{-# LANGUAGE DataKinds, TypeFamilies, KindSignatures, CPP, TemplateHaskell #-} ----------------------------------------------------------------------------- -- | -- Module : Data.Singletons.CustomStar -- Copyright : (C) 2013 Richard Eisenberg -- License : BSD-style (see LICENSE) -- Maintainer : Richard Eisenberg (eir@cis.upenn.edu) -- Stability : experimental -- Portability : non-portable -- -- This file implements 'singletonStar', which generates a datatype @Rep@ and associated -- singleton from a list of types. The promoted version of @Rep@ is kind @*@ and the -- Haskell types themselves. This is still very experimental, so expect unusual -- results! -- ---------------------------------------------------------------------------- module Data.Singletons.CustomStar ( singletonStar ) where import Language.Haskell.TH import Language.Haskell.TH.Syntax ( Quasi(..) ) import Data.Singletons.Util import Data.Singletons.Promote import Data.Singletons.Singletons import Control.Monad #if __GLASGOW_HASKELL__ >= 707 import Data.Singletons.Decide import Data.Singletons.Instances import Data.Singletons.Eq import Unsafe.Coerce #endif {- The SEq instance here is tricky. The problem is that, in GHC 7.8+, the instance of type-level (==) for * is not recursive. Thus, it's impossible, say, to get (Maybe a == Maybe b) ~ False from (a == b) ~ False. There are a few ways forward: 1) Define SEq to use our own Boolean (==) operator, instead of the built-in one. This would work, but feels wrong. 2) Use unsafeCoerce. We do #2. Also to note: because these problems don't exist in GHC 7.6, the generation of Eq and Decide for 7.6 is entirely normal. Note that mkCustomEqInstances makes the SDecide and SEq instances in GHC 7.8+, but the type-level (==) instance in GHC 7.6. This is perhaps poor design, but it reduces the amount of CPP noise. -} -- | Produce a representation and singleton for the collection of types given. -- -- A datatype @Rep@ is created, with one constructor per type in the declared -- universe. When this type is promoted by the singletons library, the -- constructors become full types in @*@, not just promoted data constructors. -- -- For example, -- -- > $(singletonStar [''Nat, ''Bool, ''Maybe]) -- -- generates the following: -- -- > data Rep = Nat | Bool | Maybe Rep deriving (Eq, Show, Read) -- -- and its singleton. However, because @Rep@ is promoted to @*@, the singleton -- is perhaps slightly unexpected: -- -- > data instance Sing (a :: *) where -- > SNat :: Sing Nat -- > SBool :: Sing Bool -- > SMaybe :: SingRep a => Sing a -> Sing (Maybe a) -- -- The unexpected part is that @Nat@, @Bool@, and @Maybe@ above are the real @Nat@, -- @Bool@, and @Maybe@, not just promoted data constructors. -- -- Please note that this function is /very/ experimental. Use at your own risk. singletonStar :: Quasi q => [Name] -- ^ A list of Template Haskell @Name@s for types -> q [Dec] singletonStar names = do kinds <- mapM getKind names ctors <- zipWithM (mkCtor True) names kinds let repDecl = DataD [] repName [] ctors [''Eq, ''Show, ''Read] fakeCtors <- zipWithM (mkCtor False) names kinds eqInstances <- mkCustomEqInstances fakeCtors singletonDecls <- singDataD True [] repName [] fakeCtors [''Show, ''Read #if __GLASGOW_HASKELL__ < 707 , ''Eq #endif ] return $ repDecl : eqInstances ++ singletonDecls where -- get the kinds of the arguments to the tycon with the given name getKind :: Quasi q => Name -> q [Kind] getKind name = do info <- reifyWithWarning name case info of TyConI (DataD (_:_) _ _ _ _) -> fail "Cannot make a representation of a constrainted data type" TyConI (DataD [] _ tvbs _ _) -> return $ map extractTvbKind tvbs TyConI (NewtypeD (_:_) _ _ _ _) -> fail "Cannot make a representation of a constrainted newtype" TyConI (NewtypeD [] _ tvbs _ _) -> return $ map extractTvbKind tvbs TyConI (TySynD _ tvbs _) -> return $ map extractTvbKind tvbs PrimTyConI _ n _ -> return $ replicate n StarT _ -> fail $ "Invalid thing for representation: " ++ (show name) -- first parameter is whether this is a real ctor (with a fresh name) -- or a fake ctor (when the name is actually a Haskell type) mkCtor :: Quasi q => Bool -> Name -> [Kind] -> q Con mkCtor real name args = do (types, vars) <- evalForPair $ mapM kindToType args let ctor = NormalC ((if real then reinterpret else id) name) (map (\ty -> (NotStrict, ty)) types) if length vars > 0 then return $ ForallC (map PlainTV vars) [] ctor else return ctor -- demote a kind back to a type, accumulating any unbound parameters kindToType :: Quasi q => Kind -> QWithAux [Name] q Type kindToType (ForallT _ _ _) = fail "Explicit forall encountered in kind" kindToType (AppT k1 k2) = do t1 <- kindToType k1 t2 <- kindToType k2 return $ AppT t1 t2 kindToType (SigT _ _) = fail "Sort signature encountered in kind" kindToType (VarT n) = do addElement n return $ VarT n kindToType (ConT n) = return $ ConT n kindToType (PromotedT _) = fail "Promoted type used as a kind" kindToType (TupleT n) = return $ TupleT n kindToType (UnboxedTupleT _) = fail "Unboxed tuple kind encountered" kindToType ArrowT = return ArrowT kindToType ListT = return ListT kindToType (PromotedTupleT _) = fail "Promoted tuple kind encountered" kindToType PromotedNilT = fail "Promoted nil kind encountered" kindToType PromotedConsT = fail "Promoted cons kind encountered" kindToType StarT = return $ ConT repName kindToType ConstraintT = fail $ "Cannot make a representation of a type that has " ++ "an argument of kind Constraint" kindToType (LitT _) = fail "Literal encountered at the kind level" mkCustomEqInstances :: Quasi q => [Con] -> q [Dec] mkCustomEqInstances ctors = do #if __GLASGOW_HASKELL__ >= 707 let ctorVar = error "Internal error: Equality instance inspected ctor var" sCtors <- evalWithoutAux $ mapM (singCtor ctorVar) ctors decideInst <- mkEqualityInstance StarT sCtors sDecideClassDesc a <- qNewName "a" b <- qNewName "b" let eqInst = InstanceD [] (AppT (ConT ''SEq) (kindParam StarT)) [FunD '(%:==) [Clause [VarP a, VarP b] (NormalB $ CaseE (foldExp (VarE '(%~)) [VarE a, VarE b]) [ Match (ConP 'Proved [ConP 'Refl []]) (NormalB $ ConE 'STrue) [] , Match (ConP 'Disproved [WildP]) (NormalB $ AppE (VarE 'unsafeCoerce) (ConE 'SFalse)) [] ]) []]] return [decideInst, eqInst] #else mapM mkEqTypeInstance [(c1, c2) | c1 <- ctors, c2 <- ctors] #endif