Generalize MonadIO to MonadBase
iavor.diatchki at gmail.com
Tue Apr 20 19:21:49 EDT 2010
A common way to get different "base" monads is to "newtype" existing
monads. Here is an example:
> newtype CustomMonad a = CM (StateT Int IO a) deriving (Monad)
> instance BaseM CustomMonad CustomMonad where inBase = id
> ... define some custom operations on CustomMonad...
The "newtype" provides an "abstraction barrier", which hides the
implementation of the CustomMonad. If you add more layers on top of
"CustomMonad" when you use "inBase" it will only lift up to
CustomMonad. This is useful if, for example, CustomMonad needs to
perform some IO actions, but for one reason or another it does not
allow for the execution of arbitrary IO computations.
Note that when you are implementing the operations of CustomMonad, you
can use "inBase" again, this time to lift all the way down to IO.
Basically, by using "newtypes", you can kind of "punctuate" a tall
tower of monads, an use "inBase" to jump from one stack to the next.
Hope that this helps,
On Tue, Apr 20, 2010 at 2:45 PM, Evan Laforge <qdunkan at gmail.com> wrote:
> I haven't really been following this discussion, but I use
> Identity-backed monads all the time. Usually some combination of
> StateT, WriterT, ErrorT and ReaderT.
> Are there any "base" monads other than IO and Identity?
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