Re: pure-fns in byte-opt.el

[Stefan Monnier <monnier@iro.umontreal.ca>]

> A side-effect-free function is one where all of the following is true:
> - After exiting the function (whether normally or nonlocally), the
> `match-data' are `equal' to the `match-data' before entering the function.
> - After exiting the function, all dynamic variables, their default values,
> their symbol cells, etc., are `eq' to the respective values before entering
> the function. Exceptions are variables only used for debugging/tracing,
> such as `cons-cells-consed'.
> - All buffer contents, markers, point values etc. are
> `equal-including-properties' to their previous values.
> - The same buffers and threads still exist.

I don't think we want to enumerate all the manners that side-effects can
occur, since we will inevitably forget some (and the list will get out
of date).

I think we should define it more as something like:

    side-effect-free means: if the result of the function is not used,
    calling the function or not does not affect the behavior of the program,
    except that the function may signal an error.

    side-effect-and-error-free means: like side-effect-free but cannot
    signal an error.  This means that the optimizer can eliminate calls
    to those functions if the result is not used.

    pure: like side-effect-free but additionally, the function always
    returns the same value when called with the same arguments, so the
    compiler can precompute the call if it knows the arguments.


-- Stefan