Re: [Haskell-cafe] Problem with TemplateHaskell

2 Nov 2011


      On Tue, Nov 1, 2011 at 5:42 PM, Magicloud Magiclouds
 wrote:
...
Hi,
 I have code as following, to make a toDocument function (using
Data.Bsin.=:) for a data structure.
bson :: DecsQ -> DecsQ
bson decsq = do
 decs <- decsq
 let datad = head decs
     DataD _ _ _ cons _ = datad
     to = mkName "toDocument"
     from = mkName "fromDocument"
 fund <- mapM (\con ->
                case con of
                  RecC n types -> do
                    let nvs = map (\(nv, _, _) ->
                                    nv
                                  ) types
                    funD to [clause [conP n $ map varP nvs]
                             (normalB $ listE $ map (\nv ->
                                                      infixE (Just $
litE $ stringL $ show nv)
                                                             (varE $
mkName "=:")
                                                             $ Just $
appE (varE $ mkName "val")
    $ varE nv
                                                    ) nvs) []]
              ) cons
 return (datad : fund)
 Testing code is as:
data T = T { a :: Int
          , b :: Char }
*TH> runQ (bson [d|data T = T {a :: Int, b :: Char}|])
[DataD [] T_0 [] [RecC T_1 [(a_2,NotStrict,ConT
GHC.Types.Int),(b_3,NotStrict,ConT GHC.Types.Char)]] [],FunD
toDocument [Clause [ConP T_1 [VarP a_2,VarP b_3]] (NormalB (ListE
[InfixE (Just (LitE (StringL "a_2"))) (VarE =:) (Just (AppE (VarE val)
(VarE a_2))),InfixE (Just (LitE (StringL "b_3"))) (VarE =:) (Just
(AppE (VarE val) (VarE b_3)))])) []]]
 So you see that, it changed the name from T/a/b to T_0/T_1/a_2/b_3.
Why is that? I did not have code to modify original data declaration.
--
竹密岂妨流水过
山高哪阻野云飞
Here is the code it actually generated:
test.hs:1:1: Splicing declarations
    bson [d| data T = T {a :: Int, b :: String} |]
  ======>
    test.hs:(7,3)-(8,38)
    data T_a1XY = T_a1XZ {a_a1Y0 :: Int, b_a1Y1 :: String}
    toName (T_a1XZ a_a1Y0 b_a1Y1)
      = [("a_1627397516" =: a_a1Y0), ("b_1627397517" =: b_a1Y1)]

How to avoid the name changing?
-- 
竹密岂妨流水过
山高哪阻野云飞