wiring: Wiring, promotion and demotion of types.

This package provides the ability to join up or wire instances of types (such as ReaderT, WriterT and RWST) with some semi-automatic conversions.

ReaderT Example

With ReaderT there might be two functions already written that we wish to compose:

userLookup :: Int -> ReaderT (Resource1, Database1) IO User
ordersLookup :: Int -> ReaderT (Database2, Resource1) IO [String]

Ordinarily these could not be used together because their environment type differs, which reduces the utility of ReaderT as a method of dependency injection.

However with use of the wire method from the Wirable typeclass it becomes possible to use these two together with little pain.

composedLookup :: Int -> ReaderT (Resource1, Database1, Database2) IO String
composedLookup userId = do
  user    <- wire $ userLookup userId
  orders  <- wire $ ordersLookup userId
  return $ describeOrders user orders

Tuples

Built in are instances of Wirable for transforming tuples to their individual fields, like taking a from a tuple (a, b, c).

As well as producing new tuples from existing tuples, starting with a tuple of (a, b, c) and returning a tuple of (c, a).

Other Types

WriterT and RWST are similarly well supported, including the ability to promote from ReaderT/WriterT up to RWST.

Some existing functionality is extended like for instance with the wiredAsk and wiredTell functions.

• Control
  • Monad
    • RWS
      • Lazy
        • Control.Monad.RWS.Lazy.Wiring
      • Strict
        • Control.Monad.RWS.Strict.Wiring
    • Reader
      • Class
        • Control.Monad.Reader.Class.Wiring
      • Control.Monad.Reader.Wiring
    • State
      • Lazy
        • Control.Monad.State.Lazy.Wiring
      • Strict
        • Control.Monad.State.Strict.Wiring
    • Trans
      • RWS
        • Lazy
          • Control.Monad.Trans.RWS.Lazy.Wiring
        • Strict
          • Control.Monad.Trans.RWS.Strict.Wiring
      • Reader
        • Control.Monad.Trans.Reader.Wiring
      • State
        • Lazy
          • Control.Monad.Trans.State.Lazy.Wiring
        • Strict
          • Control.Monad.Trans.State.Strict.Wiring
      • Writer
        • Lazy
          • Control.Monad.Trans.Writer.Lazy.Wiring
        • Strict
          • Control.Monad.Trans.Writer.Strict.Wiring
    • Control.Monad.Wiring
    • Writer
      • Lazy
        • Control.Monad.Writer.Lazy.Wiring
      • Strict
        • Control.Monad.Writer.Strict.Wiring