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```
{-
Copyright (c) John P Mayer Jr, 2013
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
-}
module Control.State where
{-| -}
type alias State s a = { runState : s -> (a, s) }
get : State s s
get = State <| \s -> (s, s)
put : s -> State s ()
put s' = State <| \s -> ((), s')
evalState : State s a -> s -> a
evalState st = fst << st.runState
execState : State s a -> s -> s
execState st = snd << st.runState
{- Monad Instance -}
returnS : a -> State s a
returnS x = State <| \s -> (x, s)
bindS : State s a -> (a -> State s b) -> State s b
bindS st f =
State <|
\s -> let (x, s') = st.runState s
in (f x).runState s'
sequenceS : List (State s a) -> State s (List a)
sequenceS ms =
let k m m' =
bindS m (\x ->
bindS m' (\xs ->
returnS <| x :: xs))
in List.foldr k (returnS []) ms
mapMS : (a -> State s b) -> List a -> State s (List b)
mapMS f = sequenceS << List.map f
{- Functor Instance (derived) -}
fmapS : (a -> b) -> State s a -> State s b
fmapS f st =
bindS st (\x ->
returnS <| f x)
updateS : (a -> a) -> State a ()
updateS f = bindS get (\s -> put <| f s)
```