@hackage multistate0.1.2

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

             -- an IO action wrapped by a MultiState
             -- containing both a Char and a [Char].
examplePrint :: MultiStateT (Cons [Char] (Cons Char Null)) IO ()
examplePrint = do
  c  <- mGet             -- inferred to be :: m Char
  cs <- mGet             -- inferred to be :: m [Char]
  lift $ putStrLn (c:cs)

This computation can be executed in the following way:

main = evalMultiStateT
     $ withMultiState 'H'
     $ withMultiState "ello, World!"
     $ examplePrint

( you can find this example as an executable in the package. )

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.