#10804: Rules conditional on strictess properties -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: feature | Status: new request | Priority: low | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Hi, This is taking note of a rough idea, i.e. something that I might want to investigate more deeply some day. The current libraries contain this code, which makes the compiler switch from `foldl` to `foldl'` for certain types where `min` is known to be strict in the first argument. {{{#!hs minimum :: (Ord a) => [a] -> a {-# INLINE [1] minimum #-} minimum [] = errorEmptyList "minimum" minimum xs = foldl1 min xs {-# RULES "minimumInt" minimum = (strictMinimum :: [Int] -> Int); "minimumInteger" minimum = (strictMinimum :: [Integer] -> Integer) #-} strictMinimum :: (Ord a) => [a] -> a strictMinimum [] = errorEmptyList "minimum" strictMinimum xs = foldl1' min xs }}} Is is a bit sad that this can only be done by listing explicit instances. What I would like to see is that it sufficies to write {{{#!hs minimum :: (Ord a) => [a] -> a {-# INLINE [1] minimum #-} minimum [] = errorEmptyList "minimum" minimum xs = foldl1 min xs }}} and then there are general rules taking care of choosing the right `foldl` variant: {{{#!hs {-# RULES "strict foldl" forall f. mumble_mumble f ==> foldl f = foldl' f "strict foldl1" forall f. mumble_mumble f ==> foldl1 f = foldl1' f #-} }}} The `mumble_mumble` would be some way of specifying the required strictness properties of `f`, which would be checked use the strictness analyzer. A simple `isStrict f` is probably not sufficient (The information that `f` is strict in the first argument also depends on how many arguments we pass to f, as `min ⊥ ≠ ⊥`, but `∀x. min ⊥ x = ⊥`). Maybe `∀x. min ⊥ x = ⊥`, or some ASCIIfication of it, is indeed the proper sytanx.... This is related to #9137, which also wants conditional rewrite rules with certain compiler-checked aspectes of some of the matched variables. I think there is also a ticket (or at least an idea) about rewrite rules being conditional on some equation (e.g. (not very useful) `RULES forall f (*). (forall x y. x * y ≡ y * x) ==> foldl1 (*) (reverse xs) == foldl (*) xs`). I guess the rather vague #9601 covers that. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10804 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler