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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