Re: [Haskell-cafe] Stacked return

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 Monadinstance. 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-Over... 2013/11/23 Dmitry Bogatov

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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 , 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.