[Haskell-cafe] Extensible states

[Fumiaki Kinoshita]

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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