It looks like there is no difference, for
uses each
underneath.
$ irb
>> for x in nil
>> puts x
>> end
NoMethodError: undefined method `each' for nil:NilClass
from (irb):1
>> nil.each {|x| puts x}
NoMethodError: undefined method `each' for nil:NilClass
from (irb):4
Like Bayard says, each is more idiomatic. It hides more from you and doesn't require special language features. Per Telemachus's Comment
for .. in ..
sets the iterator outside the scope of the loop, so
for a in [1,2]
puts a
end
leaves a
defined after the loop is finished. Where as each
doesn't. Which is another reason in favor of using each
, because the temp variable lives a shorter period.