[Camillo]

I agree, the examples in the article look terribly confusing. And they're inconsistent with Swift's own for loop syntax: https://developer.apple.com/library/content/documentation/Sw...

    for item in groceries { print item.department }

versus:

    Dictionary(grouping: groceries by: { item in item.department })

The people who chose this syntax probably said "hey, it's VARIABLE in EXPRESSION, same thing right?". But the semantics are completely different!

In the for loop it's "for PRODUCT in SOURCE": the expression is evaluated first, and the variable is assigned with each of its items in turn. In the lambda or whatever it is, it's "{ SOURCE in PRODUCT }": first the variable is assigned, then the expression is evaluated based on it. The data flow is the opposite!

This is just objectively bad language design.