Re: Proposal: add 'state' to the MonadState class

26 Sep 2011


      Yes.

On Mon, Sep 26, 2011 at 11:58 AM, Antoine Latter  wrote:
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On Mon, Sep 26, 2011 at 10:41 AM, Antoine Latter 
wrote:
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On Mon, Sep 26, 2011 at 9:25 AM, Twan van Laarhoven 
wrote:
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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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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