The proc/gen/fun distinction isn't entirely an optimisation issue, its a semantic one, heavily related to the implementation model. In principle fun and gen use the machine stack for return addresses whilst proc uses a heap allocated list which allows cooperative multi-tasking using channels for communication: such so-called spaghetti stacks are easy to swap. Machine stacks can only be swapped in a conforming way in C/C++ by using pre-emptive threads which is precisely what we're trying to avoid.

I said "entirely" above because semantics and optimisation are heavily intertwined in any system.

The comment about "syntactic sugar" is valid: there are at least four operators with different precedences all meaning "application of function to arguments". However note that in most languages this is true anyhow: x + y is really just add (x,y). Finding a good set of squiggles and marks that's acceptable to many people isn't easy.

However the syntax is defined in the library, in user space, so you can add your own grammar or design your own domain specific sub-language, and add your favorite squiggles that way. Any contributions to making the standard syntax simpler would be welcome: I'm constantly struggling with this issue because I'm well aware syntax matters, especially early in learning a language.