problem</h1> |
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| 311 | <p>The solution to the problem is quite similar to how SBCL does its streams. First of all, the pretty printer will |
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| 312 | no longer replace stream functions. The stream functionality will be based on closures in the slots of |
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| 313 | the structure-object representing the stream, and those closures will invoke low-level i/o functions that |
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| 314 | are stream-specific.</p> |
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| 315 | <p>The pretty printer will just setup closures that will extract the underlying stream |
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| 316 | object from a pprint-wrapped stream, and invoke its low-level functions. If pprint wrapping isn't present, |
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| 317 | the slots will contain closures that directly invoke low-level functions of streams. Gray streams will |
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| 318 | still replace the stream functions, because it's capable of invoking the replaced functions.</p> |
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| 319 | <p>In addition to these changes, it is planned that the stream function primitives will be moved from the Stream |
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| 320 | java class to a streamfunctions library, allowing the stream functions to be written in lisp rather than java. |
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| 321 | There's an ongoing aspiration to increase the lisp/java code ratio of ABCL, and this new design allows for that.</p> |
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| 322 | <img alt="pprint-solution.png" src="pprint-solution.png" /> |
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| 323 | </div> |
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| 324 | </div> |
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| 325 | </body> |
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| 326 | </html> |
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