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large performance regression in type checker speed in 7.8

    • Closed Issue created by Carter Schonwald @carter

    it was recently demonstrated on the haskell-cafe mailing list that the following code takes measurably longer to type check in GHC 7.8.2 than in GHC 7.6.

    http://www.haskell.org/pipermail/haskell-cafe/2014-June/114562.html

    the program example is as follows

    begin :: Monad m => (m () -> t) -> t
begin cont = cont (return ())
 
a :: IO a -> (IO () -> t) -> t
a m cont = cont (m >> putStrLn "a")
 
end :: t -> t
end m = m
 
main = begin a a a a a a a a a a a a a a a a a a end

    takes about a second to type check in ghc 7.8, and is "instant" in 7.6 ()

    each additional a doubles the time to type check the program

    main = begin a a a a a a a a a a a a a a a a a a a a a a a a end

    takes longer than I have the patience to wait :) (so more than 5 seconds)

    the author of the email notes that a related program

      main = id id id id id id id id id id id
         id id id id id id id id id id id (return ())

    has the exponential complexity in both 7.8.2 and 7.6.2

    This It could very well be that some of the type checker changes between 7.8 and 7.6 enlarged the space of programs that trip up exponential worst case behavior, but one way or another understanding *why* this happened

    Edited
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