I think the use of 'idiomatic' that daviddaviddavid had in mind is this one[1]:
1. a. Peculiar to or characteristic of a given language.
b. Characterized by proficient use of idiomatic expressions: a foreigner who speaks idiomatic English.
That use of the word 'idiomatic' is very common, and although it's related to the word idiom, it's not quite the same thing. An idiom, as you say, is a phrase that you can't guess the meaning of just by glossing each of the individual words (a favorite example - in Puerto Rico, though not all Spanish-speaking countries, a flatterer is a 'lambeojo', literally 'eye-licker'). But someone who speaks Spanish idiomatically or who writes idiomatic French does so 'like a native'. It's a good thing. So when someone says that 'each' is 'idiomatic Ruby', that's probably what they have in mind.
All of that said, I can understand wanting to teach beginners the for loop first. But I don't think a Ruby course should finish without looking at each. If a person plans to read any real Ruby, they had better be ready for lots of iteration with each.
1. a. Peculiar to or characteristic of a given language. b. Characterized by proficient use of idiomatic expressions: a foreigner who speaks idiomatic English.
That use of the word 'idiomatic' is very common, and although it's related to the word idiom, it's not quite the same thing. An idiom, as you say, is a phrase that you can't guess the meaning of just by glossing each of the individual words (a favorite example - in Puerto Rico, though not all Spanish-speaking countries, a flatterer is a 'lambeojo', literally 'eye-licker'). But someone who speaks Spanish idiomatically or who writes idiomatic French does so 'like a native'. It's a good thing. So when someone says that 'each' is 'idiomatic Ruby', that's probably what they have in mind.
All of that said, I can understand wanting to teach beginners the for loop first. But I don't think a Ruby course should finish without looking at each. If a person plans to read any real Ruby, they had better be ready for lots of iteration with each.
[1]: http://www.thefreedictionary.com/idiomatic