From: Robert Klemme <shortcutter@...>
Date: 2007-09-04T14:30:04+09:00
Subject: Re: with

On 03.09.2007 23:50, Matthias W�chter wrote:
> Robert Klemme schrieb:
>> Somewhere above you completely lost me - it is especially unclear to me
>> what "a_very_long_calculation" is supposed to be.  Is this a variable
>> name?  Is this a placeholder for a long expression?
> 
> It's the latter.
> 
> Suppose you do a calculation like the following:
> 
> a=["abc","thing"].
>      map{|x| x.length}.
>      inject(0){|sum,x|sum+x}.
>      to_f**0.5
> 
> (this is only for the sake of example, don't try to make sense out of it).
> 
> When I want to insert a function between .map and .inject, I can simply do that:
> 
> a=["abc","thing"].
>      map{|x| x.length}.
>      map{|x| x>5 ? 5 : x}.
>      inject(0){|sum,x|sum+x}.
>      to_f**0.5
> 
> I don't have to change any part of the expression before or after my change, it's just an *insert*.
> 
> But I can't do the same for values that are not arrays. Say, after .to_f I want to set bounds on the value so that the square root function does not raise an exception. Then .with is useful.
> 
> a=["abc","thing"].
>      map{|x| x.length}.
>      map{|x| x>5 ? 5 : x}.
>      inject(0){|sum,x|sum+x}.
>      to_f.
>      with{|f| f<0.0 ? 0.0 : f}**0.5
> 
> Otherwise I'd have to split execution at this place, assign to a temporary variable, make my calculation and continue the expression later on.
> 
> temp=["abc","thing"].
>      map{|x| x.length}.
>      map{|x| x>5 ? 5 : x}.
>      inject(0){|sum,x|sum+x}.
>      to_f
> if temp<0.0
>   temp = 0.0
> end
> a=temp**0.5
> 
> I like the 'with' version better.
> 
>> Anyway, I do wonder whether all this can be nicely solved by using local
>> variables.  Basically #with does nothing else than binding a value to a
>> local variable with limited scope.
> 
> Ruby tries to avoid local variables wherever it can by chaining iterators together like in file-open-each-line-gsub. I tried to allow a block to be used for non-enumerable types as well.
> 
> What .with does is producing a little space for writing a block instead of requiring to reorder the whole function just for adding this little piggy-bag change.
> 
> Does it make sense now?

Yes, thank you for the explanation!  I know now what you were after.  I 
am not sure I'd subscribe to the Ruby "tries to avoid local variables".

Kind regards

	robert