derive-0.1.2: Data/DeriveTraversal.hs
{-
This module is not written/maintained by the usual Data.Derive author.
MAINTAINER: Twan van Laarhoven
EMAIL: "twanvl" ++ "@" ++ "gmail" ++ "." ++ "com"
Please send all patches to this module to Neil (ndmitchell -at- gmail),
and CC Twan.
-}
-- NOTE: Cannot be guessed as it relies on type information
-- | Derives 'Functor' and similair classes, as discussed on the Haskell-prime mailing list:
-- <http://www.mail-archive.com/haskell-prime@haskell.org/msg02116.html>.
module Data.DeriveTraversal(
TraveralType(..), defaultTraversalType,
traversalDerivation1,
traversalInstance, traversalInstance1,
deriveTraversal
) where
import Language.Haskell.TH.All
import Data.List
import qualified Data.Set as S
import Control.Monad.Writer
import Control.Applicative
---------------------------------------------------------------------------------
-- Stuff that should be in a library
instance Monoid w => Applicative (Writer w) where
pure = return
(<*>) = ap
---------------------------------------------------------------------------------
-- Information datatype, public interface
-- | An expression representing a traversal of a subpart of the data
type Trav = Exp
-- | What kind of traversal are we deriving?
data TraveralType = TraveralType
{ traversalArg :: Int -- ^ On what position are we traversing?
, traversalCo :: Bool -- ^ covariant?
, traversalName :: String -- ^ name of the traversal function
, traversalId :: Trav -- ^ Identity traversal
, traversalDirect :: Trav -- ^ Traversal of 'a'
, traversalFunc :: String -> Trav -> Trav -- ^ Apply the sub-traversal function
, traversalPlus :: Trav -> Trav -> Trav -- ^ Apply two non-identity traversals in sequence
, traverseArrow :: Trav -> Trav -> Trav -- ^ Traverse a function type
, traverseTuple :: [Exp] -> Exp -- ^ Construct a tuple from applied traversals
, traverseCtor :: String -> [Exp] -> Exp -- ^ Construct a data type from applied traversals
, traverseFunc :: Pat -> Exp -> Clause -- ^ Construct a clause of the traversal function
}
defaultTraversalType = TraveralType
{ traversalArg = 1
, traversalCo = False
, traversalName = undefined -- prevent warnings
, traversalId = id'
, traversalDirect = l0 "f"
, traversalFunc = l1
, traversalPlus = (.:)
, traverseArrow = fail "Cannot derive traversal over function types"
, traverseTuple = TupE
, traverseCtor = lK
, traverseFunc = undefined
}
data RequiredInstance = RequiredInstance
{ requiredDataArg :: Name -- ^ What argument of the current data type?
, requiredPosition :: Int -- ^ What argument position of that type?
}
deriving (Eq, Ord)
-- | Monad that collects required instances
type WithInstances a = Writer (S.Set RequiredInstance) a
---------------------------------------------------------------------------------
-- Deriving traversals
-- | Derivation for a Traversable like class with just 1 method
traversalDerivation1 :: TraveralType -> String -> Derivation
traversalDerivation1 tt nm = derivation (traversalInstance1 tt nm) (className (traversalArg tt))
where className n = nm ++ (if n > 1 then show n else "")
-- | Instance for a Traversable like class with just 1 method
traversalInstance1 :: TraveralType -> String -> DataDef -> [Dec]
traversalInstance1 tt nm dat = traversalInstance tt nm dat [deriveTraversal tt dat]
-- | Instance for a Traversable like class
traversalInstance :: TraveralType -> String -> DataDef -> [WithInstances Dec] -> [Dec]
traversalInstance tt nameBase dat bodyM
| dataArity dat == 0 = []
| otherwise = [InstanceD ctx head body]
where
(body, required) = runWriter (sequence bodyM)
