RE: [External] : Re: function arguments optional default values

[Drew Adams <drew.adams@oracle.com>]

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> >> > Does elisp support function arguments optional default values ?
> >> I solve that problem with following:
> >> (defun my-function (arg &optional other-arg)
> >>   (let* ((other-arg (or other-arg my-default)))
> >>     other-arg))
> > Simpler - no reason for another binding,
> > just change the value (it's a local var).
> >
> > (defun my-function (arg &optional other-arg)
> >   (setq other-arg  (or other-arg my-default))
> >   ...)
> 
> FWIW, assuming you're using `lexical-binding` (which you should), this
> tends to result in less efficient byte-code.
> 
> The difference is marginal in most cases (and efficiency of ELisp code
> is irrelevant in most cases as well), but the point is that bindings
> don't "cost" as much as mutation.
> 
> Here is how this usually plays out: with (setq x (or x <default>)) the
> evaluation pushes the result of (or x <default>) on the stack, does the
> `setq` which consumes the value on the stack and modifies the value of
> `x`, which itself lives on the stack.
> 
> With (let ((x1 (or x <default>))) ...), the beginning is the same,
> pushing the result of (or x <default>) on the stack, but then the
> execution of the `let` is the "free" act of keeping that value on the
> stack (i.e. doing nothing), and instead of referring to the stack
> element that holds `x`, we will simply refer to the stack element that
> holds `x1`.
> [ There is still a cost to `let` in the fact that the stack grows a bit
>   more, of course, but it's usually of no consequence.  ]
> ... 
>
> The difference is more drastic when the variable is captured by
> a closure because our closure conversion is too naïve to understand that
> the `setq` happens "once and forall" before we capture the variable, so
> it presumes that the var may still be mutated after the closure is
> constructed, which means that the closure can't just hold a copy of the
> var's value but really needs to hold a "reference to the variable",
> which we do by storing the variable's value inside a cons-cell:
> ...

Interesting!  Thanks.

I wonder what the case is with various
Common Lisp implementations.

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