[Haskell-cafe] TH instance code.

[Michael Snoyman]

On Tue, Jun 22, 2010 at 6:24 PM, Andy Stewart <lazycat.manatee at gmail.com>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
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