Control.Monad.Operational

Synopsis

To write a monad, use the Program type.

To write a monad transformer, use the ProgramT type.

For easier interoperability, the Program type is actually a type synonym and defined in terms of ProgramT.

Overview

The basic idea for implementing monads with this libary is to think of monads as sequences of primitive instructions. For instance, imagine that you want to write a web application with a custom monad that features an instruction

 askUserInput :: CustomMonad UserInput

which sends a form to the remote user and waits for the user to send back his input

To implement this monad, you decide that this instruction is a primitive, i.e. should not be implemented in terms of other, more basic instructions. Once you have chosen your primitives, collect them in a data type

data CustomMonadInstruction a where
    AskUserInput :: CustomMonadInstruction UserInput

Then, obtain your custom monad simply by applying the Program type constructor

 type CustomMonad a = Program CustomMonadInstruction a

The library makes sure that it is an instance of the Monad class and fulfills all the required laws.

Essentially, the monad you now obtained is just a fancy list of primitive instructions. In particular, you can pattern match on the first element of this list. This is how you implement an interpret or run function for your monad. Note that pattern matching is done using the view function

runCustomMonad :: CustomMonad a -> IO a
runCustomMonad m = case view m of
    Return a            -> return a -- done, return the result
    AskUserInput :>>= k -> do
        b <- waitForUserInput       -- wait for external user input
        runCustomMonad (k b)        -- proceed with next instruction

The point is that you can now proceed in any way you like: you can wait for the user to return input as shown, or you store the continuation k and retrieve it when your web application receives another HTTP request, or you can keep a log of all user inputs on the client side an replay them, and so on. Moreover, you can implement different run functions for one and the same custom monad, which is useful for testing. Also note that the result type of the run function does not need to be a monad at all.

In essence, your custom monad allows you to express your web application as a simple imperative program, while the underlying implementation can freely map this to an event-drived model or some other control flow architecture of your choice.

The possibilities are endless. More usage examples can be found here: https://github.com/HeinrichApfelmus/operational/tree/master/doc/examples#readme

Monad

type Program instr = ProgramT instr Identity

The abstract data type Program instr a represents programs, i.e. sequences of primitive instructions.

• The primitive instructions are given by the type constructor instr :: * -> *.
• a is the return type of a program.

Program instr is always a monad and automatically obeys the monad laws.

singleton :: instr a -> ProgramT instr m a

Program made from a single primitive instruction.

type ProgramView instr = ProgramViewT instr Identity

View type for inspecting the first instruction. It has two constructors Return and :>>=. (For technical reasons, they are documented at ProgramViewT.)

view :: Program instr a -> ProgramView instr a

View function for inspecting the first instruction.

Example usage

Stack machine from "The Operational Monad Tutorial".

    data StackInstruction a where
        Push :: Int -> StackInstruction ()
        Pop  :: StackInstruction Int

    type StackProgram a = Program StackInstruction a
    type Stack b        = [b]

    interpret :: StackProgram a -> (Stack Int -> a)
    interpret = eval . view
        where
        eval :: ProgramView StackInstruction a -> (Stack Int -> a)
        eval (Push a :>>= is) stack     = interpret (is ()) (a:stack)
        eval (Pop    :>>= is) (a:stack) = interpret (is a ) stack
        eval (Return a)       stack     = a

Note that since ProgramView is a GADT, the type annotation for eval is mandatory.

interpretWithMonad :: forall instr m b. Monad m => (forall a. instr a -> m a) -> Program instr b -> m b

Utility function that extends a given interpretation of instructions as monadic actions to an interpration of Programs as monadic actions.

This function can be useful if you are mainly interested in mapping a Program to different standard monads, like the state monad. For implementing a truly custom monad, you should write your interpreter directly with view instead.

Monad transformer

data ProgramT instr m a

The abstract data type ProgramT instr m a represents programs over a base monad m, i.e. sequences of primitive instructions and actions from the base monad.

• The primitive instructions are given by the type constructor instr :: * -> *.
• m is the base monad, embedded with lift.
• a is the return type of a program.

ProgramT instr m is a monad transformer and automatically obeys both the monad and the lifting laws.

Instances

MonadReader r m => MonadReader r (ProgramT instr m)

MonadState s m => MonadState s (ProgramT instr m)

MonadTrans (ProgramT instr)

Monad m => Monad (ProgramT instr m)

Monad m => Functor (ProgramT instr m)

Monad m => Applicative (ProgramT instr m)

MonadIO m => MonadIO (ProgramT instr m)

data ProgramViewT instr m a where

View type for inspecting the first instruction. This is very similar to pattern matching on lists.

• The case (Return a) means that the program contains no instructions and just returns the result a.
• The case (someInstruction :>>= k) means that the first instruction is someInstruction and the remaining program is given by the function k.

Constructors

Return :: a -> ProgramViewT instr m a

:>>= :: instr b -> (b -> ProgramT instr m a) -> ProgramViewT instr m a

viewT :: Monad m => ProgramT instr m a -> m (ProgramViewT instr m a)

View function for inspecting the first instruction.

Example usage

List monad transformer.

    data PlusI m a where
        Zero :: PlusI m a
        Plus :: ListT m a -> ListT m a -> PlusI m a

    type ListT m a = ProgramT (PlusI m) m a

    runList :: Monad m => ListT m a -> m [a]
    runList = eval <=< viewT
        where
        eval :: Monad m => ProgramViewT (PlusI m) m a -> m [a]
        eval (Return x)        = return [x]
        eval (Zero     :>>= k) = return []
        eval (Plus m n :>>= k) =
            liftM2 (++) (runList (m >>= k)) (runList (n >>= k))

Note that since ProgramView is a GADT, the type annotation for eval is mandatory.

liftProgram :: Monad m => Program instr a -> ProgramT instr m a

Lift a plain sequence of instructions to a sequence of instructions over a monad m. This is the counterpart of the lift function from MonadTrans.

It can be defined as follows:

    liftProgram = eval . view
        where
        eval :: ProgramView instr a -> ProgramT instr m a
        eval (Return a) = return a
        eval (i :>>= k) = singleton i >>= liftProgram . k