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Comment by arunix | Hacker News Reader

Can you say more about the latter i.e. lack of monad composition ("why can't I use monads for everything") ?

After you learn how to solve states, input data, output data, error handling, parsing context, resource mocking, guaranteed resource release, and a lot of other things just by declaring the correct monad when you call your code, people tend to want to solve several of those at the same time.

But for solving several of them, you need to compose your monads, and there is no good general way to do that. There are context dependent ways to compose some of them, but you are always thinking about their compatibility and they often require that you specialize your code.

> But for solving several of them, you need to compose your monads, and there is no good general way to do that.

It's (provably, I think) impossible for fully general monads, but for traversable-but-otherwise-general monads you can do[0]:

  newtype (%) (f1::k1->Type) (f2::k2->k1) (a::k2) = Comp { unComp :: (f1 (f2 a)) }
  instance (Monad f1,Monad f2,Traversable f2) => Monad (f1 % f2) where
    join (Comp a)  = Comp $ a >>= (map join . mapA (unComp    ))
    (Comp a) >>= k = Comp $ a >>= (map join . mapA (unComp . k))

(For that matter, you only need f2 to be traversable; f1 can be any monad at all.)

0: copied from my own code, so it might need other adjustment than s/map/fmap/ and s/mapA/traverse/.

That instance is type-correct but unlawful: https://stackoverflow.com/q/42284879/7509065

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