Nicolas Frisby wrote: | I wrote a combination reader/writer monad (a la the RWS monad in the | mtl) and I find myself wanting to use multiple instances of it in the | same stack of transformers. The functional dependencies prevent this | from working out. The class is called MonadRW and the transformer is | called RWT. | | I find myself wishing I could import the same module twice, but | instead of having two names for the same entity, I want the | definitions proper of the module to be duplicated: I want two separate | MonadRW classes and two separate RWT transformers. Since the module | integrates the MonadRW and RWT transformer with the mtl, I would then | only need to lift the instances of the instantiated MonadRW classes | through the other RWTs. | | I'm rather unfamiliar with Template Haskell, but it sounds like it | might fit the bill. Although, if I recall correctly, instances and | type declarations splicing are yet to be implemented in GHC? | | Does anyone have any advice how to do this? I'm envisioning some | preprocessing. I'd like to know if someone has developed a tool for | this already or if this path has been attempted.
Sounds familiar :-).
I wrote some TH code to get "named" versions of the StateT monad
transformer. Example usage:
> {-# OPTIONS_GHC -fth #-}
> module Test where
>
> import NamedStateT
>
> data Settings = ...
>
> $(namedStateT "Settings" $ Just $ ConT ''Settings)
>
> q :: (MonadSettings m,MonadIO m) => m ()
> q = do
> settings <- getSettings
> liftIO $ print settings
Notes:
- The second argument to namedStateT is the type of the state. |Nothing|
keeps it parameterised, like |StateT| itself.
- The big comment shows the code that would be generated by a typical
splicing, although the code may have been updated at some point without
changing the example accordingly.
- Not all instances and helper functions from Control.Monad.State are in
there; |modify| seems to be missing, for instance. If you need any, I can
add them (or you can do it yourself).
- I just saw I also have a NamedReaderT module, but it has never been
used, so it may not work. Yell if you think you can use it.
Kind regards,
Arie
--
Familiarity with the Haskell language is assumed, but no prior experience
with warm, fuzzy things is required.
NamedStateT.hs
Description: Binary data
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