Proposal: add 'state' to the MonadState class

[Edward Kmett]

On Mon, Sep 26, 2011 at 10:41 AM, Antoine Latter <aslatter at gmail.com> wrote:

> On Mon, Sep 26, 2011 at 9:25 AM, Twan van Laarhoven <twanvl at gmail.com>
> wrote:
> > Hello list,
> >
> >
> > Currently the 'state' function is only defined for the State type (aka
> > StateT Identity):
> >
> >    state :: (s -> (a, s)) -> State s a
> >    state f = StateT (Identity . f)
> >
> > But this function makes sense for all MonadState instances. I therefore
> > propose to either add it to the MonadState class:
> >
> >    class MonadState s m | m -> s where
> >        ...
> >        state :: (s -> (a,s)) -> m a
> >        state f = do s <- get
> >                     let (a,s') = f s
> >                     set s'
> >                     return a
> >
>

I am 100% behind this change.

It has been on the short list of changes that I want to make to mtl.

The main reason that I like it aside from the fact that it witnesses the
canonical monad homomorphism from State is that it yields more efficient
definitions for functions like modify, and the often-needed but needlessly
expensive operations where you bump a counter _and_ want the result for
something like a fresh variable supply.

I would like to encourage further public discussion.
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