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@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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