I'm using the TypeFamilies extension to generate types that are quite large. GHC can handle these large types fine when they are created manually, but when type families get involved, GHC's performance dies. It's doing in quadratic time what looks to me like it should be linear time. I don't know if this is expected behavior, if I'm doing something wrong, or if this is a GHC bug. I've attached a code sample below that demonstrates the problem. Types.hs generates other haskell files. The first parameter is the size of the type (which is type list of that length), and the second specifies which test to run. All tests generate the same type in the end, but some use type families and some don't. Here's an example of running it: These tests show quadratic time when using the type family. I have to increase the context stack size to be greater than the recursion depth of the type family. I don't know if this is a bad sign or to be expected. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.3 $ ghc Types $ ./Types 200 a > test.hs && time ghc test.hs > /dev/null -fcontext-stack=250 real 0m2.973s $ ./Types 300 a > test.hs && time ghc test.hs > /dev/null -fcontext-stack=350 real 0m6.018s $ ./Types 400 a > test.hs && time ghc test.hs > /dev/null -fcontext-stack=450 real 0m9.995s $ ./Types 500 a > test.hs && time ghc test.hs > /dev/null -fcontext-stack=550 real 0m15.645s Without the type family, I get MUCH better performance: $ ./Types 10000 d > test.hs && time ghc test.hs > /dev/null real 0m2.271s ------------------------ -- Types.hs below ------------------------ import System.Environment code :: Int -> String -> String code i test = concat $ map (++"\n") $ [ "{-# LANGUAGE TypeOperators,DataKinds, KindSignatures,TypeFamilies,PolyKinds #-}" , "import GHC.TypeLits" , "data Nat1 = Zero | Succ Nat1" , "type family Replicate1 (n :: Nat1) (x::a) :: [a]" , "type instance Replicate1 Zero x = '[]" , "type instance Replicate1 (Succ n) x = x ': (Replicate1 n x)" , "class Class a where" , " f :: a -> a" , "data Data (xs::a) = X | Y" , " deriving (Read,Show)" , "main = print test1" ] ++ case head test of 'a' -> [ "instance (xs ~ Replicate1 ("++mkNat1 i++") ()) => Class (Data xs) where" , " f X = Y" , " f Y = X" , "test1 = f (X :: Data ( Replicate1 ("++mkNat1 i++") () ))" ] 'b' -> [ "instance (xs ~ ("++mkList i++") ) => Class (Data xs) where" , " f X = Y" , " f Y = X" , "test1 = f (X :: Data ( Replicate1 ("++mkNat1 i++") () ))" ] 'c' -> [ "instance (xs ~ Replicate1 ("++mkNat1 i++") ()) => Class (Data xs) where" , " f X = Y" , " f Y = X" , "test1 = f (X :: Data ( ("++mkList i++") ))" ] otherwise -> [ "instance (xs ~ ("++mkList i++") ) => Class (Data xs) where" , " f X = Y" , " f Y = X" , "test1 = f (X :: Data ( ("++mkList i++") ))" ] mkList :: Int -> String mkList 0 = " '[] " mkList i = " () ': " ++ mkList (i-1) mkNat1 :: Int -> String mkNat1 0 = " Zero " mkNat1 i = " Succ ( " ++ mkNat1 (i-1) ++ ")" main = do numstr : test : xs <- getArgs let num = read numstr :: Int putStrLn $ code num test