v-- I had accidentally elided the _'s before the t's in the quasiquotation before.

What you're looking for is something like:

deriveVariable _t = [d|
    instance Variable $_t where
        toVariant = toVariant . show
        fromVariant x = fmap (\v -> read v :: $_t) $ fromVariant x|]

deriveVariable (conT ''PageType)
deriveVariable (conT ''Int)
deriveVariable (conT ''Maybe `appT` conT ''Char)
...




On Tue, Jun 22, 2010 at 11:24 AM, Andy Stewart <lazycat.manatee@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

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