From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Philipp Stephani Newsgroups: gmane.emacs.bugs Subject: bug#42147: 28.0.50; pure vs side-effect-free, missing optimizations? 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Juli 2020 um 23:00 Uhr schrieb Mattias Engdeg=C3=A5rd : > > 25 juli 2020 kl. 22.11 skrev Philipp Stephani : > > > The manual states that > > "This function [concat] always constructs a new string that is not > > =E2=80=98eq=E2=80=99 to any existing string" so I don't see how it coul= d ever be pure. > > Actually that part of the manual was corrected fairly recently, as that s= tatement hasn't been true for decades. > > More to the point, the current set of functions marked as 'pure' are real= ly the superset 'pure-absent-mutation': functions that are pure when it can= be assumed that the arguments are not modified. This assumption can be bas= ed on physical immutability (integers), by convention (string constants), o= r anything else the compiler can prove such as control flow. > > There is also the question of what equality to use and here the answer is= probably 'equal' since we are only dealing with immutables. (The return va= lues of these functions cannot be considered mutable for obvious reasons, s= o make-string is out.) > > If you want to rename the property accordingly then I won't object. In an= y case, it is certainly a good idea to be precise about what the various se= ts really mean. It looks like these are really two separate concepts, which should be documented separately and should get separate function properties: - Fully-pure functions are guaranteed to either return results that are "eql" or signal error data that is "equal-including-properties" when given arguments that are pairwise "eql", for all possible arguments, mutable or not. - Pure-unless-mutable functions are a strict superset whose restrictions to immutable arguments are guaranteed to either return results or signal error data that are "equal-including-properties" when given arguments that are immutable and pairwise "equal-including-properties". Both categories must be free of observable side effects as well. > > There are also some functions declared 'pure' that appear to have side ef= fects: kbd, package-get-version > kbd can and should be made pure-unless-mutable at least. package-get-version should be neither, but should be marked using a third to-be-introduced property (e.g. "eval-when-compile") to state the intention without resorting to hacks.