Safe Haskell | Safe-Infered |
---|

- type Program instr = ProgramT instr Identity
- singleton :: instr a -> ProgramT instr m a
- type ProgramView instr = ProgramViewT instr Identity
- 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

# 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 not that the result 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 ProgramView instr = ProgramViewT instr IdentitySource

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 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.

# Monad transformer

data ProgramT instr m a Source

The abstract data type

represents programs
over a base monad `ProgramT`

instr m a`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.

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. 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`

.

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.

*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 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