[Haskell-beginners] Effective use of nested Monads

[David Hinkes]

On Sun, Feb 10, 2013 at 9:17 PM, Brent Yorgey <byorgey at seas.upenn.edu>wrote:

> On Sun, Feb 10, 2013 at 02:53:18PM -0800, David Hinkes wrote:
> > Hi haskell-beginners,
> >
> > I'm starting to come to the idea of exposing a Monad as a means of
> > controlling an API.  So, I've started creating my own Monad data types
> > based on classical monads.  However, I'm running into a problem regarding
> > creating monad definitions when using nested Monads.
> >
> > For example:
> >
> > newtype Example m o = Example {
> >   runExample :: State Int (m o)
> > }
> >
> > Is there a clean way to make Example a monad?
>
> Actually, there isn't!  This is one way in which monads turn out to be
> *too* powerful: they don't compose very well.  If m and n are monads,
> then their composition (that is, a type like newtype Composed a =
> Composed (m (n a))) is *not* necessarily a monad!  So "nesting" monads
> in this way is usually not a good idea.
>
> What you want are called "monad transformers", which give you a way to
> "compose" certain monads (though it's more complicated than just
> nesting them).  You can do your Example type something like this:
>
>   newtype Example m o = Example {
>     runExample :: StateT Int m o
>   }
>
> where StateT is the State monad transformer, defined in the
> 'transformers' package (and also exported from the 'mtl' package).  I
> refer you to the typeclassopedia for more information and links to
> further reading:
>
  http://www.haskell.org/haskellwiki/Typeclassopedia#Monad_transformers


Thanks Brent, I'll try to  re-organize around the transforms.
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