Could iteratee depend on mtl instead of transformers?

[Dan Doel]

On Tuesday 17 November 2009 9:56:21 pm Antoine Latter wrote:
> Every time this comes up I grow paranoid that there is some sort of
> general overhead to evaluating an expression of type "StateT s
> Identity a" vs "State s a".
> 
> So I finally benchmarked it using the example code listed in
> Control.Monad.State, and found no difference whatsoever.
> 
> Here's the source:
>  http://hpaste.org/fastcgi/hpaste.fcgi/view?id=12259#a12259
> 
> I guess parsec is the odd one out, where there is a difference. Or I
> was measuring something else back when I had parsec benchmarks set up.

At least in the State case, everything involved is a newtype, so:

  StateT s Identity a

should be represented identically to:

  s -> Identity (a, s) = s -> (a, s)

Which is exactly what State s a is. From there it's just up to the compiler to 
eliminate no-ops like:

  return :: a -> a -- return :: a -> Identity a
  return a = a

which shouldn't be too arduous. Building up stacks of novel monads can 
introduce additional indirection over flattening them into a direct 
implementation (like StateT s (WriterT w ...) versus RWST r s w ...), but 
thankfully, Identity shouldn't introduce such overhead.

-- Dan