Stierhof: copy from old wiki

2018-04-11T12:23:59Z

copy from old wiki

New page

Function nesting (or functions define functions) is, in principle, not possible:
<pre>
define fun() {
define fun2() { return 5; };
return fun2();
};
</pre>

This approach results in an error. But one can may use "eval" to define a nested function:
<pre>
define fun() {
eval("define fun2() { return 5; };");
return eval("fun2()");
};
</pre>

The nested function can also be made ''private''.

In that way, functions can be defined dynamically during runtime.
'''However''', there are some '''disadvantages''' one should be aware of:
* The "eval"uated string is parsed by the S-lang interpreter only during the call of eval. That means syntax errors are '''not''' detected beforehand, which could unnecessarily crash your script!
* Never ever pass external input into eval-statements -- at least not directly, but best not at all! If not handled properly, malicious code can be '''injected''' into your process or script, which is a known computer '''attack''' (see also this [http://en.wikipedia.org/wiki/Code_injection article])

Note that the function defined in the eval-Statement is by no means "nested" (in the sense of: "internal to the function"), as the following example illustrates:
<pre>
define f(x)
{
if(x == 0)
{
eval("define nested() { return 42; }");
}
return eval("nested()");
}

define g()
{
eval("define nested() { return `f's 'nested' function was hijacked by g`; }");
}

variable x;
_for x (0, 2) vmessage("f(%d) = %S", x, f(x));
g();
_for x (3, 5) vmessage("f(%d) = %S", x, f(x));
define nested() { throw DataError; }
_for x (6, 9) vmessage("f(%d) = %S", x, f(x));
</pre>

[[Category:Isis / Slang]]

Function nesting - Revision history

Stierhof: copy from old wiki