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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>10.6 Breaks</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" 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class="tocviewselflink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running Racket</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">▼</a></td><td>10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">10.1 </td><td><a href="values.html" class="tocviewlink" data-pltdoc="x">Multiple Values</a></td></tr><tr><td align="right">10.2 </td><td><a href="exns.html" class="tocviewlink" data-pltdoc="x">Exceptions</a></td></tr><tr><td align="right">10.3 </td><td><a href="Delayed_Evaluation.html" class="tocviewlink" data-pltdoc="x">Delayed Evaluation</a></td></tr><tr><td align="right">10.4 </td><td><a href="cont.html" class="tocviewlink" data-pltdoc="x">Continuations</a></td></tr><tr><td align="right">10.5 </td><td><a href="contmarks.html" class="tocviewlink" data-pltdoc="x">Continuation Marks</a></td></tr><tr><td align="right">10.6 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Breaks</a></td></tr><tr><td align="right">10.7 </td><td><a href="Exiting.html" class="tocviewlink" data-pltdoc="x">Exiting</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._break-enabled%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">break-<wbr></wbr>enabled</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">parameterize-<wbr></wbr>break</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._current-break-parameterization%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">current-<wbr></wbr>break-<wbr></wbr>parameterization</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call-with-break-parameterization%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call-<wbr></wbr>with-<wbr></wbr>break-<wbr></wbr>parameterization</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._break-parameterization~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">break-<wbr></wbr>parameterization?</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="contmarks.html" title="backward to "10.5 Continuation Marks"" data-pltdoc="x">← prev</a> <a href="control.html" title="up to "10 Control Flow"" data-pltdoc="x">up</a> <a href="Exiting.html" title="forward to "10.7 Exiting"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""breakhandler"">10.6<tt> </tt><a name="(part._breakhandler)"></a><a name="(idx._(gentag._179._(lib._scribblings/reference/reference..scrbl)))"></a>Breaks</h4><p>A <a name="(tech._break)"></a><span style="font-style: italic">break</span> is an asynchronous exception, usually triggered
through an external source controlled by the user, or through the
<span class="RktSym"><a href="threads.html#%28def._%28%28quote._~23~25kernel%29._break-thread%29%29" class="RktValLink" data-pltdoc="x">break-thread</a></span> procedure. For example, the user may type Ctl-C
in a terminal to trigger a break. On some platforms, the Racket
process may receive <a name="(idx._(gentag._180._(lib._scribblings/reference/reference..scrbl)))"></a><span class="stt">SIGINT</span>, <a name="(idx._(gentag._181._(lib._scribblings/reference/reference..scrbl)))"></a><span class="stt">SIGHUP</span>,
or <a name="(idx._(gentag._182._(lib._scribblings/reference/reference..scrbl)))"></a><span class="stt">SIGTERM</span>; the latter two correspond to hang-up and
terminate breaks as reflected by <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3abreak~3ahang-up%29%29" class="RktValLink" data-pltdoc="x">exn:break:hang-up</a></span> and
<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3abreak~3aterminate%29%29" class="RktValLink" data-pltdoc="x">exn:break:terminate</a></span>, respectively. Multiple breaks may be
collapsed into a single exception, and multiple breaks of different
kinds may be collapsed to a single “strongest” break, where a
hang-up break is stronger than an interrupt break, and a terminate
break is stronger than a hang-up break.</p><p>A break exception can only occur in a
thread while breaks are enabled. When a break is detected and enabled,
the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3abreak%29%29" class="RktValLink" data-pltdoc="x">exn:break</a></span> (or <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3abreak~3ahang-up%29%29" class="RktValLink" data-pltdoc="x">exn:break:hang-up</a></span> or
<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3abreak~3aterminate%29%29" class="RktValLink" data-pltdoc="x">exn:break:terminate</a></span>) exception is raised
in the thread sometime afterward; if breaking
is disabled when <span class="RktSym"><a href="threads.html#%28def._%28%28quote._~23~25kernel%29._break-thread%29%29" class="RktValLink" data-pltdoc="x">break-thread</a></span> is called, the break is
suspended until breaking is again enabled for the thread. While a
thread has a suspended break, additional breaks are ignored.</p><p>Breaks are enabled through the <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._break-enabled%29%29" class="RktValLink" data-pltdoc="x">break-enabled</a></span> parameter-like
procedure and through the <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29" class="RktStxLink" data-pltdoc="x">parameterize-break</a></span> form, which is
analogous to <span class="RktSym"><a href="parameters.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize%29%29" class="RktStxLink" data-pltdoc="x">parameterize</a></span>. The <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._break-enabled%29%29" class="RktValLink" data-pltdoc="x">break-enabled</a></span>
procedure does not represent a parameter to be used with
<span class="RktSym"><a href="parameters.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize%29%29" class="RktStxLink" data-pltdoc="x">parameterize</a></span>, because changing the break-enabled state of a
thread requires an explicit check for breaks, and this check is
incompatible with the tail evaluation of a <span class="RktSym"><a href="parameters.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize%29%29" class="RktStxLink" data-pltdoc="x">parameterize</a></span>
expression’s body.</p><p>Certain procedures, such as <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%2Fenable-break%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait/enable-break</a></span>,
enable breaks temporarily while performing a blocking action. If
