119 lines
5.5 KiB
HTML
119 lines
5.5 KiB
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Trap Interface (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Traps.html" rel="up" title="Traps">
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<link href="Low_002dLevel-Traps.html" rel="next" title="Low-Level Traps">
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<link href="VM-Hooks.html" rel="prev" title="VM Hooks">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Trap-Interface">
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<div class="nav-panel">
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<p>
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Next: <a href="Low_002dLevel-Traps.html" accesskey="n" rel="next">Low-Level Traps</a>, Previous: <a href="VM-Hooks.html" accesskey="p" rel="prev">VM Hooks</a>, Up: <a href="Traps.html" accesskey="u" rel="up">Traps</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsubsection" id="Trap-Interface-1"><span>6.26.4.2 Trap Interface<a class="copiable-link" href="#Trap-Interface-1"> ¶</a></span></h4>
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<p>The capabilities provided by hooks are great, but hooks alone rarely
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correspond to what users want to do.
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</p>
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<p>For example, if a user wants to break when and if control reaches a
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certain source location, how do you do it? If you install a “next”
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hook, you get unacceptable overhead for the execution of the entire
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program. It would be possible to install an “apply” hook, then if the
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procedure encompasses those source locations, install a “next” hook,
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but already you’re talking about one concept that might be implemented
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by a varying number of lower-level concepts.
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</p>
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<p>It’s best to be clear about things and define one abstraction for all
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such conditions: the <em class="dfn">trap</em>.
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</p>
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<p>Considering the myriad capabilities offered by the hooks though, there
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is only a minimum of functionality shared by all traps. Guile’s current
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take is to reduce this to the absolute minimum, and have the only
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standard interface of a trap be “turn yourself on” or “turn yourself
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off”.
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</p>
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<p>This interface sounds a bit strange, but it is useful to procedurally
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compose higher-level traps from lower-level building blocks. For
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example, Guile defines a trap that calls one handler when control enters
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a procedure, and another when control leaves the procedure. Given that
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trap, one can define a trap that adds to the next-hook only when within
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a given procedure. Building further, one can define a trap that fires
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when control reaches particular instructions within a procedure.
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</p>
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<p>Or of course you can stop at any of these intermediate levels. For
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example, one might only be interested in calls to a given procedure. But
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the point is that a simple enable/disable interface is all the
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commonality that exists between the various kinds of traps, and
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furthermore that such an interface serves to allow “higher-level”
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traps to be composed from more primitive ones.
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</p>
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<p>Specifically, a trap, in Guile, is a procedure. When a trap is created,
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by convention the trap is enabled; therefore, the procedure that is the
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trap will, when called, disable the trap, and return a procedure that
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will enable the trap, and so on.
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</p>
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<p>Trap procedures take one optional argument: the current frame. (A trap
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may want to add to different sets of hooks depending on the frame that
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is current at enable-time.)
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</p>
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<p>If this all sounds very complicated, it’s because it is. Some of it is
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essential, but probably most of it is not. The advantage of using this
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minimal interface is that composability is more lexically apparent than
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when, for example, using a stateful interface based on GOOPS. But
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perhaps this reflects the cognitive limitations of the programmer who
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made the current interface more than anything else.
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Low_002dLevel-Traps.html">Low-Level Traps</a>, Previous: <a href="VM-Hooks.html">VM Hooks</a>, Up: <a href="Traps.html">Traps</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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