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copied!<p>A full APPLY is difficult in a static language.</p> <p>In Lisp APPLY applies a function to a list of arguments. Both the function and the list of arguments are arguments to APPLY. </p> <ul> <li><p>APPLY can use any function. That means that this could be any result type and any argument types.</p></li> <li><p>APPLY takes arbitrary arguments in arbitrary length (in Common Lisp the length is restricted by an implementation specific constant value) with arbitrary and possibly different types.</p></li> <li><p>APPLY returns any type of value that is returned by the function it got as an argument.</p></li> </ul> <p>How would one type check that without subverting a static type system?</p> <p>Examples:</p> <pre class="lang-lisp prettyprint-override"><code>(apply #'+ '(1 1.4)) ; the result is a float. (apply #'open (list "/tmp/foo" :direction :input)) ; the result is an I/O stream (apply #'open (list name :direction direction)) ; the result is also an I/O stream (apply some-function some-arguments) ; the result is whatever the function bound to some-function returns (apply (read) (read)) ; neither the actual function nor the arguments are known before runtime. ; READ can return anything </code></pre> <p>Interaction example:</p> <pre class="lang-lisp prettyprint-override"><code>CL-USER 49 > (apply (READ) (READ)) ; call APPLY open ; enter the symbol OPEN ("/tmp/foo" :direction :input :if-does-not-exist :create) ; enter a list #<STREAM::LATIN-1-FILE-STREAM /tmp/foo> ; the result </code></pre> <p>Now an example with the function REMOVE. We are going to remove the character a from a list of different things.</p> <pre class="lang-lisp prettyprint-override"><code>CL-USER 50 > (apply (READ) (READ)) remove (#\a (1 "a" #\a 12.3 :foo)) (1 "a" 12.3 :FOO) </code></pre> <p>Note that you also can apply apply itself, since apply is a function.</p> <pre class="lang-lisp prettyprint-override"><code>CL-USER 56 > (apply #'apply '(+ (1 2 3))) 6 </code></pre> <p>There is also a slight complication because the function APPLY takes an arbitrary number of arguments, where only the last argument needs to be a list:</p> <pre class="lang-lisp prettyprint-override"><code>CL-USER 57 > (apply #'open "/tmp/foo1" :direction :input '(:if-does-not-exist :create)) #<STREAM::LATIN-1-FILE-STREAM /tmp/foo1> </code></pre> <p>How to deal with that?</p> <ul> <li><p>relax static type checking rules</p></li> <li><p>restrict APPLY</p></li> </ul> <p>One or both of above will have to be done in a typical statically type checked programming language. Neither will give you a fully statically checked and fully flexible APPLY.</p>

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