Re: master 0cbcc62: 'assoc' is not side-effect-free; constprop its pure subset

["Mattias Engdegård" <mattiase@acm.org>]

1 nov. 2020 kl. 10.47 skrev Andrea Corallo via Emacs development discussions. <emacs-devel@gnu.org>:

> just ot mention, would be nice if we design this to be accessible so we
> can use it in the native compiler as well maintaining it just in one
> place.

It wasn't really meant seriously, but a simple system may look like

EFFECT ::=
   nil        ; no effect (subset of all effects)
 | *          ; any effect (superset of all effects)
 | signal     ; signal any condition (error etc)
 | read-env   ; read from the global environment
 | N          ; effect of function passed as argument N, N≥1
 | (union EFFECT...)  ; union of effects

Then today's declarations correspond to the effects:

pure              ~  signal
side-effect-free  ~  (union signal read-env)
error-free        ~  read-env

and the effects of 'assoc' would be (union signal 3).

More fine-grained effects could be added, such as throws, mutation of arguments, reading data referenced by arguments, etc.

It all depends on what is useful for the compiler(s) and related tools.