Commit f3a522ff authored by Petr Pudlak's avatar Petr Pudlak

Add common functions for working with TH's Type

In particular, functions for extracting types of arguments from a
function type, and for uncurrying functions with an arbitrary number of
Signed-off-by: default avatarPetr Pudlak <>
Reviewed-by: default avatarKlaus Aehlig <>
parent f20038fd
......@@ -750,6 +750,7 @@ HS_LIB_SRCS = \
src/Ganeti/Storage/Utils.hs \
src/Ganeti/THH.hs \
src/Ganeti/THH/PyType.hs \
src/Ganeti/THH/Types.hs \
src/Ganeti/Types.hs \
src/Ganeti/UDSServer.hs \
{-# 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
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
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 []
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 []
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
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)
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