Alistair Bayley wrote:
A couple of days ago I had need for: concatM :: Monad m => [m [a]] -> m [a] concatM = liftM concat . sequence but found no such thing in the std libs
It used to be in an older version of Haskell, but it was removed in Haskell 98. Bulat Ziganshin wrote:
2. it's not widely used.
Bulat is justifiably concerned about backwards compatibility issues when changing the core standard libraries, including when adding new functions. Despite Bulat's best efforts, there has been a trend lately of adding those kinds of things. If so, nowadays it can be made more general. In this case, you could at least generalize it to: concatM :: (Monad m, Traversable t, Foldable t, MonadPlus p) => t (m (p a)) -> m (p a) or concatA :: (Applicative f, Traversable t, Foldable t, MonadPlus p) => t (f (p a)) -> f (p a) I'm sure someone can do better than that. :) Regards, Yitz