[Haskell-cafe] Stateful iteratees

[Ertugrul Soeylemez]

Hello fellow Haskellers,

I'm trying to solve a very practical problem:  I need a stateful
iteratee monad transformer.  Explicit state passing is very inconvenient
and would destroy the elegance of my library.

There are two approaches to this:

  1. type MyT a m = Iteratee a (StateT MyConfig m)
  2. type MyT a m = StateT MyConfig (Iteratee a m)

Both work well except in two very specific corner cases:

  - I need to convert the transformer to 'Iteratee a m', i.e. remove the
    state layer.  This is obviously trivial with the second variant, but
    seems very difficult with the first one, if it's possible at all.

  - I need to use control structures of Iteratee like catchError.  This
    is obviously trivial with the first variant, but very inconvenient
    with the second, because I would need to reinvent many wheels.

Does someone know a cleaner, more elegant solution?  Encapsulating the
state in the iteratee's input type is not an option.

Many thanks in advance.


Greets,
Ertugrul


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