breaks are enabled for a thread, and if a break is triggered for the
thread but not yet delivered as an <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3abreak%29%29" class="RktValLink" data-pltdoc="x">exn:break</a></span> exception, then
the break is guaranteed to be delivered before breaks can be disabled
in the thread. The timing of <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3abreak%29%29" class="RktValLink" data-pltdoc="x">exn:break</a></span> exceptions is not
guaranteed in any other way.</p><p>Before calling a <span class="RktSym"><a href="exns.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._with-handlers%29%29" class="RktStxLink" data-pltdoc="x">with-handlers</a></span> predicate or handler, an
exception handler, an error display handler, an error escape handler,
an error value conversion handler, or a <span class="RktSym">pre-thunk</span> or
<span class="RktSym">post-thunk</span> for a <span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span>, the call is
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29" class="RktStxLink" data-pltdoc="x">parameterize-break</a></span>ed to disable breaks. Furthermore, breaks
are disabled during the transitions among handlers related to
exceptions, during the transitions between <span class="RktSym">pre-thunk</span>s and
<span class="RktSym">post-thunk</span>s for <span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span>, and during other
transitions for a continuation jump. For example, if breaks are
disabled when a continuation is invoked, and if breaks are also
disabled in the target continuation, then breaks will remain disabled
from the time of the invocation until the target continuation
executes unless a relevant <span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> <span class="RktSym">pre-thunk</span> or
<span class="RktSym">post-thunk</span> explicitly enables breaks.</p><p>If a break is triggered for a thread that is blocked on a nested
thread (see <span class="RktSym"><a href="threads.html#%28def._%28%28quote._~23~25kernel%29._call-in-nested-thread%29%29" class="RktValLink" data-pltdoc="x">call-in-nested-thread</a></span>), and if breaks are enabled
in the blocked thread, the break is implicitly handled by transferring
it to the nested thread.</p><p>When breaks are enabled, they can occur at any point within execution,
which makes certain implementation tasks subtle. For example, assuming
breaks are enabled when the following code is executed,</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="exns.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._with-handlers%29%29" class="RktStxLink" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="exns.html#%28def._%28%28quote._~23~25kernel%29._exn~3abreak~3f%29%29" class="RktValLink" data-pltdoc="x">exn:break?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void%29%29" class="RktValLink" data-pltdoc="x">void</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>then it is <span style="font-style: italic">not</span> the case that a <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=guide&rel=void_undefined.html&version=6.7" class="RktModLink Sq" data-pltdoc="x"><span class="nobreak"><span class="RktRes">#<void></span></span></a> result means the
semaphore was decremented or a break was received, exclusively. It is
possible that <span style="font-style: italic">both</span> occur: the break may occur after the
semaphore is successfully decremented but before a <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=guide&rel=void_undefined.html&version=6.7" class="RktModLink Sq" data-pltdoc="x"><span class="nobreak"><span class="RktRes">#<void></span></span></a>
result is returned by <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span>. A break exception will
never damage a semaphore, or any other built-in construct, but many
built-in procedures (including <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span>) contain
internal sub-expressions that can be interrupted by a break.</p><p>In general, it is impossible using only <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait</a></span> to
implement the guarantee that either the semaphore is decremented or an
exception is raised, but not both. Racket therefore supplies
<span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%2Fenable-break%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait/enable-break</a></span> (see <a href="semaphore.html" data-pltdoc="x">Semaphores</a>),
which does permit the implementation of such an exclusive guarantee:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29" class="RktStxLink" data-pltdoc="x">parameterize-break</a></span><span class="hspace"> </span><span class="RktVal">#f</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="exns.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._with-handlers%29%29" class="RktStxLink" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="exns.html#%28def._%28%28quote._~23~25kernel%29._exn~3abreak~3f%29%29" class="RktValLink" data-pltdoc="x">exn:break?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void%29%29" class="RktValLink" data-pltdoc="x">void</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%2Fenable-break%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait/enable-break</a></span><span class="hspace"> </span><span class="RktSym">s</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>In the above expression, a break can occur at any point until breaks
are disabled, in which case a break exception is propagated to the
enclosing exception handler. Otherwise, the break can only occur
within <span class="RktSym"><a href="semaphore.html#%28def._%28%28quote._~23~25kernel%29._semaphore-wait%2Fenable-break%29%29" class="RktValLink" data-pltdoc="x">semaphore-wait/enable-break</a></span>, which guarantees that if
a break exception is raised, the semaphore will not have been
decremented.</p><p>To allow similar implementation patterns over blocking port
operations, Racket provides <span class="RktSym"><a href="Byte_and_String_Input.html#%28def._%28%28quote._~23~25kernel%29._read-bytes-avail%21%2Fenable-break%29%29" class="RktValLink" data-pltdoc="x">read-bytes-avail!/enable-break</a></span>,
