ExplicitDict.hs

module ExplicitDict where

-- explicit type class definition
data Mon m =
  Mon
  { mon_op :: m -> m -> m
  , mon_unit :: m
  }

-- monoid instance of (*, 1)
intMulMon :: Mon Int
intMulMon =
  Mon
  { mon_op = (*)
  , mon_unit = 1
  }

-- monoid instance of (+, 0)
intAddMon :: Mon Int
intAddMon =
  Mon
  { mon_op = (+)
  , mon_unit = 0
  }

---

-- explicit type class definition
data Collects e ce =
  Collects
  { empty :: ce
  , insert :: e -> ce -> ce
  , member :: e -> ce -> Bool
  }

data Eq_ a =
  Eq { runEq :: a -> a -> Bool }


listCol :: Eq_ e           -- ^ explicitly pass Eq instance
        -> Collects e [e]  -- ^ result Collects instance
listCol eq =
  Collects
  { empty = []
  , insert = (:)
  , member = \x -> any (runEq eq x)
  }