Implement monads by specifying primitive instructions and their operational semantics.

This package is based on the "The Operational Monad Tutorial", published in Issue 15 of The Monad.Reader http://themonadreader.wordpress.com/.

You are reading the API reference. For more thorough documentation including design and implementation notes as well as a correctness proof, please consult the included documentation in `docs/Documentation.html`

, also available at http://projects.haskell.org/operational/Documentation.html .

This API reference includes only basic example code. More intricate examples are available in the `docs/examples`

directory, also available at http://projects.haskell.org/operational/examples.html.

- type Program instr a = ProgramT instr Identity a
- singleton :: instr a -> ProgramT instr m a
- type ProgramView instr a = ProgramViewT instr Identity a
- view :: Program instr a -> ProgramView instr a
- data ProgramT instr m a
- data ProgramViewT instr m a where
- 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)
- liftProgram :: Monad m => Program instr a -> ProgramT instr m a

# Basic usage

type Program instr a = ProgramT instr Identity aSource

The abstract data type 'Program instr a' represents programs.

- The type constructor
`instr :: * -> *`

indexes the primitive instructions. -
`a`

is the return type of a program.

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

instr

type ProgramView instr a = ProgramViewT instr Identity aSource

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 aSource

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

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.

# Monad transformer

data ProgramT instr m a Source

The abstract data type

represents programs.
`ProgramT`

instr m a

- The type constructor
`instr :: * -> *`

indexes the primitive instructions. -
`m`

is the base monad, embedded with`lift`

. -
`a`

is the return type of a program.

is a monad transformer and
automatically obeys both the monad and the lifting laws.
`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 whereSource

View type for inspecting the first instruction.

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)Source

View function for inspecting the first instruction.

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

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

*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.