Hi, Am Donnerstag, den 09.03.2017, 15:21 +0000 schrieb Simon Peyton Jones via ghc-devs:
+-- The "mapFB" rule optimises compositions of map and +-- the "mapFB/id" rule get rids of 'map id' calls. +-- (Any similarity to the Functor laws for [] is expected.)
Yes, obviously. But did you have a use-case, or did you just to this on principle? If you don't have this rule what goes wrong. You must have had a proximate reason for adding it.
Just on principle. I saw "map id" in some code dump and thought that this should not be there.
Oh. And I've just realised that mapFB c id will turn into \x y. c x y as soon as you inline mapFB. So why do you need a RULE to do that? Why not leave it for the inliner?
Because we have to keep mapFB around to rewrite it back with this rule "mapList" [1] forall f. foldr (mapFB (:) f) [] = map f This is explained in Note [Inline FB functions] in GHC.List, which is referenced from the {-# INLINE [0] mapFB #-} pragma. Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • https://www.joachim-breitner.de/ XMPP: nomeata@joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org