@hackage multistate0.1.3.2

Introduction

When using multiple ReaderT's or StateT's in the same monad stack, it becomes necessary to lift the operations in order to affect a specific transformer. Using heterogenous lists (type level functions), a GADT and a corresponding type class, this package provides transformers that remove that necessity: MultiReaderT/MultiStateT can contain a heterogenous list of values.

The type inferred for the getter/setter determines which value is read/written.

Example

simpleExample :: IO ()
simpleExample = evalMultiStateT
              $ withMultiState 'H'
              $ withMultiState "ello, World!"
              $ do
  -- the monad here is MultiStateT (Cons [Char] (Cons Char Null)) IO
  let combinedPrint = do       -- no type signature necessary
        c  <- mGet             -- type of mGet inferred to be m Char
        cs <- mGet             --              inferred to be m [Char]
        lift $ putStrLn (c:cs)
  combinedPrint
  mSet 'J'                     -- similarly for the setter
  combinedPrint

The output is:

Hello, World!
Jello, World!

( you can find both this and a more complex example in an executable in the package. )

Error Messages

If you try to execute an action that requires a specific type in the state, but the current state does not contain that type, the error message is something like

Control.Monad.MultiState.ContainsType Foo Null

where Foo is the missing type.

Known Deficits

This package currently lacks a complete set of "lifting instances", i.e. instance definitions for classes such as mtl's MonadWriter "over" the newly introduced monad transformers, as in

instance (MonadWriter w m) => MonadWriter w (MultiStateT c m) where ..

These "lifting instances" would be necessary to achieve full compatability with existing transformers. Ping me if you find anything specific missing.