[erlang-questions] newbie question of if guards
Sun Dec 10 23:23:04 CET 2006
Chris Rathman writes:
> I'm probably missing the obvious, but I can't figure out how to use the
> value of a function as a condition within an if construct. In the
> sqrt_iter function below, the compiler gives me an illegal guard error -
> where good_enough is a function that returns a boolean.
Erlang does not allow calling arbitrary functions from guards.
One way to achieve what you're attempting below is to use 'case'
square(X) -> X * X.
good_enough(Guess, X) -> abs(square(Guess) - X) < 0.001.
average(X, Y) -> (X + Y) / 2.
improve(Guess, X) -> average(Guess, X / Guess).
sqrt_iter(Guess, X) ->
case good_enough(Guess, X) of
true -> Guess;
_ -> sqrt_iter(improve(Guess, X), X)
The mailing list archives and the FAQ contain some commentary as to
why Erlang restricts guards in that way.
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