devdocs/elisp/primitive-function-type.html


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 <h4 class="subsection">Primitive Function Type</h4>  <p>A <em>primitive function</em> is a function callable from Lisp but written in the C programming language. Primitive functions are also called <em>subrs</em> or <em>built-in functions</em>. (The word “subr” is derived from “subroutine”.) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a <em>special form</em> (see <a href="special-forms">Special Forms</a>). </p> <p>It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, <strong>we discourage redefinition of primitive functions</strong>. </p> <p>The term <em>function</em> refers to all Emacs functions, whether written in Lisp or C. See <a href="function-type">Function Type</a>, for information about the functions written in Lisp. </p> <p>Primitive functions have no read syntax and print in hash notation with the name of the subroutine. </p> <div class="example"> <pre class="example">(symbol-function 'car)          ; <span class="roman">Access the function cell</span>
                                ;   <span class="roman">of the symbol.</span>
     ⇒ #&lt;subr car&gt;
(subrp (symbol-function 'car))  ; <span class="roman">Is this a primitive function?</span>
     ⇒ t                       ; <span class="roman">Yes.</span>
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