phi

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Traversable
-- Copyright   :  (c) 2020-2021 EMQ Technologies Co., Ltd.
-- License     :  BSD-style (see the LICENSE file)
--
-- Maintainer  :  Feng Lee, feng@emqx.io
--                Yang M, yangm@emqx.io
-- Stability   :  experimental
-- Portability :  portable
--
-- The Traversable typeclass.
--
-----------------------------------------------------------------------------
module Data.Traversable
  ( class Traversable
  , traverse, sequence, mapM
  ) where

import Data.Foldable (class Foldable)
import Control.Monad (class Applicative, class Functor, class Monad, map, pure, liftA2)
import Data.Functor ((<$>))

class (Functor t, Foldable t) => Traversable t where
  traverse :: forall a b m. Applicative m => (a -> m b) -> t a -> m (t b)
  sequence :: forall a m. Applicative m => t (m a) -> m (t a)

instance Traversable List where
  traverse = traverseDefault
  sequence = sequencListImpl

mapM :: forall a b m t. Monad m => Traversable t => (a -> m b) -> t a -> m (t b) 
mapM f l = sequence (map f l)

sequencListImpl :: forall a m. Applicative m => List (m a) -> m (List a)
sequencListImpl [] = pure []
sequencListImpl [x|xs] = liftA2 (\a b -> [a | b] ) x (sequencListImpl xs)

traverseDefault
  :: forall t a b m
   . Traversable t
  => Applicative m
  => (a -> m b)
  -> t a
  -> m (t b)
traverseDefault f ta = sequence (f <$> ta)