[Haskell-cafe] Extensible states

Alberto G. Corona agocorona at gmail.com
Tue May 5 09:40:05 UTC 2015


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)


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....

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