Magic (programming)

<h2 id="referential-opacity">Referential opacity</h2>
Main article: <a href="/facts/Referential_transparency/g0zARbAD">Referential transparency</a>
"Magic" refers to <a href="/facts/Procedure_(computer_science)/9vFJrg0A">procedures</a> which make calculations based on data not clearly provided to them, by accessing other modules, memory positions or global <a href="/facts/Variable_(programming)/VoCKtwml">variables</a> that they are not supposed to (in other words, they are not referentially transparent). According to most recent <a href="/facts/Software_architecture/1Hokp0Ed">software architecture models</a>, even when using <a href="/facts/Structured_programming/zA8b8dP1">structured programming</a>, it is usually preferred <a href="/facts/Pure_function/xveknjJ3">to make each function behave the same way every time</a> the same arguments are passed to it, thereby following one of the basic principles of <a href="/facts/Functional_programming/cdxqPEIM">functional programming</a>. When a function breaks this rule, it is often said to contain "magic".
A simplified example of negative magic is the following code in <a href="/facts/PHP/vU6YRJX0">PHP</a>:

function magic()
{
 global $somevariable;

echo $somevariable;
}

$somevariable = true;

magic();

While the code above is clear, if it is seen in a large project, it is often hard to understand where the function magic() gets its value from. It is preferred to write that code using the following concept:

function noMagic($myvariable)
{
 echo $myvariable;
}

$somevariable = true;

noMagic($somevariable);

<h2 id="non-orthogonality">Non-orthogonality</h2>
<blockquote>Any SV [scalar value] may be magical, that is, it has special features that a normal SV does not have.— <a href="/facts/Larry_Wall/gF8DqA5h">Larry Wall</a>, perlguts manual page,<a class="footnote-ref" id="fnref:1" href="#fn:1">1</a> Perl 5</blockquote>
This definition of magic or magical can be extended to a <a href="/facts/Data_type/fcx5hgVL">data type</a>, code fragment, keyword, or machine address that has properties not shared by otherwise identical objects. The magical properties may or may not be documented.

<ul><li>In <a href="/facts/ANSI_C/rPKeEuPm">ISO C</a>, file handles (of type FILE) cannot be safely copied as their addresses<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a> may be magic. That is, the runtime environment may place original file handles in a <a href="/facts/Hard-coded/LfJyYHn7">hard-coded</a> address range, and not provide file handle behaviour to a user-created copy at another address. Consequently, the standard library routines accept pointers to file handles, of type FILE *, instead.</li>
<li>In Perl 5, the statement while(<file_handle>) implicitly assigns the line read from the file by <file_handle> to the variable $_, and applies the defined() function to the expression so that any successfully read string, even "0" or the <a href="/facts/Empty_string/T7udJlmQ">empty string</a>, evaluates as <a href="/facts/Truth_value/SY4e2w8l">true</a> and continues the while() loop. This does not happen to <file_handle> anywhere else, or to while() with any other control expression.<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a></li>
<li>In an <a href="/facts/Emulator/FMHE8qDj">emulator</a>, especially one in development, the emulated machine's <a href="/facts/System_call/NW44AFJr">system call</a> points may be magic; when they are called, the emulator may run <a href="/facts/Native_code/bgdQ1Fxf">native code</a> for convenience, speed or access to physical hardware, and set up the emulated CPU and memory as if it had executed the original code.
<ul><li>For instance, the CALL statement of <a href="/facts/BBC_BASIC/7N5Pjzoe">BBC BASIC</a> V treats the system call addresses of <a href="/facts/Acorn_MOS/l2tee2RP">Acorn MOS</a> magically; instead of attempting to branch to ARM code at those addresses, it raises a <a href="/facts/Interrupt/Mm6C4rpc">software interrupt</a> in <a href="/facts/RISC_OS/Pc96teso">RISC OS</a> equivalent to the system call.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a>[non-primary source needed] The effect is to emulate Acorn MOS sufficiently for 8-bit BASIC programs not containing assembly language to run without modification.</li></ul></li>
<li>Also in BBC BASIC, not only does the numeric variable @% control print formatting, it accepts direct assignment of ANSI <a href="/facts/Printf/Sc6Czbp2">printf</a> format strings, normally a type mismatch error.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a></li>
<li>In <a href="/facts/JavaScript/XhhIIEsl">JavaScript</a>, evaluation of the <a href="/facts/Typeof/mCIKwDuQ">typeof</a> operator succeeds when the operand is an undeclared identifier, which would normally result in a ReferenceError.</li>
<li>Any <a href="/facts/Comment_(computer_programming)/Q8hHFq8h">comment</a> that has an effect on the code is magic.</li>
<li><a href="/facts/Memory-mapped_I%2fO/PmLNFq01">Memory-mapped I/O</a> addresses and <a href="/facts/Volatile_variable/GLgsSNDB">volatile variables</a> are also magic in this sense, although the term is not normally applied.</li></ul>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Magic_number_(programming)/Xg7mVI7V">Magic number (programming)</a></li>
<li><a href="/facts/Black_box/hJeAGnIz">Black box</a></li>
<li><a href="/facts/Cargo_cult_programming/Sf9kWpQV">Cargo cult programming</a></li>
<li><a href="/facts/Nothing-up-my-sleeve_number/AwqF2wkR">Nothing-up-my-sleeve number</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">"perlguts – perldoc.perl.org". 5 October 2014. Retrieved 18 February 2015. <a href="http://perldoc.perl.org/perlguts.html" target="_blank">http://perldoc.perl.org/perlguts.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Banahan, Mike; Brady, Declan; Doran, Mark (1991). "9.10.3 The stdio.h header file". The C book: Featuring the ANSI C standard. The Instruction Set (2nd ed.). Wokingham, England: Addison-Wesley Publishers. p. 234. ISBN 0-201-54433-4. It is not safe to copy these objects within the program; sometimes their addresses may be 'magic'. <a href="0-201-54433-4" target="_blank">0-201-54433-4</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">"perlop – perldoc.perl.org". 7 September 2010. Retrieved 17 February 2011. <a href="http://perldoc.perl.org/perlop.html" target="_blank">http://perldoc.perl.org/perlop.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">"27. Keywords" (PDF). BBC BASIC Reference Manual (1st ed.). Cambridge, England: Acorn Computers. October 1992. pp. 229, 349. ISBN 1-85250-103-0. Retrieved 9 May 2007.[dead link] <a href="1-85250-103-0" target="_blank">1-85250-103-0</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">"27. Keywords" (PDF). BBC BASIC Reference Manual (1st ed.). Cambridge, England: Acorn Computers. October 1992. pp. 229, 349. ISBN 1-85250-103-0. Retrieved 9 May 2007.[dead link] <a href="1-85250-103-0" target="_blank">1-85250-103-0</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
</ol>

Magic (programming) open-in-new

Magic (programming)