unkindpartition / monad-classes

A more flexible mtl

See this series of articles for the detailed description and motivation.

This is a more flexible version of mtl, the monad transformers library.

• You can have many layers of e.g. state transformers in your stack, and you don't have to explicitly lift your gets and puts, as soon as different state transformers carry different types of states.

  Example:

  a :: (MonadState Bool m, MonadState Int m) => m ()
  a = do
    put False -- set the boolean state
    modify (+ (1 :: Int)) -- modify the integer state

• mtl requires Θ(n²) instances (like MonadReader e (StateT s m)); monad-classes requires only Θ(n) of them (where n is the number of different transformer types).

  If you'd like to define your own monad-classes-style class, you have to write much less boilerplate code.

The status of this repo

This repo serves two purposes:

• as a reference implementation accompanying the series of articles on extensible effects
• as an actual implementation we use at Signal Vine

While this is technically open source software, I am not interested in maintaining this as an active open project. In practice this means that:

• this version of monad-classes is not on hackage
• I do not accept feature requests and pull requests. The exception is outright bugs; if you find any, please do report them.

There is a hackage package and a repo maintained by M Farkas-Dyck.

monad-classes's Issues

add io effects à la IOSpec but without it's headaches

what do you think about providing effects for typical operations within IO?

having played a bit with idris effects system, i'd totally like to have a fine-grained specification on what my IO-functions are doing.

as such, i wanted to have (at example) an OpenFile effect, that allows openFile and withFile but not (much) more. There would also be a ReadHandle and WriteHandle effect, that allows reading/seeking or writing to a handle respectively.

i would handle subtyping these effects using ConstraintsKinds (e.g. TeleType = (ConsoleWrite, ConsoleRead)).

implementing them, i would not write them by hand but use template haskell to generate them.

would you say that it's a good idea? and if so if it's in scope for monad-classes or should better be a separate package?

upload to hackage

i am really interested in using monad-classes. do you think there is still much missing before uploading to hackage? if so what do you think is missing?

except effect

i find the except effect pretty limited to use.

how did you intend it to be used? just using the functions from Control.Monad.Trans.Except kind of defeats the point of the effect imo.

looking forward, for e.g. withExcept do you think it should be along the lines of the following?

withExcept :: MonadExcept e' m => (e -> e') -> ExceptT e m a -> m a
withExcept f a = either (throw . f) return =<< runExcept a

i still have to think about whether and, if so, how to integrate Zoom with a possible catch.

catch :: MonadExcept e' m => ExceptT e m a -> (forall e' . MonadExcept e' m => e -> m a) -> m a

furthermore, what do you think about convenience functions as in the errors package, i.e. to lift Either a bs and m (Either a b)s?