DA.Action.State.Class

DA.Action.State.Class

Typeclasses

class ActionState s m where

Action m has a state variable of type s.

Rules:

• get *> ma  =  ma

• ma <* get  =  ma

• put a >>= get   =  put a $> a

• put a *> put b  =  put b

• (,) <$> get <*> get  =  get <&> \a -> (a, a)

Informally, these rules mean it behaves like an ordinary assignable variable: it doesn’t magically change value by looking at it, if you put a value there that’s always the value you’ll get if you read it, assigning a value but never reading that value has no effect, and so on.

get

: m s

Fetch the current value of the state variable.

put

: s -> m ()

Set the value of the state variable.

modify

: (s -> s) -> m ()

Modify the state variable with the given function.

default modify

: Action m => (s -> s) -> m ()

instance ActionState s (State s)