Re: [Haskell-cafe] TH instance code.

On Tue, Jun 22, 2010 at 6:24 PM, Andy Stewart wrote:

Hi all,

I have below duplicate code, but i don't know how to use TH instance code.

------------------------------> duplicate code start <------------------------------ instance Variable PageType where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: PageType) $ fromVariant x

instance Variable Int where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Int) $ fromVariant x

instance Variable (Maybe Char) where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Maybe Char) $ fromVariant x

instance Variable (Maybe Int) where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Maybe Int) $ fromVariant x

instance Variable ProcessID where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: ProcessID) $ fromVariant x

instance Variable GWindowId where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: GWindowId) $ fromVariant x ------------------------------> duplicate code end <------------------------------

Any TH expert help?

Thanks,

-- Andy

mkInstance :: Name -> Q Dec mkInstance n = do tv <- [|toVariant . show|] fv <- [|fmap read . fromVariant|] return $ InstanceD [] (ConT ''Variable `AppT` ConT n) [ FunD (mkName "toVariant") [Clause [] [NormalB tv] []] , FunD (mkName "fromVariant") [Clause [] [NormalB fv] []] ] mkInstances = mapM mkInstance mkInstances [''PageType, ''Int, ..] Haven't tried it, but it should be pretty close. Michael