module Data.Comp.Param.Derive.Difunctor
(
Difunctor,
makeDifunctor
) where
import Data.Comp.Derive.Utils
import Data.Comp.Param.Difunctor
import Language.Haskell.TH
makeDifunctor :: Name -> Q [Dec]
makeDifunctor fname = do
TyConI (DataD _ name args constrs _) <- abstractNewtypeQ $ reify fname
let coArg :: Name = tyVarBndrName $ last args
let conArg :: Name = tyVarBndrName $ last $ init args
let argNames = map (VarT . tyVarBndrName) (init $ init args)
let complType = foldl AppT (ConT name) argNames
let classType = AppT (ConT ''Difunctor) complType
constrs' :: [(Name,[Type])] <- mapM normalConExp constrs
dimapDecl <- funD 'dimap (map (dimapClause conArg coArg) constrs')
return [InstanceD [] classType [dimapDecl]]
where dimapClause :: Name -> Name -> (Name,[Type]) -> ClauseQ
dimapClause conArg coArg (constr, args) = do
fn <- newName "_f"
gn <- newName "_g"
varNs <- newNames (length args) "x"
let f = varE fn
let g = varE gn
let fp = VarP fn
let gp = VarP gn
let pat = ConP constr $ map VarP varNs
body <- dimapArgs conArg coArg f g (zip varNs args) (conE constr)
return $ Clause [fp, gp, pat] (NormalB body) []
dimapArgs :: Name -> Name -> ExpQ -> ExpQ
-> [(Name, Type)] -> ExpQ -> ExpQ
dimapArgs _ _ _ _ [] acc =
acc
dimapArgs conArg coArg f g ((x,tp):tps) acc =
dimapArgs conArg coArg f g tps
(acc `appE` (dimapArg conArg coArg tp f g `appE` varE x))
dimapArg :: Name -> Name -> Type -> ExpQ -> ExpQ -> ExpQ
dimapArg conArg coArg tp f g
| not (containsType tp (VarT conArg)) &&
not (containsType tp (VarT coArg)) = [| id |]
| otherwise =
case tp of
VarT a | a == conArg -> f
| a == coArg -> g
AppT (AppT ArrowT tp1) tp2 -> do
xn <- newName "x"
let ftp1 = dimapArg conArg coArg tp1 f g
let ftp2 = dimapArg conArg coArg tp2 f g
lamE [varP xn]
(infixE (Just ftp2)
[|(.)|]
(Just $ infixE (Just $ varE xn)
[|(.)|]
(Just ftp1)))
SigT tp' _ ->
dimapArg conArg coArg tp' f g
_ ->
if containsType tp (VarT conArg) then
[| dimap $f $g |]
else
[| fmap $g |]