Proposal: add 'state' to the MonadState class
aslatter at gmail.com
Mon Sep 26 17:58:26 CEST 2011
On Mon, Sep 26, 2011 at 9:50 AM, Edward Kmett <ekmett at gmail.com> wrote:
> 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>
>> > 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.
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.
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