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- <a name="Have-combinator-1"></a>
- <h4 class="subsubsection">2.7.16.2 Have combinator</h4>
- <p>As virtual machine interfaces to external libraries accumulate faster
- than they can be documented and may vary from one installation to
- another, it is helpful to have a way of interrogating the virtual
- machine for an up to date list of the installed libraries and
- functions. A combinator called <code>have</code> can be used to test for the
- availability of a library function. It takes the form
- </p>
- <dl compact="compact">
- <dt> <em>T34</em></dt>
- <dd><p>[[<code>have</code>]] (<code><var>x</var></code>,<code><var>y</var></code>) = <code>((nil,nil),((nil,<var>x</var>),(nil,<var>y</var>)))</code>
- </p></dd>
- </dl>
- <p>where <var>x</var> is the name of a library and <var>y</var> is the name of a
- function within the library encoded as character strings. For example,
- if <var>x</var> is <code>'mtwist'</code> and <var>y</var> is <code>'u_disc'</code> (for the
- natural random number generator function in the Mersenne twistor
- library) then <code>have(<var>x</var>,<var>y</var>)</code> is a function that returns
- a non-empty value if an only if that library is installed and that
- function is available within it. The actual argument to the function
- is ignored as the result depends only on the installed virtual machine
- configuration. In this sense, it acts like a <code>constant</code> combinator.
- </p>
- <p>One way for this combinator to be used is in code of the form
- </p>
- <table><tr><td> </td><td><pre class="example"> portable_rng =
- conditional(
- have('mtwist','u_disc'),
- library('mtwist','u_disc'),
- some_replacement_function)
- </pre></td></tr></table>
- <p>which will use the library function if available but otherwise use a
- replacement function. Code in this form makes the decision at run
- time, but it is also possible to express the function such that the
- check for library presence is made at compile time, as the following
- example shows, which will imply a slight improvement in performance.
- </p>
- <table><tr><td> </td><td><pre class="example"> non_portable_rng =
- apply(
- conditional(
- have('mtwist','u_disc'),
- constant library('mtwist','u_disc'),
- constant some_replacement_function),
- 0)
- </pre></td></tr></table>
- <p>This program would be non-portable in the sense that it would need to
- be recompiled for each installation if there were a chance that some
- of them might have the <code>mtwist</code> library and some might not,
- whereas the previous example would be binary compatible across all of
- them. <a name="DOCF2" href="avram_fot.html#FOOT2">(2)</a>
- </p>
- <p>The actual value returned by a function <code>have(foo,bar)</code> is the
- list of pairs of strings <code><(foo,bar)></code> if the function is
- available, or the empty list otherwise. A non-empty list is
- represented as a pair <code>(head,tail)</code>, and an empty list as
- <code>nil</code>. The angle bracket notation <code><a,b,c...></code> used here is
- an abbreviation for <code>(a,(b,(c...nil)))</code>.
- </p>
- <p>Either or both arguments to the <code>have</code> combinator can be a
- wildcard, which is the string containing a single asterisk,
- <a name="index-wild-cards"></a>
- <code>'*'</code>. In that case, the list of all available matching library
- names and function names will be returned. This feature can be used to
- find out what library functions are available without already knowing
- their names.
- </p>
- <p>If a library had a function named <code>'*'</code>, which clashes with
- the wild card string, the interpretation as a wild card would take
- precedence.
- </p>
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