#8099: Alternate syntax for indicating when a function is "fully applied" for purposes of inlining -------------------------------------+------------------------------------ Reporter: jberryman | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by ekmett): Replying to [comment:9 simonpj]:
PS: Edward, could you offer a couple of concrete examples?
This issue I mentioned is up on github at https://github.com/ekmett/lens/issues/183 We have a lot of code like {{{ over l f = runIdentity #. l (Identity #. f) }}} In that you can read (#.) as if it were (.), but if we switch it to {{{ over l = \f -> runIdentity #. l (Identity #. f) }}} then GHC can inline it better in a call like {{{ foo :: (a -> b) -> [a] -> [b] foo = over mapped }}} For reference: {{{ mapped f = Identity #. fmap (runIdentity #. f) }}} Unfortunately, this means that the 'f' argument isn't in scope for where clauses, etc. which makes a lot of function implementations a lot uglier when they have more structure than this trivial example. {{{ scanr1Of l f = snd . mapAccumROf l step Nothing where step Nothing a = (Just a, a) step (Just s) a = let r = f a s in (Just r, r) }}} There because f is referenced in the where clause step becomes explicitly parameterized on f, so we wind up having to either plumb the f argument in to step or we have to define step inside of a let clause inside of the lambda that takes f cluttering things up considerably. You can find dozens of examples by skimming through http://hackage.haskell.org/packages/archive/lens/3.9.0.2/doc/html/src /Control-Lens-Traversal.html#mapMOf or http://hackage.haskell.org/packages/archive/lens/3.9.0.2/doc/html/src /Control-Lens-Fold.html#foldrOf Almost any combinator with a name that ends in "Of" which currently takes more than one argument on the left hand side of the equal sign is a candidate. Our current plan is to just bite the bullet and move all the code around to get better inlining, because we've found it makes an difference in quality of the resulting core. -Edward -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8099#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler