[Haskell-cafe] Monadic tunnelling: the art of threading one monad through another

[Jules Bean]

Derek Elkins wrote:
> What you're actually showing is that these effects can be -embedded- in
> IO (i.e. that IO already supports them).  I noticed you didn't try to
> make an instance for the Cont monad.  Actually, if we added
> continuations to IO, we'd be set.  We wouldn't even need your typeclass.


Yes, precisely. Your use of the term 'embedded' parallels the fact I 
called the method 'embed'.

It's a useful technique because it enables you to give more specific 
types to your functions: to use StateT IO instead of just using IO and 
instead of using ad-hoc IORefs yourself you can use the StateT methods 
and have the IORefs behind the scenes when you need to thread through 
another monad. Similarly you can give pure types to callbacks (either 
plain State s, or the parametric forall m. StateT s m) which makes it 
easier to specify and test them. It even has applications in an 
untrusted code environment, where it's dangerous to accept callbacks of 
IO type.

Of course, you can't do Cont or List (unless I'm missing something) 
because they are both capable of duplicating the current continuation, 
and it's not possible to duplicate the entire IO state (a.k.a. the 
RealWorld#).

Jules