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-- T15226
import Control.Exception (evaluate)
-- Just in case Prelude.repeat changes for some reason.
import Prelude hiding (repeat)
import Data.Coerce
-- We want to be sure that the compiler *doesn't* know that
-- all the elements of the list are in WHNF, because if it
-- does, GHC.Core.Opt.ConstantFold may erase the seq#'s altogether.
repeat :: a -> [a]
repeat a = res
where res = a : res
{-# NOINLINE repeat #-} -- Belt *and* suspenders
newtype Foo = Foo Int
silly :: [Int] -> IO ()
silly = foldr go (pure ())
where
go x r = do
x' <- (coerce (evaluate :: Foo -> IO Foo) :: Int -> IO Int) x
evaluate (x' + 3) -- GHC should know that x' has been evaluated,
-- so this calculation will be erased entirely.
-- Otherwise, we'll create a thunk to pass to
-- evaluate.
r
main :: IO ()
-- 10,000,000 repetitions take only a twentieth of a second,
-- but allocations go up dramatically if the result is not
-- known evaluated.
main = silly $ take 10000000 $ repeat 1
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