{-# LANGUAGE TemplateHaskell, CPP #-}

{- |
This module provides an automatic Template Haskell
routine to scour data type definitions and generate
accessor objects for them automatically.
-}
module Data.Accessor.Template (
   nameDeriveAccessors, deriveAccessors,
   ) where

import qualified Data.Accessor.Basic as Accessor

import Language.Haskell.TH.Syntax
  -- (Q, Exp(VarE), Pat(VarP), Dec(ValD), Name(Name), mkOccName, occString, reify, )

import Data.Maybe (catMaybes)
import Control.Monad (guard, liftM, )



-- |@deriveAccessors n@ where @n@ is the name of a data type
-- declared with @data@ looks through all the declared fields
-- of the data type, and for each field ending in an underscore
-- generates an accessor of the same name without the underscore.
--
-- It is "nameDeriveAccessors" n f where @f@ satisfies
--
-- > f (s ++ "_") = Just s
-- > f x          = x       -- otherwise
--
-- For example, given the data type:
--
-- > data Score = Score { p1Score_ :: Int
-- >                    , p2Score_ :: Int
-- >                    , rounds   :: Int
-- >                    }
--
-- @deriveAccessors@ will generate the following objects:
--
-- > p1Score :: Accessor Score Int
-- > p1Score = Accessor p1Score_ (\x s -> s { p1Score_ = x })
-- > p2Score :: Accessor Score Int
-- > p2Score = Accessor p2Score_ (\x s -> s { p2Score_ = x })
--
deriveAccessors :: Name -> Q [Dec]
deriveAccessors n = nameDeriveAccessors n transformName
    where
    transformName s = do
        guard $ not (null s)
        guard $ last s == '_'
        return $ init s

-- |@nameDeriveAccessors n f@ where @n@ is the name of a data type
-- declared with @data@ and @f@ is a function from names of fields
-- in that data type to the name of the corresponding accessor. If
-- @f@ returns @Nothing@, then no accessor is generated for that
-- field.
nameDeriveAccessors :: Name -> (String -> Maybe String) -> Q [Dec]
nameDeriveAccessors t namer = do
    TyConI (DataD _ _name _ cons _) <- reify t
    liftM concat $ mapM makeAccs cons

    where

    makeAccs :: Con -> Q [Dec]
    makeAccs (RecC _ vars) =
        liftM catMaybes $ mapM (\ (name,_,_) -> makeAccFromName name) vars
    makeAccs (ForallC _ _ c) = makeAccs c
    makeAccs _ = return []

    transformName :: Name -> Maybe Name
    transformName (Name occ f) = do
        n <- namer (occString occ)
        return $ Name (mkOccName n) f

    makeAccFromName :: Name -> Q (Maybe Dec)
    makeAccFromName name = do
        case transformName name of
            Nothing -> return Nothing
            Just n -> liftM Just $ makeAcc name n

    -- haddock doesn't grok TH
#ifndef __HADDOCK__

    makeAcc :: Name -> Name -> Q Dec
    makeAcc name accName = do
        body <- [|
                 Accessor.fromSetGet
                    ( \x s ->
                        $( return $ RecUpdE (VarE 's) [(name, VarE 'x)] ) )
                    ( $( return $ VarE name ) )
                |]
        return $ ValD (VarP accName) (NormalB body) []

#endif