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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Skutečně lokální proměnné</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot"><link rel="home" href="index.html" title="Příručka k aplikaci Genius"><link rel="up" href="ch07.html" title="Chapter 7. Pokročilé programování v jazyce GEL"><link rel="prev" href="ch07s03.html" title="Vracení funkcí"><link rel="next" href="ch07s05.html" title="Spouštěcí procedura GEL"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Skutečně lokální proměnné</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch07s03.html">Prev</a> </td><th width="60%" align="center">Chapter 7. Pokročilé programování v jazyce GEL</th><td width="20%" align="right"> <a accesskey="n" href="ch07s05.html">Next</a></td></tr></table><hr></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="genius-gel-true-local-variables"></a>Skutečně lokální proměnné</h2></div></div></div><p lang="en">
When passing functions into other functions, the normal scoping of
variables might be undesired. For example:
</p><pre lang="en" class="programlisting">k := 10;
function r(x) = (x+k);
function f(g,x) = (
k := 5;
g(x)
);
f(r,1)
</pre><p lang="en">
you probably want the function <code class="function">r</code>
when passed as <code class="function">g</code> into <code class="function">f</code>
to see <code class="varname">k</code> as 10 rather than 5, so that
the code returns 11 and not 6. However, as written, the function
when executed will see the <code class="varname">k</code> that is
equal to 5. There are two ways to solve this. One would be
to have <code class="function">r</code> get <code class="varname">k</code> in a
private dictionary using the square bracket notation section
<a class="link" href="ch07s03.html" title="Vracení funkcí">Returning
Functions</a>.
</p><p lang="en">
But there is another solution. Since version 1.0.7 there are
true local variables. These are variables that are visible only
from the current context and not from any called functions.
We could define <code class="varname">k</code> as a local variable in the
function <code class="function">f</code>. To do this add a
<span class="command"><strong>local</strong></span> statement as the first statement in the
function (it must always be the first statement in the function).
You can also make any arguments be local variables as well.
That is,
</p><pre lang="en" class="programlisting">function f(g,x) = (
local g,x,k;
k := 5;
g(x)
);
</pre><p lang="en">
Then the code will work as expected and prints out 11.
Note that the <span class="command"><strong>local</strong></span> statement initializes
all the referenced variables (except for function arguments) to
a <code class="constant">null</code>.
</p><p lang="en">
If all variables are to be created as locals you can just pass an
asterisk instead of a list of variables. In this case the variables
will not be initialized until they are actually set.
So the following definition of <code class="function">f</code>
will also work:
</p><pre lang="en" class="programlisting">function f(g,x) = (
local *;
k := 5;
g(x)
);
</pre><p lang="en">
</p><p>Je dobrým zvykem, aby všechny funkce, které přebírají jiné funkce jako argumenty, používaly lokální proměnné. Je to způsob, jak zabránit, aby předávaná funkce neviděla implementační podrobnosti a nemátlo ji to.</p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch07s03.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch07.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch07s05.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Vracení funkcí </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Spouštěcí procedura GEL</td></tr></table></div></body></html>
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