[erlang-questions] illegal guard expression for IF illegal guard expression for "if"

[Barco You]

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.

BRs,
Barco
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