decepticon

-module(decepticon@stateful).
-compile([no_auto_import, nowarn_unused_vars, nowarn_unused_function, nowarn_nomatch]).

-export([action/1, get/0, put/1, map/2, apply/2, do/2, run/2, eval/2, exec/2, increment_state/0]).
-export_type([state/2]).

-type state(FJS, FJT) :: {state, fun((FJT) -> {FJS, FJT})}.

-spec action(FJU) -> state(FJU, any()).
action(Action_value) ->
    {state, fun(S) -> {Action_value, S} end}.

-spec get() -> state(FJY, FJY).
get() ->
    {state, fun(S) -> {S, S} end}.

-spec put(FKB) -> state(nil, FKB).
put(State_value) ->
    {state, fun(_) -> {nil, State_value} end}.

-spec map(state(FKE, FKF), fun((FKE) -> FKI)) -> state(FKI, FKF).
map(State, Map_fn) ->
    {state,
        fun(S) ->
            {A, S@1} = (erlang:element(2, State))(S),
            {Map_fn(A), S@1}
        end}.

-spec apply(state(fun((FKL) -> FKM), FKN), state(FKL, FKN)) -> state(FKM, FKN).
apply(Prev, Next) ->
    {state,
        fun(S) ->
            {F, S@1} = (erlang:element(2, Prev))(S),
            {A, S@2} = (erlang:element(2, Next))(S@1),
            {F(A), S@2}
        end}.

-spec do(state(FKU, FKV), fun((FKU) -> state(FKY, FKV))) -> state(FKY, FKV).
do(State, And_then_fn) ->
    {state,
        fun(S) ->
            {A, S@1} = (erlang:element(2, State))(S),
            (erlang:element(2, And_then_fn(A)))(S@1)
        end}.

-spec run(state(FLD, FLE), FLE) -> {FLD, FLE}.
run(State, Initial) ->
    (erlang:element(2, State))(Initial).

-spec eval(state(FLH, FLI), FLI) -> FLH.
eval(State, Initial) ->
    {A, _} = run(State, Initial),
    A.

-spec exec(state(any(), FLM), FLM) -> FLM.
exec(State, Initial) ->
    {_, S} = run(State, Initial),
    S.

-spec increment_state() -> state(nil, integer()).
increment_state() ->
    {state, fun(S) -> {nil, S + 1} end}.