You could perhaps write MyWierdMonadT and MyAnotherMonadT.

Also, probably layers package [1] is related to what you ask.

But if you actually want arbitrary nested monads like m1 (m2 (m3 (m4 a))), there's no simple way.

Consider Maybe [MyType Int]. MyType may have or may not have a Monad instance. Even more: it can have instance in one module, but not have in another. But suppose MyType has a Num instance. Then how this should work: return 2 :: Maybe [MyType Int]? Should 2 be of type Int or MyType Int?

Note that you can make an instance Num a => Num [a] which makes sense (lists as polynoms). So the same question arises even for Maybe [Int].

If you *really want to do it* (which I doubt), you can use TH (do determine how many return's to use, though it's ambiguous anyway).

[1] http://hackage.haskell.org/package/layers-0.1/docs/Documentation-Layers-Overview.html

2013/11/23 Dmitry Bogatov <KAction@gnu.org>

> Perhaps you just want monad transformers [1, 2]. If you're not familiar
> with them, you should probably read [3].
>
> With transformers you'd be able to do this (and more):
>
> $ return 2 :: IO Int
> $ return 2 :: ListT Maybe Int -- this works like Maybe [Int]
> $ return 2 :: MaybeT [] Int -- this works like [Maybe Int]

In fact, I read and used transformers a bit. If I understand correctly,
I can only do such things for monads, supported by transformers. So,

I cant get (return 2) :: MyWierdMonad (MyAnotherMonad 2)

Yes, I know, it have little practical use, but I am insterested about
teoretical part of question, magic of ghc.

--
Best regards, Dmitry Bogatov <KAction@gnu.org>,
Free Software supporter and netiquette guardian.
git clone git://kaction.name/rc-files.git --depth 1
GPG: 54B7F00D
Html mail and proprietary format attachments are forwarded to /dev/null.