[erlang-questions] newbie question of if guards
Sun Dec 10 23:31:53 CET 2006
Chris Rathman wrote:
> 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.
> 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) ->
> (good_enough(Guess, X)) -> Guess;
> true -> sqrt_iter(improve(Guess, X), X)
Unfortunately, 'if' is one of the dark corners of Erlang -- the
expressions that can appear as part of 'if' can only be guard builtins;
not user-defined functions as your good_enough function.
Your example can be written as:
sqrt_iter(Guess, X) ->
case good_enough(Guess, X) of
true -> Guess;
false -> sqrt_iter(improve(Guess, X), X)
My advice: try to forget that Erlang has 'if' -- things become much
easier if you pretend you never knew 'if' existed in Erlang and its
creator only allowed 'case' expressions.
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