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All the fields must be instances of 'Val'. + +You can also generate documents with 'selectFields', which takes a +list of functions names that of type a -> b and returns a function +of type a -> Document. + +Example: + +> {-# Language TemplateHaskell #-} +> +> import Data.Bson.Mapping +> import Data.Time.Clock +> +> data Post = Post { time :: UTCTime +> , author :: String +> , content :: String +> , votes :: Int +> } +> deriving (Show, Read, Eq, Ord) +> $(deriveBson ''Post) +> +> main :: IO () +> main = do +> now <- getCurrentTime +> let post = Post now "francesco" "lorem ipsum" 5 +> (fromBson (toBson post) :: IO Post) >>= print +> print $ toBson post +> print $ $(selectFields ['time, 'content]) post + +-} + +module Data.Bson.Mapping ( + Bson (..) + , deriveBson + , selectFields + ) where + +import qualified Data.Bson as B +import Language.Haskell.TH +import Language.Haskell.TH.Lift () + +class Bson a where + toBson :: a -> B.Document + fromBson :: Monad m => B.Document -> m a + + +deriveBson :: Name -> Q [Dec] +deriveBson type' = do + (cx, con, keys) <- bsonType + (constr, fields) <- parseCon con + let context = [ classP ''B.Val [varT key] | key <- keys ] ++ map return cx + i <- instanceD (sequence context) (mkType ''Bson [mkType type' (map varT keys)]) + [ funD 'toBson [clause [] (normalB $ selectFields fields) []] + , deriveFromBson fields constr + ] + return [i] + + where + + bsonType = do + info <- reify type' + case info of + TyConI (DataD cx _ keys [con] _) -> return (cx, con, map conv keys) + TyConI (NewtypeD cx _ keys con _) -> return (cx, con, map conv keys) + _ -> inputError + + parseCon con = do + case con of + RecC name fields -> return (name, map (\(n, _, _) -> n) fields) + _ -> inputError + + mkType con = foldl appT (conT con) + + conv (PlainTV nm) = nm + conv (KindedTV nm _) = nm + + inputError = error "deriveBson: Invalid type provided. The type must be a data with a single constructor or a newtype. The constructor must have named fields." + + deriveFromBson fields constr = do + doc <- newName "doc" + (fields', stmts) <- genStmts fields doc + let ci = noBindS $ [| return $(recConE constr fields') |] + funD 'fromBson [clause [varP doc] (normalB $ doE (stmts ++ [ci])) []] + + genStmts [] _ = return ([], []) + genStmts (f : fs) doc = do + fvar <- newName "f" + let stmt = bindS (varP fvar) $ [| B.lookup (B.u (nameBase f)) $(varE doc) |] + (fields, stmts) <- genStmts fs doc + return $ (return (f, VarE fvar) : fields, stmt : stmts) + +selectFields :: [Name] -> Q Exp +selectFields ns = do + d <- newName "d" + e <- gf d ns + lamE [varP d] (return e) + where + gf _ [] = [| [] |] + gf d (n : ns') = [| (B.u (nameBase n) B.=: $(varE n) $(varE d)) : $(gf d ns') |]