[Haskell-cafe] Extensible states

[Alberto G. Corona]

hmmm..

A form of "extensible state" constructed with an state monad with a Map
indexed by the type of the data using "typeOf" could have advantages over
extensible records. It makes the "transport" of data and the addition of
more kinds of data less cumbersome.

 Among other things it encourages good programming practices like the use
of newtypes. It is just a matter of defining two primitives:

with getData :: (MonadState TheMap m,Typeable a) => m (Maybe a)

and setData :: (MonadState TheMap m, Typeable a) => a -> m ()

 and perhaps a third : delData.

2015-05-08 7:33 GMT+02:00 Fumiaki Kinoshita <fumiexcel at gmail.com>:

> Except the performance, my extensible[0] library provides native lens
> support (label names are also lenses!) and quite easy to use. Let me show
> an example:
>
> {-# LANGUAGE TypeOperators, DataKinds, TemplateHaskell, FlexibleContexts
> #-}
> import Data.Extensible
> import Control.Lens
> import Control.Monad.State
>
> mkField "foo bar baz"
>
> statefulStuff :: State (Record '["foo" :> Int, "bar" :> Int, "baz" :>
> Float]) ()
> statefulStuff = do
>     v <- use foo
>     bar += v
>     baz .= 42
>
> main = print $ execState statefulStuff
>   $ foo @= 10 <: bar @= 0 <: baz @= 0 <: Nil
>
> I could use Vector Any internally for O(1) lookup, but seems trade-off
> between lookup and update.
>
> 2015-05-05 18:40 GMT+09:00 Alberto G. Corona <agocorona at gmail.com>:
>
>> Hi,
>>
>> Anyone used some of the extensible record packages to create a kind of
>> extensible state monad?
>>
>> I mean something that besides having "get", "gets" and "put"  would have
>> some kind of "add" and "gets":
>>
>> add :: a -> State ()
>> gets  :: State (Maybe a)
>>
>> or
>>
>> add :: LabelName -> a -> State ()
>> gets :: LabelName -> State (Maybe a)
>>
>>
>> So that I can extend the state without using additional monad
>> transformers. Monad transformers are very hard for beginners and scramble
>> error messages
>>
>> I did the first option for MFlow, hplayground and Transient packages
>> (setSData and getSData). But my solution uses a map indexed by type and
>> this requires a lookup for each access.
>>
>> I would like to know if there is an alternative with no lookups. I´m not
>> obsessed with speed but In some applications the speed may be important....
>>
>> Anyone?
>> --
>> Alberto.
>>
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>>
>>
>


-- 
Alberto.
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