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{-# LANGUAGE TemplateHaskell #-}

{-| Utility Template Haskell functions for working with types.

-}

{-

Copyright (C) 2013 Google Inc.

This program is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2 of the License, or

(at your option) any later version.

This program is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

You should have received a copy of the GNU General Public License

along with this program; if not, write to the Free Software

Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

02110-1301, USA.

-}

module Ganeti.THH.Types

  ( typeOfFun

  , funArgs

  , tupleArgs

  , uncurryVarType

  , uncurryVar

  , OneTuple(..)

  ) where

import Control.Arrow (first)

import Control.Monad (liftM)

import Language.Haskell.TH

import qualified Text.JSON as J

-- | This fills the gap between @()@ and @(,)@, providing a wrapper for

-- 1-element tuples. It's needed for RPC, where arguments for a function are

-- sent as a list of values, and therefore for 1-argument functions we need

-- this wrapper, which packs/unpacks 1-element lists.

newtype OneTuple a = OneTuple { getOneTuple :: a }

  deriving (Eq, Ord, Show)

instance Functor OneTuple where

  fmap f (OneTuple x) = OneTuple (f x)

-- The value is stored in @JSON@ as a 1-element list.

instance J.JSON a => J.JSON (OneTuple a) where

  showJSON (OneTuple a) = J.JSArray [J.showJSON a]

  readJSON (J.JSArray [x]) = liftM OneTuple (J.readJSON x)

  readJSON _               = J.Error "Unable to read 1 tuple"

-- | Returns the type of a function. If the given name doesn't correspond to a

-- function, fails.

typeOfFun :: Name -> Q Type

typeOfFun name = reify name >>= args

  where

    args :: Info -> Q Type

    args (VarI _ tp _ _) = return tp

    args _               = fail $ "Not a function: " ++ show name

-- | Splits a function type into the types of its arguments and the result.

funArgs :: Type -> ([Type], Type)

funArgs = first reverse . f []

  where

    f ts (ForallT _ _ x)            = f ts x

    f ts (AppT (AppT ArrowT t) x)   = f (t:ts) x

    f ts x                          = (ts, x)

tupleArgs :: Type -> Maybe [Type]

tupleArgs = fmap reverse . f []

  where

    f ts (TupleT _)                = Just ts

    f ts (AppT (AppT ArrowT x) t)  = f (t:ts) x

    f _  _                         = Nothing

-- | Generic 'uncurry' that counts the number of function arguments in a type

-- and constructs the appropriate uncurry function into @i -> o@.

-- It the type has no arguments, it's converted into @() -> o@.

uncurryVarType :: Type -> Q Exp

uncurryVarType = uncurryN . length . fst . funArgs

  where

    uncurryN 0 = do

      f <- newName "f"

      return $ LamE [VarP f, TupP []] (VarE f)

    uncurryN 1 = [| (. getOneTuple) |]

    uncurryN n = do

      f <- newName "f"

      ps <- mapM newName (replicate n "x")

      return $ LamE (VarP f : map VarP ps) (foldl AppE (VarE f) $ map VarE ps)

-- | Creates an uncurried version of a function.

-- If the function has no arguments, it's converted into @() -> o@.

uncurryVar :: Name -> Q Exp

uncurryVar name = do

  t <- typeOfFun name

  appE (uncurryVarType t) (varE name)