Yeah, but you can keep the effects *local* and not spread out all over the place like BrowserUk seems to be worried about.

Any form of IO is a side-effect, regardless of where you do it.

In an imperative language, 'functions' (procdures that return values if you prefer) are the major mechanism of code structuring. When I do IO to a database, I call a function (or method (same difference). When I read or write to a file, I call a function. When I read or write to a socket udp/tcp/http/ftp/telnet/et al, it is all done with functions and all of those functions that do IO have side-effects. f(a) + g(b) * h(c)

f(a) retrieves the range from the radar.

g(b) retrieves a correction factor from the wind speed indicator.

h(c) gets a scalar from a lookup table in memory. keyed upon the type of shell (HE, tracer, AP) that will load next from the belt.

The final result is used to adjust the elevation of the barrel in real time.

How do you avoid the side effects?

OK, for all pedants out there, here's a slight modification:

Avoid side-effects if at all possible. If you absolutely have to use them, make sure you try as hard as you can to have them act in as limited scope as possible.

Your example is perfect. Why in the world should f interact or depend on g or h? They shouldn't! It is a sign of poor programming if they did. BTW, I (heart) Perl, but God help any unfortunate soul who's life would depend on the correct functioning of a Perl program.

Trivial.

my $f = f(a);
my $g = g(b);
my $h = h(c);
$f + $g * $h;
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And then you fire the programmer putting such side-effects in the code.