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<h3 class="sectionHead"><span class="titlemark">6.8 </span> <a
id="x40-720006.8"></a>Lazy programming</h3>
<!--l. 856--><p class="noindent" ><a name="nip_label_sec:lazy"></a>
<!--l. 858--><p class="indent" > <span
class="pcrr7t-">nip2</span>’s programming language is <span
class="ptmri7t-">lazy</span>, that is, it delays
evaluation as long as it possibly can. For example, <span
class="pcrr7t-">error</span>
is a function which immediately halts execution of your
function and pops up an alert window. So:
<div class="verbatim" id="verbatim-61">
12 + error "wombat!"
</div>
<!--l. 864--><p class="nopar" >
<!--l. 866--><p class="noindent" >Has no value: this expression will halt with an error
message. However:
<div class="verbatim" id="verbatim-62">
false && error "lasagne!"
</div>
<!--l. 871--><p class="nopar" >
<!--l. 873--><p class="noindent" >Will evaluate to <span
class="pcrr7t-">false</span>, since <span
class="pcrr7t-">nip2 </span>knows after looking at
the left-hand-side of <span
class="pcrr7t-">&& </span>that the result must be <span
class="pcrr7t-">false</span>, and
so does not evaluate the right-hand-side.
<div class="verbatim" id="verbatim-63">
[12, error "hot chilli!"] ? 0 == 12
</div>
<!--l. 880--><p class="nopar" >
<!--l. 882--><p class="noindent" >This also evaluates completely, since the second element of
the list is never used, and therefore never evaluates.
<!--l. 886--><p class="indent" > Things become more confusing when you start calling
functions, since the arguments to a function call are also
not evaluated until the function needs that value. For
example:
<div class="verbatim" id="verbatim-64">
foldr (error "boink!") 2 [] == 2
</div>
<!--l. 892--><p class="nopar" >
<!--l. 894--><p class="noindent" >Again, this evaluates successfully, since the function is
never used by <span
class="pcrr7t-">foldr</span>.
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