[lotyrin]

There's also a disconnect with using a PL's concept of a function (procedure, calling convention) instead of the simple intellectual concept (composition) whether or not you're concerned with the mathematical concept (purity); language compilers or runtimes often perform poorly because as a matter of course every function call adds to a stack (failing to perform obvious inline or tail call optimizations), involves a dispatch that might be more expensive than the function itself (just in case inheriting code overloaded it or because every procedure lives in a run-time mutable table), or causes potentially large copies of arguments (to pretend at them being immutable by called code), etc. -- paying costs which in many cases could be avoided.