On 11/12/10 4:53 PM, Ryan Ingram wrote:
From http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/bugs.html#bugs-ghc:
GHC's inliner can be persuaded into non-termination using the standard way to encode recursion via a data type:
More specifically, since your bad2 does not look recursive, the inliner will get inline happy and apply the definition of bad1, but then it'll see a constant case match that it can reduce statically, and <loop> { okay, let's start with bad1 } bad1 b@(C f) = f b { mmm, tasty tasty sugar } bad1 = \b -> case b of C f -> f b { ha, dare you to optimize that further } { okay, now let's co compile bad2 } bad2 = bad1 (C bad1) { oh look, it's nonrecursive so let's start inlining } bad2 = (\b -> case b of C f -> f b) (C bad1) { oh look, an application we can evaluate statically } bad2 = let b = (C bad1) in case b of C f -> f b { meh, let's ignore sharing for now } bad2 = case (C bad1) of C f -> f (C bad1) { oh look, a case analysis we can evaluate statically } bad2 = let f = bad1 in f (C bad1) { oh look, f is only used once so we can substitute the let } { and if we didn't ignore sharing before, then we'd see that b is only used once now, so we could substitute that let too } bad2 = bad1 (C bad1) { oh look, it's nonrecursive so let's start inlining } -- Live well, ~wren