illegal guard expression

今天遇到一个奇怪的问题：

put(a, 1),
if
	get(a) =:= 1 ->
		ok;
	true ->
		bad
end.

* 2: illegal guard expression

查了一下官方关于Guard的文档：
The set of valid guard expressions (sometimes called guard tests) is a subset of the set of valid Erlang expressions. The reason for restricting the set of valid expressions is that evaluation of a guard expression must be guaranteed to be free of side effects.

网上也有人问道：

On Fri, Dec 2, 2011 at 10:30 AM, Barco You <> wrote:
Why does the following expression got “illegal guard expression” when compiling:
X = 0.5,
if
random:uniform() < X -> %error reported for this line
good;
true ->
bad
end.

But if I change it to following expression, it’s ok:
X = 0.5,
Ran = random:uniform(),
if
Ran < X ->
good;
true ->
bad
end.
Yes, and not only does this mean that only a limited set of functions are allowed as guard functions, random:uniform() _does_ have side effects, since it updates the seed in the process dictionary.

可见，erlang:get/1 also has side effect.