## root / src / Ganeti / THH / Types.hs @ 2992f2f7

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{-# LANGUAGE TemplateHaskell #-} |
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{-| Utility Template Haskell functions for working with types. |

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-} |

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{- |

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Copyright (C) 2013 Google Inc. |

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

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

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

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(at your option) any later version. |

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

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WITHOUT ANY WARRANTY; without even the implied warranty of |

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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |

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General Public License for more details. |

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

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

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

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02110-1301, USA. |

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-} |

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module Ganeti.THH.Types |

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( typeOfFun |

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, funArgs |

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, tupleArgs |

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, uncurryVarType |

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, uncurryVar |

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, OneTuple(..) |

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) where |

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import Control.Arrow (first) |

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import Control.Monad (liftM) |

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import Language.Haskell.TH |

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import qualified Text.JSON as J |

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

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

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

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

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newtype OneTuple a = OneTuple { getOneTuple :: a } |

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deriving (Eq, Ord, Show) |

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instance Functor OneTuple where |

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

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

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

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

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

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

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

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-- function, fails. |

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typeOfFun :: Name -> Q Type |

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typeOfFun name = reify name >>= args |

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where |

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args :: Info -> Q Type |

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args (VarI _ tp _ _) = return tp |

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

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

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

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funArgs = first reverse . f [] |

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where |

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

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

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f ts x = (ts, x) |

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tupleArgs :: Type -> Maybe [Type] |

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tupleArgs = fmap reverse . f [] |

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where |

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f ts (TupleT _) = Just ts |

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

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f _ _ = Nothing |

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

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

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

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uncurryVarType :: Type -> Q Exp |

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uncurryVarType = uncurryN . length . fst . funArgs |

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where |

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uncurryN 0 = do |

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f <- newName "f" |

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

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uncurryN 1 = [| (. getOneTuple) |] |

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uncurryN n = do |

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f <- newName "f" |

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

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return $ LamE [VarP f, TupP $ map VarP ps] |

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(foldl AppE (VarE f) $ map VarE ps) |

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-- | Creates an uncurried version of a function. |

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

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uncurryVar :: Name -> Q Exp |

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uncurryVar name = do |

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t <- typeOfFun name |

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appE (uncurryVarType t) (varE name) |