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Typeclasses

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class ActionState s m

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. Methods:

• 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.
• modify : Action m => (s -> s) -> m ()

Instances:

• instance ActionState s (State s)

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