From: "Matthias Wächter" <matthias@...>
Date: 2007-09-04T06:50:43+09:00
Subject: Re: with

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?

- Matthias