On Mon, Sep 26, 2011 at 10:41 AM, Antoine Latter wrote:

On Mon, Sep 26, 2011 at 9:25 AM, Twan van Laarhoven 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.

Yes. On Mon, Sep 26, 2011 at 11:58 AM, Antoine Latter wrote:

...

On Mon, Sep 26, 2011 at 10:41 AM, Antoine Latter wrote:

...

On Mon, Sep 26, 2011 at 9:25 AM, Twan van Laarhoven 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

On Mon, Sep 26, 2011 at 9:50 AM, Edward Kmett wrote: therefore

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...

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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.

If 'modify' is re-written to use this proposed class method, does that mean I can finally make "modify" safe to use in "BackwardStateT"?

Not that I actually use "BawkardStateT", but it is a fun demo.

Antoine

On Sat, Jan 21, 2012 at 4:13 PM, Bas van Dijk wrote:

What happend to this proposal?

This was my mistake. For some reason I had thought part of this change had belonged in transformers, so I let it get backed up on that, but upon looking back at it, it is entirely an mtl concern, and doesn't require anything out of Ross.

Edward, could you also list yourself as maintainer of mtl and list the repository in the cabal file[1]?

Sure thing. -Edward