Personally, I'd argue there's only one concept -- pass-by-value -- with a whole lot of language-specific syntactic sugar as to what is getting passed by value. But I'll call myself out on that being even more pedantic than the current discussion.
Having thought about it for a minute or two, it seems pretty silly to me . Types are all about restricting the set of values an expression can have, and restricting the set of operations you can perform on an expression, such that any operation you're allowed to perform on a value will always do something sensible. Untyped languages don't do that, which makes them quite different from typed languages. But then i've only thought about it for a minute or two.
I could imagine a genuinely unityped language. Perhaps one where every value is a vector of real numbers with an infinite number of zeroes on the end, and the operations are piecewise arithmetic, dot products, interleaving, appending, etc. That would behave quite differently to the languages where you get "undefined is not a function" and friends.
I think Harper would take issue with it being explained this way, but one can think of Ruby and Python being languages where everything is a pointer to a HashMap and everything else is just sugar on top of that (similar to the above argument about Java and the sugar with the pointers).
I don't think Harper would like that way of explaining it because it's over-focused on an example instead of sticking to the concept and because it conflates (statically-knowable) types with (runtime) classes, which is big no-no with him.
This is in contrast to, say, Java, where (2/3) can be part of a valid program, but ("abc"/3) cannot, so 2 and "abc" cannot belong to the same equivalence class.
Yes. Python e.g. is more unityped (union typed, things have a type, but are handled as a union among several options) than untyped. tcl, on the other hand, is more untyped (everything is the same thing: string-like).
The whole “pass by x” really confuses things. It’s really more “represented by foo” underlying the passing that is important.