2010/9/14 Kevin Jardine <[email protected]>:
> I would like to use some macro system (perhaps Template Haskell?) to
> reduce this to something like
>
> defObj MyType
>
> I've read through some Template Haskell documentation and examples,
> but I find it intimidatingly hard to follow. Does anyone has some code
> suggestions or pointers to something similar?
The solutions first:
-------------------------------------------------
{-# LANGUAGE TemplateHaskell #-}
module T(mkNewType) where
import Language.Haskell.TH
decls = [d|newtype TempDecl = TempDecl Int deriving (Eq,Ord,Show)|]
decl = do
[d] <- decls
runIO $ print d -- just to show inetrnals
return d
mkNewType :: String -> Q [Dec]
mkNewType n = do
d <- decl
let name = mkName n
return $ (\x -> [x]) $ case d of
(NewtypeD cxt _ argvars (NormalC _ args) derivings) ->
NewtypeD cxt name argvars (NormalC name args) derivings
--------------------------------------
I took perfectly valid declaration, dissected it using case analysis
and changed relevant parts.
And an example client:
-------------------------------------
{-# LANGUAGE TemplateHaskell #-}
import T
$(mkNewType "A")
-------------------------------------
It all work together.
I studied how to use Template Haskell that way: I obtained
declarations of what I need, printed them and looked through
documentation for relevant data types and constructors. It's not
harder that any other library in Haskell, actually.
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