Off the top of my head, here are languages in use that have a feature for pass-by-reference semantics: C#, Ada, C++ (though in this language you can get ahold o

Off the top of my head, here are languages in use that have a feature for pass-by-reference semantics: C#, Ada, C++ (though in this language you can get ahold of the reference as a value if you want). Of course, to implement pass by reference, you tend to have to pass pointers around, but the point is that the language gives the illusion that the identifiers within a function's scope are locations specified by the caller. I guess a takeaway is that in designing a language, one has to consider the relationship between an identifier, the mutable cell associated with the identifier, and the value stored in that cell. It seem that in common languages the identifier refers to the cell, but if the identifier is used in a value context, it evaluates to the value in the cell. Languages with pass-by-reference semantics let you make your own & function, so to speak. (A weird example in C of this relationship is that an identifier for an array refers to a cell that contains the whole array, but, if you use the identifier in a value context, it becomes a pointer to the first element. One aspect of it not actually being a pointer is that you cannot replace its value with the pointer to another array.) Anyway, an everything-must-be-a-first-class-object hard-liner would insist that pass-by-reference is the way of the past. But, sometimes it is not economical to get everything to be a first-class object. For instance, you might decide it is not worth trying to create a pointer type for referring to members of a packed struct, but it is not too hard to make a pass-by-reference feature for this (via copying).