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haskelltypesmonadsstate-monad

What does the get and put function do?

From our lecture notes:

get' state = (state, state)
put' item state = ((), item)     -- () is void value

data State s a = State (s -> (a, s))

-- Functions get and put:        -- (sic!)

get :: State s s
get = State get'

put :: s -> State s ()
put item = State (put' item)

I am totally lost in these two functions get and put.

First, there is no arrow in the type signature of get:

get :: State s s

What does it mean?

What does s mean in both get and put? Are they state?

Solution

  • Remember that State s a is essentially a function s -> (a, s), that is, a function that takes a state and returns a value of some type a and a new state.

    So get :: State s s is a s -> (s, s), a function that simply returns the current state.

    put :: s -> s -> ((), s) is implemented as s -> _ -> ((), s), a function that takes a given state, ignores the current state, returns the given state, and produces no new values.