src/Ganeti/THH/Types.hs

{-# LANGUAGE TemplateHaskell, DeriveFunctor #-}

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

-}

{-

Copyright (C) 2013 Google Inc.
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-}

module Ganeti.THH.Types
  ( typeOfFun
  , funArgs
  , tupleArgs
  , argumentType
  , uncurryVarType
  , uncurryVar
  , curryN
  , OneTuple(..)
  ) where

import Control.Arrow (first)
import Control.Monad (liftM, replicateM)
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, Functor)
-- 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

-- | Given a type of the form @m a@, this function extracts @a@.
-- If the given type is of another form, it fails with an error message.
argumentType :: Type -> Q Type
argumentType (AppT _ t) = return t
argumentType t          = fail $ "Not a type of the form 'm a': " ++ show t

-- | 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 <- replicateM n (newName "x")
      return $ LamE [VarP f, TupP $ 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)

-- | Generic 'curry' that constructs a curring function of a given arity.
curryN :: Int -> Q Exp
curryN 0 = [| ($ ()) |]
curryN 1 = [| (. OneTuple) |]
curryN n = do
  f <- newName "f"
  ps <- replicateM n (newName "x")
  return $ LamE (VarP f : map VarP ps)
             (AppE (VarE f) (TupE $ map VarE ps))