I'm trying to write some template haskell which will transform: $(buildCP 0) into \(SimpleM d1 d2 d3) (SimpleM _ _ _) -> (SimpleM d1 d2 d3) $(buildCP 1) into \(SimpleM _ d2 d3) (SimpleM d1 _ _) -> (SimpleM d1 d2 d3) $(buildCP 1) into \(SimpleM d1 _ d3) (SimpleM _ d2 _) -> (SimpleM d1 d2 d3) and so on. Ultimately I want to generalize this to more variables. I can't seem to get anything to substitute for the pattern variables in a lambda. Is there a straightforward way of doing this? Below is what I've been playing with to try to make this work. Thanks, Patrick. --- module THTest where import Language.Haskell.TH import qualified Data.Bits type Policy = Int data Management = SimpleM Policy Policy Policy deriving Show -- Compiles - but no substitution for the "aX" and "bX" variables buildCP :: Int -> ExpQ buildCP k = [|\(SimpleM a1 a2 a3) (SimpleM b1 b2 b3) -> (SimpleM $e1 $e2 $e3) |] where (e1,a1,b1) = bitToExprs 0 k (e2,a2,b2) = bitToExprs 1 k (e3,a3,b3) = bitToExprs 2 k -- Won't compile: buildCP2 :: Int -> ExpQ buildCP2 k = [|\(SimpleM $a1 $a2 $a3) (SimpleM $b1 $b2 $b3) -> (SimpleM $e1 $e2 $e3) |] where (e1,a1,b1) = bitToExprs 0 k (e2,a2,b2) = bitToExprs 1 k (e3,a3,b3) = bitToExprs 2 k cp1 0 = \(SimpleM d1 d2 d3) (SimpleM _ _ _) -> (SimpleM d1 d2 d3) {- -- idea is to use in calls like this: cp0 0 = $(buildCP 0) -- should be \(SimpleM d1 d2 d3) (SimpleM _ _ _) -> (SimpleM d1 d2 d3) cp0 1 = $(buildCP 1) -} -- There is also a template haskell [p| ... |] syntax, but not yet implemented ... bitToExprs:: Int -> Int -> (ExpQ,PatQ,PatQ) bitToExprs n k = if Data.Bits.testBit (k::Int) (n::Int) then (e,v1,v2) else (e,v2,v1) where v1 = return WildP v2 = return $ VarP (mkName name) e = return $ VarE (mkName name) name = "d" ++ (show $ n + 1) {- -- ulitmate goal is something like this with 10ish d variables: -- cp0 0 (SimpleM d1 d2 d3 m1) (SimpleM _ _ _ m2) = (SimpleM d1 d2 d3 (me1 m1 m2)) cp0 1 (SimpleM d1 d2 _ m1) (SimpleM _ _ d3 m2) = (SimpleM d1 d2 d3 (me2 m1 m2)) cp0 2 (SimpleM d1 _ d3 m1) (SimpleM _ d2 _ m2) = (SimpleM d1 d2 d3 (me1 m1 m2)) cp0 3 (SimpleM d1 _ _ m1) (SimpleM _ d2 d3 m2) = (SimpleM d1 d2 d3 (me2 m1 m2)) cp0 4 (SimpleM _ d2 d3 m1) (SimpleM d1 _ _ m2) = (SimpleM d1 d2 d3 (me1 m1 m2)) cp0 5 (SimpleM _ d2 _ m1) (SimpleM d1 _ d3 m2) = (SimpleM d1 d2 d3 (me2 m1 m2)) cp0 6 (SimpleM _ _ d3 m1) (SimpleM d1 d2 _ m2) = (SimpleM d1 d2 d3 (me1 m1 m2)) cp0 7 (SimpleM _ _ _ m1) (SimpleM d1 d2 d3 m2) = (SimpleM d1 d2 d3 (me2 m1 m2)) cp0 _ _ _ = (trace "cp0 error" undefined) -}