[Haskell-cafe] More Flexible Monad Transformer OR Bad Coding Style

[Gábor Lehel]

2010/8/9 Edward Z. Yang <ezyang at mit.edu>:
> Excerpts from Gábor Lehel's message of Mon Aug 09 15:39:49 -0400 2010:
>> Actually, while I haven't even used monad transformers before (just
>> read about them a lot), I was thinking that something like this might
>> be the way to solve the lift . lift . lift . lift . foo problem on the
>> one hand, and by wrapping the 'contents' (e.g. the environment of a
>> reader monad) of every level of the stack in a unique newtype (if the
>> type isn't otherwise unique), the problem of "what if I want to use
>> the same transformer more than once, how do I disambiguate them". (Do
>> I have roughly the right idea?)
>
> In fact, what you describe is already in use by current monad transformer
> libraries. :-)  Take for example StateT in mtl [1]:
>
> MonadWriter w m => MonadWriter w (StateT s m)
> MonadError e m => MonadError e (StateT s m)
> Monad m => MonadState s (StateT s m)
> MonadReader r m => MonadReader r (StateT s m)
> MonadTrans (StateT s)
> Monad m => Monad (StateT s m)
> Monad m => Functor (StateT s m)
> MonadFix m => MonadFix (StateT s m)
> MonadPlus m => MonadPlus (StateT s m)
> MonadIO m => MonadIO (StateT s m)
> MonadCont m => MonadCont (StateT s m)
>
> By default, it comes with all of these instances, so that if you use a function
> that relies on say MonadReader and not Reader, no lifting is necessary.  However,
> newtyping every reader monad because you want to use multiple copies of them
> isn't as good, because you still have to manually add all of these instances.

What I meant was to make certain that the 'r' in `ReaderT r m a`, say,
is a unique type in every case, so that when you write `foo ::
MonadReader SomethingOrOther m => m a -> m a` (for example), it's
always unambiguous which MonadReader in the stack you might've meant.
(If it's not... would you get an overlapping instances error, or
what?)

>
> Cheers,
> Edward
>
> [1] http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/Control-Monad-State-Lazy.html
>



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