#13698: Add a more complete example for the special SPEC argument to the user guide -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: SpecConstr Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In the section for `-fspec-constr` in the user guide there is an example of how to use `SPEC` to force a loop to be unrolled by spec constr. However, this argument doesn't do anything until `foldl` is called with a specific argument. It wasn't clear to me exactly how this worked as the example was incomplete. Below is a complete file which shows how the optimisation unrolls the list completely such that `res2 = 6` in core. {{{ {-# LANGUAGE BangPatterns #-} {-# LANGUAGE ExistentialQuantification #-} module Foo where import GHC.Types data Stream a = forall s. Stream !(s -> Step a s) -- a stepper function !s -- an initial state data Step a s = Yield a !s | Skip !s | Done s2 = Stream s [1,2,3] s [] = Done s (x:xs) = Yield x xs res2 = Foo.foldl (+) 0 s2 foldl :: (a -> b -> a) -> a -> Stream b -> a {-# INLINE foldl #-} foldl f z (Stream step s) = foldl_loop SPEC z s where foldl_loop !sPEC z s = case step s of Yield x s' -> foldl_loop sPEC (f z x) s' Skip s' -> foldl_loop sPEC z s' Done -> z }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13698 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler