state.md

State Monad

Represents a calculation which relies on a value provided later, which will be updated during calculation.

Construction

This type has one public constructor state that takes 1 argument, which is a function of type s -> (t, s).

This type has numerous factories as well:

• State.mreturn
• State.get (see below) -- retrieve the state
• State.gets (see below) -- query the state
• State.put (see below) -- replace the state
• State.modify (see below) -- change the state with a function

Usage

Please see example1 and example2

Interface

Aside from the standard fmap, ap, and mbind, the State type has a few more helpers.

#get(), #gets(f), #put(s), #modify(f)

These methods exist to provide a more pleasant code structure than using the constructors directly. It is similar to the convenience methods on the Reader, and shares the same limitations which is namely that a call to get or gets is only good for the function it is used in. If a value is required by a subsequent chained call, then nesting will be required.

#run(state)

This is used to execute the monad with an initial state.

#evalState(state)

This is used to execute the monad with an initial state and return the result.

#execState(state)

This is used to execute the monad with an initial state and return the final state.