On Wed, Jul 15, 2015 at 4:37 AM, Marko Rauhamaa <ma...@pacujo.net> wrote: > Steven D'Aprano <st...@pearwood.info>: > >> Tail call behaviour is not undefined in Python. There is a strong >> convention (followed by all implementations I know of) to follow the >> reference implementation's (CPython) behaviour, which is not to >> optimize tail calls. But even if an implementation chooses to not >> follow that, it's not *undefined behaviour*. It's just >> implementation-dependent behaviour. > > But in Scheme, tail call elimination is mandatory and thus can be relied > on. Unless that promise is made, you can't write code that depends on
I think I'm beginning to understand. Tail call elimination is necessary to cope with a system of programming that has deliberately excluded certain other options (eg mutables, in pure functional languages), but is not as important in a more fully-featured language. Constant folding would be the same way; it's not absolutely critical, and if Python doesn't support it for some particular expression then it's not going to kill you, but in SPL [1] it would be a vital efficiency improvement. Scheme needs TCE because of the way you have to write code for Scheme to be even capable of executing it; Python doesn't, because you can write your code to not need TCE. [1] https://en.wikipedia.org/wiki/Shakespeare_(programming_language)#Constants_and_Assignment_of_Values ChrisA -- https://mail.python.org/mailman/listinfo/python-list
