[Haskell-cafe] TH instance code.

[Edward Kmett]

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 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
>
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