Monads

[Christopher Milton]

Mark,

I'm no expert, but does it help to start from withStateT?

> withStateT :: (s -> s) -> StateT s m a -> StateT s m a
> withStateT f m = StateT $ runStateT m . f

There are some notes about computations and lifting
state transformers in

Modular Denotational Semantics for Compiler Construction
Sheng Liang, Paul Hudak
http://citeseer.nj.nec.com/liang96modular.html

Monad Transformers and Modular Interpreters
Sheng Liang, Paul Hudak, Mark Jones
http://citeseer.nj.nec.com/liang95monad.html

Don't mind me: I just couldn't control the vestiges of
librarianship lurking in my dark, lost soul...

Dobrego Nowego Roku!

Chris Milton (no, not MLton:-)

--- Mark Carroll <mark at chaos.x-philes.com> wrote:
> Omitting the typeclass bit, I'm trying to write something like
> (s1 -> s2) -> StateT s1 m () -> StateT s2 m a -> StateT s1 m a
> 
> That is, it sequences two StateT computations, providing a way to
> translate from the first's state to the second to keep the chain
> going.
> 
> I can easily write something for when s1 and s2 are the same, and my
> understanding of much of Control.Monad.* remains tenuous at best, but if
> it's easy for anyone to provide me with some tips, then I thought I should
> mention that it'd certainly be helpful.
> 
> And Happy New Year, everyone!
> 
> -- Mark
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