[Haskell-cafe] Template haskell problem

15 Jun 2007

      I want to write a a template haskell function deriving show for data types without constructor.

  -- example
  -- data A a
  -- the derived instance:
  -- instance (Show a) =>  Show (A a) where
  --   show _ = "A " ++ show (undefined :: a)

  deriveShowNoConstructors :: Name -> Q [ Dec ]
  deriveShowNoConstructors name = do
      TyConI (DataD [] _ typeParams [] []) <- reify name
      runIO $ putStrLn $ show typeParams
      runIO $ hFlush stdout
      return [ InstanceD (contexts typeParams)
                         (AppT (ConT nShow) (iOf (ConT name) typeParams))
                         [ FunD (mkName "show") 
                                [Clause [WildP]
                                        (NormalB (foldr (\a n -> InfixE (Just a) (VarE (mkName "++")) (Just n)) 
                                             (LitE (StringL (pprint name)))  
                                             (map und typeParams))) -- <<< replacing occurs here (see below)
                                        [] ]
                         ] ]
    where contexts = map (\n -> AppT (ConT nShow) (VarT n) )   -- ( ... ) =>
          iOf = foldr (\n a -> AppT a (VarT n) )                         -- ( ... ) where
          nShow = mkName "Show"
          und n = AppE (VarE (mkName "show")) (SigE (VarE (mkName "undefined")) (ForallT [n] [] (VarT n))) -- undefined :: typePara

But when trying to run it with:

  data A a b c
  [ .. module change ]
  $( do l <- deriveShowNoConstructors ''A 
        runIO $ putStrLn $ pprint l
        runIO $ hFlush stdout
        return l )

I get the error:

  src-HListMissing/HListMissingTest.hs|29| 3:
  ||     Ambiguous type variable `a[ad8]' in the constraint:
  ||       `Show a[ad8]'
  || 	arising from use of `show'
  || 	at src-HListMissing/HListMissingTest.hs:(29,3)-(32,13)
  ||     Probable fix: add a type signature that fixes these type variable(s)
  || 
  src-HListMissing/HListMissingTest.hs|29| 3:
  ||     Ambiguous type variable `b[ada]' in the constraint:
  ||       `Show b[ada]'
  || 	arising from use of `show'
  || 	at src-HListMissing/HListMissingTest.hs:(29,3)-(32,13)
  ||     Probable fix: add a type signature that fixes these type variable(s)
  || 
  src-HListMissing/HListMissingTest.hs|29| 3:
  ||     Ambiguous type variable `c[adc]' in the constraint:
  ||       `Show c[adc]'
  || 	arising from use of `show'
  || 	at src-HListMissing/HListMissingTest.hs:(29,3)-(32,13)
  ||     Probable fix: add a type signature that fixes these type variable(s)

When replacing typeParams with []
(thus only  show _ = "Class" instead of show _ = "Class" ++ (undefined:: a) ++ ...
it compiles and prints the pretty printed abstract syntax tree as expected:

  || instance (Show a_0, Show b_1, Show c_2) => Show (THStuff.A c_2
  ||                                                            b_1
  ||                                                            a_0)
  ||     where show _ = "THStuff.A"

Do you see what I am doing wrong?
The function 
  und :: Name -> Exp 
  und n = AppE (VarE (mkName "show")) (SigE (VarE (mkName "undefined")) (ForallT [n] [] (VarT n)))
should create the (show $ undefined :: a)
part.

Marc Weber