{-# LANGUAGE TemplateHaskell #-}
--------------------------------------------------------------------------------
-- |
-- Module      :  Data.Comp.Derive.Equality
-- Copyright   :  (c) 2010-2011 Patrick Bahr
-- License     :  BSD3
-- Maintainer  :  Patrick Bahr <paba@diku.dk>
-- Stability   :  experimental
-- Portability :  non-portable (GHC Extensions)
--
-- Automatically derive instances of @EqF@.
--
--------------------------------------------------------------------------------
module Data.Comp.Derive.Equality
    (
     EqF(..),
     makeEqF
    ) where

import Data.Comp.Derive.Utils
import Language.Haskell.TH hiding (Cxt, match)


{-| Signature equality. An instance @EqF f@ gives rise to an instance
  @Eq (Term f)@. -}
class EqF f where

    eqF :: Eq a => f a -> f a -> Bool

{-| Derive an instance of 'EqF' for a type constructor of any first-order kind
  taking at least one argument. -}
makeEqF :: Name -> Q [Dec]
makeEqF :: Name -> Q [Dec]
makeEqF Name
fname = do
  Just (DataInfo Cxt
_cxt Name
name [TyVarBndr]
args [Con]
constrs [DerivClause]
_deriving) <- Q Info -> Q (Maybe DataInfo)
abstractNewtypeQ (Q Info -> Q (Maybe DataInfo)) -> Q Info -> Q (Maybe DataInfo)
forall a b. (a -> b) -> a -> b
$ Name -> Q Info
reify Name
fname
  let argNames :: Cxt
argNames = (TyVarBndr -> Type) -> [TyVarBndr] -> Cxt
forall a b. (a -> b) -> [a] -> [b]
map (Name -> Type
VarT (Name -> Type) -> (TyVarBndr -> Name) -> TyVarBndr -> Type
forall b c a. (b -> c) -> (a -> b) -> a -> c
. TyVarBndr -> Name
tyVarBndrName) ([TyVarBndr] -> [TyVarBndr]
forall a. [a] -> [a]
init [TyVarBndr]
args)
      complType :: Type
complType = (Type -> Type -> Type) -> Type -> Cxt -> Type
forall (t :: * -> *) b a.
Foldable t =>
(b -> a -> b) -> b -> t a -> b
foldl Type -> Type -> Type
AppT (Name -> Type
ConT Name
name) Cxt
argNames
      preCond :: Cxt
preCond = (Type -> Type) -> Cxt -> Cxt
forall a b. (a -> b) -> [a] -> [b]
map (Name -> Cxt -> Type
mkClassP ''Eq (Cxt -> Type) -> (Type -> Cxt) -> Type -> Type
forall b c a. (b -> c) -> (a -> b) -> a -> c
. (Type -> Cxt -> Cxt
forall a. a -> [a] -> [a]
: [])) Cxt
argNames
      classType :: Type
classType = Type -> Type -> Type
AppT (Name -> Type
ConT ''EqF) Type
complType
  Dec
eqFDecl <- Name -> [ClauseQ] -> DecQ
funD 'eqF  ([Con] -> [ClauseQ]
eqFClauses [Con]
constrs)
  [Dec] -> Q [Dec]
forall (m :: * -> *) a. Monad m => a -> m a
return [Cxt -> Type -> [Dec] -> Dec
mkInstanceD Cxt
preCond Type
classType [Dec
eqFDecl]]
      where eqFClauses :: [Con] -> [ClauseQ]
eqFClauses [Con]
constrs = (Con -> ClauseQ) -> [Con] -> [ClauseQ]
forall a b. (a -> b) -> [a] -> [b]
map ((Name, Int) -> ClauseQ
genEqClause((Name, Int) -> ClauseQ) -> (Con -> (Name, Int)) -> Con -> ClauseQ
forall b c a. (b -> c) -> (a -> b) -> a -> c
.Con -> (Name, Int)
abstractConType) [Con]
constrs
                                   [ClauseQ] -> [ClauseQ] -> [ClauseQ]
forall a. [a] -> [a] -> [a]
++ [Con] -> [ClauseQ]
forall (t :: * -> *) a. Foldable t => t a -> [ClauseQ]
defEqClause [Con]
constrs
            defEqClause :: t a -> [ClauseQ]
defEqClause t a
constrs
                | t a -> Int
forall (t :: * -> *) a. Foldable t => t a -> Int
length t a
constrs  Int -> Int -> Bool
forall a. Ord a => a -> a -> Bool
< Int
2 = []
                | Bool
otherwise = [[PatQ] -> BodyQ -> [DecQ] -> ClauseQ
clause [PatQ
wildP,PatQ
wildP] (ExpQ -> BodyQ
normalB [|False|]) []]
            genEqClause :: (Name, Int) -> ClauseQ
genEqClause (Name
constr, Int
n) = do
              [Name]
varNs <- Int -> String -> Q [Name]
newNames Int
n String
"x"
              [Name]
varNs' <- Int -> String -> Q [Name]
newNames Int
n String
"y"
              let pat :: Pat
pat = Name -> [Pat] -> Pat
ConP Name
constr ([Pat] -> Pat) -> [Pat] -> Pat
forall a b. (a -> b) -> a -> b
$ (Name -> Pat) -> [Name] -> [Pat]
forall a b. (a -> b) -> [a] -> [b]
map Name -> Pat
VarP [Name]
varNs
                  pat' :: Pat
pat' = Name -> [Pat] -> Pat
ConP Name
constr ([Pat] -> Pat) -> [Pat] -> Pat
forall a b. (a -> b) -> a -> b
$ (Name -> Pat) -> [Name] -> [Pat]
forall a b. (a -> b) -> [a] -> [b]
map Name -> Pat
VarP [Name]
varNs'
                  vars :: [Exp]
vars = (Name -> Exp) -> [Name] -> [Exp]
forall a b. (a -> b) -> [a] -> [b]
map Name -> Exp
VarE [Name]
varNs
                  vars' :: [Exp]
vars' = (Name -> Exp) -> [Name] -> [Exp]
forall a b. (a -> b) -> [a] -> [b]
map Name -> Exp
VarE [Name]
varNs'
                  mkEq :: Exp -> Exp -> ExpQ
mkEq Exp
x Exp
y = let (ExpQ
x',ExpQ
y') = (Exp -> ExpQ
forall (m :: * -> *) a. Monad m => a -> m a
return Exp
x,Exp -> ExpQ
forall (m :: * -> *) a. Monad m => a -> m a
return Exp
y)
                             in [| $x' == $y'|]
                  eqs :: ExpQ
eqs = [ExpQ] -> ExpQ
listE ([ExpQ] -> ExpQ) -> [ExpQ] -> ExpQ
forall a b. (a -> b) -> a -> b
$ (Exp -> Exp -> ExpQ) -> [Exp] -> [Exp] -> [ExpQ]
forall a b c. (a -> b -> c) -> [a] -> [b] -> [c]
zipWith Exp -> Exp -> ExpQ
mkEq [Exp]
vars [Exp]
vars'
              Exp
body <- if Int
n Int -> Int -> Bool
forall a. Eq a => a -> a -> Bool
== Int
0
                      then [|True|]
                      else [|and $eqs|]
              Clause -> ClauseQ
forall (m :: * -> *) a. Monad m => a -> m a
return (Clause -> ClauseQ) -> Clause -> ClauseQ
forall a b. (a -> b) -> a -> b
$ [Pat] -> Body -> [Dec] -> Clause
Clause [Pat
pat, Pat
pat'] (Exp -> Body
NormalB Exp
body) []