[Haskell-cafe] Monad transformer responsibilities

Antoine Latter aslatter at gmail.com
Fri Jun 5 21:45:04 EDT 2009

On Fri, Jun 5, 2009 at 6:38 PM, wren ng thornton<wren at freegeek.org> wrote:

> 4) Define a newtype of MyMonadT Parsec and declare instances of MyMonad and
> Parsec for it.
> Yes, I know Parsec is (an alias for) a data type, not a type class. But for
> the general problem, using newtype wrappers is often the best solution when
> it's possible. This is one of the reasons why it's good to define type
> classes for specialty monads rather than hard-wiring the types of functions
> to use the one concrete instance. (For instance, this is one of the things
> that rocks about the LogicT library; you can add a StateT on top of a
> MonadLogic and it all works great.)
> For the actual case of Parsec, you could try defining a GenParser class and
> giving it all the combinators as methods, but that's a good deal of work and
> may not scale to your task.

If you need a head start:


I'm not too happy with what turned out to be the class methods. But
I'd need to retire certain parts of the Parsec API to clean it up.


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