Issue #8398 has been updated by jeremyevans0 (Jeremy Evans).


You probably think your case statement is translated to:

case
when (String === 1) && (proc { |x| x.blah } === 1)
  :something
else
  :no
end

I believe ruby actually translates it to:

case
when (String && proc { |x| x.blah }) === 1
  :something
else
  :no
end

As String is not false or nil, it's fairly obvious why proc is evaluated.  As you can see, using && in case expressions with values doesn't generally make sense.

Note that if you don't provide a value for case, the short circuiting works as you expect:

case
when (String === 1) && proc{|x| p x} === 1
 :yes
else
 :no
end

I suppose ruby could be modified to do what you want, but it could break existing code.  Note that for || expressions, there is already special case syntax for short circuiting:

case 1
when String, proc { |x| x.blah } # String === 1 || proc { |x| x.blah } === 1
  :something
else
  :no
end
----------------------------------------
Bug #8398: case/when shouldn't try to evaluate all AND joined conditions if one of preceding was falsy 
https://bugs.ruby-lang.org/issues/8398#change-39282

Author: goshakkk (Gosha Arinich)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: 1.9.3, 2.0.0
Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN


=begin

Steps to reproduce:

 case 1
 when String && proc { |x| x.blah }
   :something
 else
   :no
 end

Here is how I thought it works: goes to the first 'when', evals the first condition ((({1 === String}))), sees it's false and proceeds to the next (({when})) (or (({else}))) and happily returns (({:no})).

As it turns out, even if one of AND joined conditions was false, ruby would still try to eval other conditions, and as soon as it calls the block with 1, (({NoMethodError})) gets thrown as there is no (({#blah})) on Integer.

I believe this is incorrect behavior and conditions with AND in case/when should behave like everywhere else.

=end



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