Hi, a common pattern is x `elem` [a,b,c] or x `elem` "/!#" instead of x == a || x == b || x == c or x == '/' || x == '!' || x == '#'.
This used to be also what hlint suggested, although that hint was removed https://github.com/ndmitchell/hlint/issues/31. Upon closer inspection it seems that the compiler is not optimizing the former to the latter (more efficient, as allocation-free) form, which mildly surprised me. I guess the problem is described in this note in GHC/Base.hs: -- The foldr/cons rule looks nice, but it can give disastrously -- bloated code when commpiling -- array (a,b) [(1,2), (2,2), (3,2), ...very long list... ] -- i.e. when there are very very long literal lists -- So I've disabled it for now. We could have special cases -- for short lists, I suppose. -- "foldr/cons" forall k z x xs. foldr k z (x:xs) = k x (foldr k z xs) Now I am quite demanding of my compiler, and in particular I expect it to make a more informed choice here. Is there a good way of making an informed choice here? Up to what length of the list is it a good idea to do this? And can we implement it? Maybe simply with a few static rules for list lengths up to a certain, short length? (Maybe I should try that on a branch and see what perf.haskell.org has to say about it.) Greetings, Joachim -- Joachim “nomeata” Breitner m...@joachim-breitner.de • https://www.joachim-breitner.de/ XMPP: nome...@joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nome...@debian.org
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