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     36 <h2 class="chapter">12 Promises</h2>
     38 <p><a name="index-promises-155"></a>
     39 <!-- TODO xref to $lazy, memoize, force -->
     40 A promise is an object that represents the potential to determine a
     41 value.  The value may be the result of an arbitrary computation that
     42 will not be performed until the value must be determined (constructor
     43 <code>$lazy</code>); or, in advanced usage, the value may be determined
     44 before the promise is constructed (constructor <code>memoize</code>).
     46    <p>The value determined by a promise is obtained by forcing it
     47 (applicative <code>force</code>).  A given promise cannot determine
     48 different values on different occasions that it is forced.  Also, if a
     49 promise determines its value by computation, and that computation has
     50 already been completed, forcing the promise again will produce the
     51 previously determined result without re-initiating the computation to
     52 determine it.
     54    <p>The Kernel data type promise is encapsulated.
     56 <!-- TODO add xref to eq? and equal? -->
     57    <p>The general rules for predicate <code>eq?</code> only require it to
     58 distinguish promises if they can exhibit different behavior; the
     59 resulting leeway for variation between implementations is similar, in
     60 both cause and effect, to that for <code>eq?</code>-ness of operatives.  For
     61 example, if two promises, constructed on different occasions, would
     62 perform the same computation to determine their values, and that
     63 computation has no side-effects and must always return the same value,
     64 the promises may or may not be <code>eq?</code>.  Two promises are
     65 <code>equal?</code> iff they are <code>eq?</code>.
     67 <div class="defun">
     68 &mdash; Applicative: <b>promise?</b> (<var>promise? . objects</var>)<var><a name="index-promise_003f-156"></a></var><br>
     69 <blockquote><p>  The primitive type predicate for type promise.  <code>promise?</code>
     70 returns true iff all the objects in <code>objects</code> are of type
     71 promise. 
     72 </p></blockquote></div>
     74 <div class="defun">
     75 &mdash; Applicative: <b>force</b> (<var>force object</var>)<var><a name="index-force-157"></a></var><br>
     76 <blockquote><p>  If <code>object</code> is a promise, applicative <code>force</code> returns the
     77 value determined by promise; otherwise, it returns <code>object</code>.
     79         <p>The means used to force a promise depend on how the promise was
     80 constructed.  The description of each promise constructor specifies
     81 how to force promises constructed by that constructor. 
     82 </p></blockquote></div>
     84 <div class="defun">
     85 &mdash; Operative: <b>$lazy</b> (<var>$lazy expression</var>)<var><a name="index-g_t_0024lazy-158"></a></var><br>
     86 <blockquote><p>  Operative <code>$lazy</code> constructs and returns a new object of type
     87 promise, representing potential evaluation of expression in the
     88 dynamic environment from which <code>$lazy</code> was called.
     90         <p>When the promise is forced, if a value has not previously been
     91 determined for it, <code>expression</code> is evaluated in the dynamic
     92 environment of the constructing call to <code>$lazy</code>.  If, when the
     93 evaluation returns a result, a value is found to have been determined
     94 for the promise during the evaluation, the result is discarded in
     95 favor of the previously determined value; otherwise, the result is
     96 forced, and the value returned by that forcing becomes the value
     97 determined by the promise.
     99      <!-- TODO add xref to tail context -->
    100         <p>Forcing an undetermined lazy promise (i.e., a promise constructed by
    101 $lazy for which no value has yet been determined) may cause a
    102 sequential series of evaluations, each of which returns a promise that
    103 is forced and thus initiates the next evaluation in the series.  The
    104 implementation must support series of this kind with unbounded length
    105 (i.e., unbounded number of sequential evaluations).
    107      <!-- TODO add xref to eq? -->
    108         <p>Note that forcing concerns the value determined by a given promise,
    109 not the result of evaluating a given expression in a given
    110 environment. Distinct promises (judged by <code>eq?</code> represent
    111 different occasions of evaluation; so, even if they do represent
    112 evaluation of the same expression in the same environment, forcing one
    113 does not necessarily determine the value for the other, and actual
    114 evaluation will take place the first time each of them is forced. 
    115 </p></blockquote></div>
    117 <div class="defun">
    118 &mdash; Applicative: <b>memoize</b> (<var>memoize object</var>)<var><a name="index-memoize-159"></a></var><br>
    119 <blockquote><p>  Applicative <code>memoize</code> constructs and returns a new object of
    120 type promise, representing memoization of <code>object</code>.  Whenever the
    121 promise is forced, it determines <code>object</code>. 
    122 </p></blockquote></div>
    124 <div class="defun">
    125 &mdash; Operative: <b>$delay</b> (<var>$delay &lt;expression&gt;</var>)<var><a name="index-g_t_0024delay-160"></a></var><br>
    126 <blockquote><p>Operative <code>delay</code> behaves as the composition of <code>$lazy</code> and
    127 <code>memoize</code>, that is:
    128      <pre class="example">          ($delay &lt;expr&gt;) == ($lazy (memoize &lt;expr&gt;))
    129 </pre>
    130         <p>SOURCE NOTE: this is taken from r7rs. 
    131 </p></blockquote></div>
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