is_int is doing the work of a type checker?

I mean I'm not an Elixer guy (not at all) so it's a bit opaque to me how that's not an enforcement of specific types (albeit the firstname, lastname can be anything?)

Once we get into the function itself, if two incompatible types, say a string and a filehandle.. (just random attempt, you may, if you choose, point to why the two types must have some alignment), what happens - the function dies?

That's my core issue with dynamic typing, you might argue that the function's explosion has a restricted blast radius, but that's not the point, the point is that the function was called, and was unable to compute whatever, and that's a bug.