ctx = [ lK (className p) (VarT n : vars 's' (p - 1))
| RequiredInstance n p <- S.toList required
]
vrs = vars 't' (dataArity dat)
(vrsBefore,(_:vrsAfter)) = splitAt (length vrs - traversalArg tt) vrs
className n = nameBase ++ (if n > 1 then show n else "")
head = lK (className (traversalArg tt)) (lK (dataName dat) vrsBefore : vrsAfter)
-- | Derive a 'traverse' like function
deriveTraversal :: TraveralType -> DataDef -> WithInstances Dec
deriveTraversal tt dat = fun
where
fun = funN (traversalNameN tt (traversalArg tt)) <$> body
args = argPositions dat
body = mapM (deriveTraversalCtor tt args) (dataCtors dat)
-- | Derive a clause of a 'traverse' like function for a constructor
deriveTraversalCtor :: TraveralType -> ArgPositions -> CtorDef -> WithInstances Clause
deriveTraversalCtor tt ap ctor = do
tTypes <- mapM (deriveTraversalType tt ap) (ctorTypes ctor)
return $ traverseFunc tt (ctp ctor 'a')
$ traverseCtor tt (ctorName ctor) (zipWith AppE tTypes (ctv ctor 'a'))
-- | Derive a traversal for a type
deriveTraversalType :: TraveralType -> ArgPositions -> Type -> WithInstances Trav
deriveTraversalType tt ap (ForallT _ _ _) = fail "forall not supported in traversal deriving"
deriveTraversalType tt ap (AppT (AppT ArrowT a) b)
= traverseArrow tt <$> deriveTraversalType tt{traversalCo = not $ traversalCo tt} ap a
<*> deriveTraversalType tt ap b
deriveTraversalType tt ap (AppT a b) = deriveTraversalApp tt ap a [b] -- T a b c ...
deriveTraversalType tt ap ListT = return $ traversalId tt -- []
deriveTraversalType tt ap (ConT n) = return $ traversalId tt -- T
deriveTraversalType tt ap (VarT n) -- a
| ap n /= traversalArg tt = return $ traversalId tt
| traversalCo tt = fail "tyvar used in covariant position"
| otherwise = return $ traversalDirect tt
-- | Find all arguments to a type application, then derive a traversal
deriveTraversalApp :: TraveralType -> ArgPositions -> Type -> [Type] -> WithInstances Trav
deriveTraversalApp tt ap (AppT a b) args = deriveTraversalApp tt ap a (b : args)
deriveTraversalApp tt ap tycon args
| isTupleT tycon = do -- (a,b,c)
tArgs <- mapM (deriveTraversalType tt ap) args
return $
if (all (== traversalId tt) tArgs) then
traversalId tt
else
LamE [TupP (vars 't' (length args))]
(traverseTuple tt $ zipWith AppE tArgs (vars 't' (length args)))
| otherwise = do -- T a b c
tCon <- deriveTraversalType tt ap tycon
tArgs <- mapM (deriveTraversalType tt ap) args
-- need instances?
case tycon of
VarT n | ap n == traversalArg tt -> fail "kind error: type used type constructor"
| otherwise -> tell $ S.fromList
[ RequiredInstance n i
| (t,i) <- zip (reverse tArgs) [1..]
, t /= traversalId tt
]
_ -> return ()
-- combine non-id traversals
let nonId = [ traverseArg tt i t
| (t,i) <- zip (reverse tArgs) [1..]
, t /= traversalId tt
]
return $ case nonId of
[] -> traversalId tt -- no interesting arguments to type con
_ -> foldl1 (traversalPlus tt) nonId
-- | Lift a traversal to the argument of a type constructor
traverseArg :: TraveralType -> Int -> Trav -> Trav
traverseArg tt n e = traversalFunc tt (traversalNameN tt n) e
traversalNameN :: TraveralType -> Int -> String
traversalNameN tt n = traversalName tt ++ (if n > 1 then show n else "")
-- | Information on argument positions
type ArgPositions = Name -> Int
-- | Position of an argument in the data type
-- In the type "data X a b c"
-- positions are: a -> 3, b -> 2, c -> 1
argPositions :: DataDef -> Name -> Int
argPositions dat = \nm -> case elemIndex nm args of
Nothing -> error "impossible: tyvar not in scope"
Just k -> length args - k
where args = ex_args dat