[Haskell-cafe] Problem with TemplateHaskell

1 Nov 2011


      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.
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[Haskell-cafe] Problem with TemplateHaskell

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