[erlang-questions] newbie question of if guards

Chris Rathman <>
Mon Dec 11 01:37:58 CET 2006


Thanks for the answers (from Mathias and Kostis)!  That gets me further 
along. 

As long as I'm asking annoying newbie questions and I'm on the pedagogic 
subject of square roots....  :-)

I'm trying to figure out the way Erlang does nested functions.  From 
what I gather, anonymous functions would be the way to go.  
Unfortunately, I can't figure out how to make the lambda functions be 
recursive, and searches for 'Erlang' and 'fun' don't narrow it down 
enough.  Here's what I am trying to do:

sqrt(X) ->
   Good_Enough =
      fun (Guess) ->
         abs(square(Guess) - X) < 0.001
      end,

   Improve =
      fun (Guess) ->
         average(Guess, X / Guess)
      end,

   Sqrt_Iter =
      fun (Guess) ->
         case (Good_Enough(Guess)) of
            true -> Guess;
            false -> Sqrt_Iter(Improve(Guess))
         end
      end,

   Sqrt_Iter(1.0, X).

The recursive call in Sqrt_Iter gives me an error informing me that it 
is unbound.  I can understand why it is unbound, but I was wondering if 
there's something similar to CaML's 'rec' modifier?  Or is there another 
Erlang idiom for achieving this effect?

Thanks,
Chris Rathman

Matthias Lang wrote:
> Erlang does not allow calling arbitrary functions from guards.
>
> One way to achieve what you're attempting below is to use 'case'
> instead:
> ....
> Matthias
>
>
>   




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