Menu
Emacs lisp is a dialect of LISP and especially Scheme. Most of scheme interpreters do have a optimization of Tail Recursion, but emacs lisp doens't. I searched the reason in `info elisp' for a while, but I fail to find it.
P.S. Yes, there is other iteration syntax in elisp like `while', but I still cannot find a good reason why they didn't implement tail recursion like other scheme interpreters.
204
Some compilers for languages such as Common Lisp and C perform a limited form of "tail call optimization," but Scheme's treatment of tail calls, is more general, and standardized, so you can use recursion more freely in your programs, without fear of stack overflow if you code your routines tail-recursively.
As always, it depends As a rule of thumb; tail-recursive functions are faster if they don't need to reverse the result before returning it. That's because that requires another iteration over the whole list. Tail-recursive functions are usually faster at reducing lists, like our first example.
If the return statement of the recursive function is a call to itself, i.e return recursiveFunction() and nothing else, then the javascript engine will be able to optimize the tail call and not grow the stack.
Emacs Lisp was created in the 1980's. The Lisp dialect that the Emacs author (Richard Stallman) was most familiar with at the time was MIT Maclisp, and Emacs Lisp is very similar to it. It used dynamic variable scoping and did not have lexical closures or tail recursion optimization, and Emacs Lisp is the same.
Emacs Lisp is very different from Scheme. The biggest difference is that Scheme is lexically scoped, but this was only recently added to Emacs Lisp, and it has to be enabled on a per-file basis (by putting ;;; -*- lexical-binding: t -*- on the first line) because doing it by default would cause many incompatibilities.
There has been some work to replace Emacs Lisp with Guile, a Scheme dialect. But it's not clear whether it will ever reach fruition.
140
Emacs Lisp has had dynamic scoping as its main/only scoping rule for the first 25 years of its life. Dynamic scoping is basically incompatible with optimization of tail-recursion, so until Emacs-24 (which introduced lexical scoping) there was very little to no interest in this optimization.
Nowadays, ELisp could benefit sometimes from optimization of tail recursion, and there's been some patches submitted to do that, but that hasn't yet been integrated. The lack of tail-recursion optimization as well as the relatively inefficient implementation of function calls has influenced ELisp style, such that recursion is not used very often, which in turns reduces the benefits of adding the optimization of tail calls.
24
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
