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{-# LANGUAGE UnboxedTuples #-}
module Main where
data T = MkT !Int Int
-- An expensive recursive function
g :: Int -> Int -> (# Int, Int #)
g x 0 = (# x, 33 #)
g x n = g (x+n) (n-1)
-- 'foo' calls 'h' often
foo h 0 = 0
foo h n = h n `seq` foo h (n-1)
main = print (foo (MkT (case g 1 200 of (# a,b #) -> a))
200)
{- In main, we don't want to eta-expand the MkT to
(\x. MkT (case g 1 200 of (# a,b #) -> a) x)
because then that call to g may be made more often
The faffing with unboxed tuples is to defeat full
laziness which would otherwise lift the call to g
out to top level
Before fixing #19347, running this program gave
2,012,096 bytes allocated in the heap
after it gave
101,712 bytes allocated in the heap
-}
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