OneTuple.hs

-- |OneTuple fills the /tuple gap/ with a singleton tuple.
--
-- OneTuple /does not support/ the usual parenthesized tuple syntax.
--
-- OneTuple
--
--   * has the expected laziness properties
--
--   * can be pattern-matched
--
--   * ships with instances for several standard type classes,
--     including all those supported by H98-standard tuples
--
--   * requires no language extensions, except for hierarchical modules

module Data.Tuple.OneTuple (OneTuple(OneTuple), only) where

import Control.Applicative (Applicative (..))
import Control.Monad       (ap)
import Control.Monad.Fix   (MonadFix (..))
import Data.Foldable       (Foldable (..))
import Data.Ix             (Ix (..))
import Data.Monoid         (Monoid (..))
import Data.Semigroup      (Semigroup (..))
import Data.Traversable    (Traversable (..))

-- |OneTuple is the singleton tuple data type.
data OneTuple a
    = OneTuple a  -- ^ singleton tuple constructor
    deriving (Eq,Ord,Bounded,Show,Read)

-- |The 'only' function extracts the OneTuple's only member.
-- (Compare to 'fst' and 'snd'.)
only :: OneTuple a -- ^ takes a singleton tuple argument
     -> a          -- ^ returns the only element in the tuple
only (OneTuple x) = x

instance (Enum a) => Enum (OneTuple a) where
    succ = fmap succ
    pred = fmap pred
    toEnum = pure . toEnum
    fromEnum (OneTuple x) = fromEnum x

instance (Ix a) => Ix (OneTuple a) where
    range   (OneTuple x, OneTuple y) = map OneTuple (range (x,y))
    index   (OneTuple x, OneTuple y) (OneTuple z) = index   (x,y) z
    inRange (OneTuple x, OneTuple y) (OneTuple z) = inRange (x,y) z

instance Foldable OneTuple where
    fold (OneTuple m) = m
    foldMap f (OneTuple x) = f x
    foldr f b (OneTuple x) = f x b
    foldl f a (OneTuple x) = f a x
    foldr1 _f (OneTuple x) = x
    foldl1 _f (OneTuple x) = x

instance Traversable OneTuple where
    traverse f (OneTuple x) = fmap OneTuple (f x)
    sequenceA (OneTuple x) = fmap OneTuple x

instance Functor OneTuple where
    fmap f (OneTuple x) = OneTuple (f x)

instance Applicative OneTuple where
    pure = OneTuple

    OneTuple f <*> OneTuple x = OneTuple (f x)
    _ *> x = x
    x <* _ = x

instance Monad OneTuple where
    return = pure
    (>>) = (*>)
    (OneTuple x) >>= f = f x

instance (Semigroup a) => Semigroup (OneTuple a) where
    OneTuple x <> OneTuple y = OneTuple (x <> y)

instance (Monoid a) => Monoid (OneTuple a) where
    mempty = OneTuple mempty
    mappend (OneTuple x) (OneTuple y) = OneTuple (mappend x y)

instance MonadFix OneTuple where
    mfix f = let a = f (only a) in a