[Haskell-cafe] Template Haskell - substitution in pattern in a
dezgeg at gmail.com
Sun Jan 3 22:35:33 EST 2010
2010/1/4 Patrick Caldon <patc at pessce.net>
> 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.
Here's something pretty generic that gets the patterns right:
module THTest where
type Policy = Int
data Management = SimpleM Policy Policy Policy
buildCP :: Name -> Int -> Int -> ExpQ
buildCP ctor nVars nth = do names <- replicateM nVars $ newName "pat"
let p1 = replaceAt nth WildP $ map VarP names
p2 = replaceAt nth (VarP $ names!!nth) $ replicate
return $ LamE [ConP ctor p1, ConP ctor p2] (ListE $
map VarE names)
replaceAt :: Int -> a -> [a] -> [a]
replaceAt pos x xs = let (first,_:rest) = splitAt pos xs
in first ++ [x] ++ rest
-- for example:
doFst = $(buildCP 'SimpleM 3 0)
doFst (SimpleM 1 2 3) (SimpleM 4 5 6) ==> [4,2,3]
(returns a list because it's easier to do. Modifying it to return
SimpleM left as an exercise :)
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