Documentation

data Bind t f Source

Monadic actions are terms composed from the following constructors

Constructors

Bind t f

Instances

(RunBind m s x, RunState y s w) => RunBind (Bind m (x -> y)) s w
(RunState m s a, RunState t s b) => RunState (Bind m (a -> t)) s b

data Return v Source

Constructors

Return v

Instances

RunBind (Return a) s a
RunState (Return a) s a

data Get Source

Constructors

Get

Instances

RunBind Get s s
RunState Get s s

data Put v Source

Constructors

Put v

Instances

RunBind (Put s) s ()
RunState (Put s) s ()

class RunBind t s a => RunState t s a | t s -> a whereSource

examples of terms

An example of an ill-formed term

An example of an ill-typed term The following term is ill-typed because it attempts to store both a boolean and a character in the state. Our state can have only one type.

The interpreter of monadic actions It takes the term t and the initial state of the type s and returns the final state and the resulting value. The type of the result, a, is uniquely determined by the term and the state

Methods

runst :: t -> s -> (s, a)Source

Instances

RunState Get s s
RunState (Put s) s ()
RunState (Return a) s a
RunState (Statte s a) s a
(RunState m s a, RunState t s b) => RunState (Bind m (a -> t)) s b

class RunBind m s a whereSource

Interpretation of the Bind action requires an auxiliary class This is due to the polymorphism of the monadic bind, which has the type

 m a -> (a -> m b) -> m b

Note the polymorphism both in a (value type of the input monadic action) and b (value type of the resulting action)

Methods

runbind :: RunState t s b => m -> (a -> t) -> s -> (s, b)Source

Instances

RunBind Get s s
RunBind (Put s) s ()
RunBind (Return a) s a
RunBind (Statte s a) s a
(RunBind m s x, RunState y s w) => RunBind (Bind m (x -> y)) s w

data Statte s a Source

We can now run (interpret) our sample terms

term2 denoted the action of negating the current state and returning the original state

Now, we show that our term representation of the state monad is an instance of MonadState

Constructors

forall t . RunState t s a => Statte t

Instances

MonadState s (Statte s)
Monad (Statte s)
RunBind (Statte s a) s a
RunState (Statte s a) s a

as_statte :: Statte s a -> Statte s aSource

We can write computations expressed by term1 and term2 using the normal monadic syntax

The following identity function is to resolve polymorphism, to tell that monadic terms below should use our representation of MonadState