<span class="RktSym"><a href="Byte_and_String_Output.html#%28def._%28%28quote._~23~25kernel%29._write-bytes-avail%2Fenable-break%29%29" class="RktValLink" data-pltdoc="x">write-bytes-avail/enable-break</a></span>, and other procedures.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._break-enabled))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._break-enabled%29%29" class="RktValDef RktValLink" data-pltdoc="x">break-enabled</a></span></span><span class="RktPn"></span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="RktPn">(</span><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._break-enabled%29%29" class="RktValDef RktValLink" data-pltdoc="x">break-enabled</a></span></span><span class="hspace"> </span><span class="RktVar">on?</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void~3f%29%29" class="RktValLink" data-pltdoc="x">void?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">on?</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Gets or sets the break enabled state of the current thread. If
<span class="RktVar">on?</span> is not supplied, the result is <span class="RktVal">#t</span> if breaks are
currently enabled, <span class="RktVal">#f</span> otherwise. If <span class="RktVar">on?</span> is supplied
as <span class="RktVal">#f</span>, breaks are disabled, and if <span class="RktVar">on?</span> is a true
value, breaks are enabled.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/more-scheme..rkt)._parameterize-break))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">parameterize-break</a></span></span><span class="hspace"> </span><span class="RktVar">boolean-expr</span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Evaluates
<span class="RktVar">boolean-expr</span> to determine whether breaks are initially
enabled while evaluating the <span class="RktVar">body</span>s in sequence. The result
of the <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29" class="RktStxLink" data-pltdoc="x">parameterize-break</a></span> expression is the result of the last
<span class="RktSym">expr</span>.</div></p><p>Like <span class="RktSym"><a href="parameters.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize%29%29" class="RktStxLink" data-pltdoc="x">parameterize</a></span> (see <a href="parameters.html" data-pltdoc="x">Parameters</a>), a fresh
<a href="eval-model.html#%28tech._thread._cell%29" class="techoutside" data-pltdoc="x"><span class="techinside">thread cell</span></a> (see <a href="threadcells.html" data-pltdoc="x">Thread Cells</a>) is allocated to
hold the break-enabled state of the continuation, and calls to
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._break-enabled%29%29" class="RktValLink" data-pltdoc="x">break-enabled</a></span> within the continuation access or modify the
new cell. Unlike parameters, the break setting is not inherited by new
threads.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/private/more-scheme..rkt)._current-break-parameterization))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._current-break-parameterization%29%29" class="RktValDef RktValLink" data-pltdoc="x">current-break-parameterization</a></span></span><span class="RktPn"></span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._break-parameterization~3f%29%29" class="RktValLink" data-pltdoc="x">break-parameterization?</a></span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Analogous to <span class="RktPn">(</span><span class="RktSym"><a href="parameters.html#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._current-parameterization%29%29" class="RktValLink" data-pltdoc="x">current-parameterization</a></span><span class="RktPn">)</span> (see
<a href="parameters.html" data-pltdoc="x">Parameters</a>); it returns a break parameterization
(effectively, a thread cell) that holds the current continuation’s
break-enable state.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((lib._racket/private/more-scheme..rkt)._call-with-break-parameterization))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call-with-break-parameterization%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-with-break-parameterization</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">break-param</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">thunk</span><span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">break-param</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._break-parameterization~3f%29%29" class="RktValLink" data-pltdoc="x">break-parameterization?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">thunk</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">Analogous to <span class="RktPn">(</span><span class="RktSym"><a href="parameters.html#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call-with-parameterization%29%29" class="RktValLink" data-pltdoc="x">call-with-parameterization</a></span><span class="stt"> </span><span class="RktSym">parameterization</span><span class="stt"> </span><span class="RktVar">thunk</span><span class="RktPn">)</span> (see <a href="parameters.html" data-pltdoc="x">Parameters</a>), calls <span class="RktVar">thunk</span> in a
continuation whose break-enabled state is in <span class="RktVar">break-param</span>. The
<span class="RktVar">thunk</span> is <span style="font-style: italic">not</span> called in tail position with respect to
the <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call-with-break-parameterization%29%29" class="RktValLink" data-pltdoc="x">call-with-break-parameterization</a></span> call.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/private/more-scheme..rkt)._break-parameterization~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._break-parameterization~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">break-parameterization?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">v</span> is a break parameterization as produced by
<span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._current-break-parameterization%29%29" class="RktValLink" data-pltdoc="x">current-break-parameterization</a></span>, <span class="RktVal">#f</span> otherwise.</div></p><p class="SHistory">Added in version 6.1.1.8 of package <span class="stt">base</span>.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="contmarks.html" title="backward to "10.5 Continuation Marks"" data-pltdoc="x">← prev</a> <a href="control.html" title="up to "10 Control Flow"" data-pltdoc="x">up</a> <a href="Exiting.html" title="forward to "10.7 Exiting"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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