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
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.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe