let's consider this example Alice has 1.hs like this v :: (,) Int Bool v = (,) 0 False main :: IO () main = putStrLn "print some magic number" >> print ( fst v ) Bob download 1.hs from Alice Bob would like to change 1.hs to 2.hs Bob want the programs [ 1.hs 2.hs ] to behave the same the reason that Bob want to modify 1.hs may be Bob does not like the type (,) in 1.hs Bob has Vector_2.hs module Vector_2 where data Vector_2 a b = C a b x1_of :: Vector_2 a b -> a x1_of ( C x1 _ ) = x1 x2_of :: Vector_2 a b -> b x2_of ( C _ x2 ) = x2 Bob like his Vector_2.hs Bob thinks the Vector_2 shall be good enough to replace (,) in 1.hs i want to create a translator.hs for Bob Bob tell the translator.hs that Bob want to replace (,) with Vector_2 in 1.hs translator.hs automatically translate 1.hs to 2.hs 2.hs shall look like this import Vector_2 ( Vector_2 ) import qualified Vector_2 v :: Vector_2 Int Bool v = Vector_2.C 0 False main :: IO () main = putStrLn "print some magic number" >> print ( Vector_2.x1_of v ) what shall i do ?