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use std::marker::PhantomData;
pub struct State<'state, S, A> {
run: Box<Fn(S) -> (S, A) + 'state>,
state_type: PhantomData<S>,
content_type: PhantomData<A>,
}
impl<'state, S: 'state + Clone + Copy, A: 'state> State<'state, S, A> {
pub fn new<F>(f: F) -> State<'state, S, A>
where
F: Fn(S) -> (S, A) + 'state,
{
State {
run: Box::new(f),
state_type: PhantomData,
content_type: PhantomData,
}
}
pub fn run(&self, s: S) -> (S, A) {
(self.run)(s)
}
pub fn map<B: 'state, G>(self, f: G) -> State<'state, S, B>
where
G: Fn(A) -> B + 'state,
{
let h = move |s: S| {
let (s1, a) = (self.run)(s);
(s1, f(a))
};
State::new(h)
}
pub fn flat_map<B: 'state, G>(self, f: G) -> State<'state, S, B>
where
G: Fn(A) -> State<'state, S, B> + 'state,
{
let h = move |s: S| {
let (s1, a) = (self.run)(s);
(f(a).run)(s1)
};
State::new(h)
}
pub fn get(self) -> State<'state, S, S> {
let f = move |s: S| {
let s2 = s.clone();
(s, s2)
};
State::new(f)
}
pub fn gets<F>(self, f: F) -> State<'state, S, A>
where
F: Fn(S) -> A + 'state,
{
let g = move |s: S| {
let s2 = s.clone();
(s, f(s2))
};
State::new(g)
}
pub fn put(self, s: S) -> State<'state, S, ()> {
let s2 = s.clone();
let f = move |_| (s2, ());
State::new(f)
}
pub fn modify<F>(self, f: F) -> State<'state, S, ()>
where
F: Fn(S) -> S + 'state,
{
let g = move |s| (f(s), ());
State::new(g)
}
}
#[cfg(test)]
mod tests {
use super::State;
#[derive(Debug, Clone, Copy)]
struct Account {
balance: i32,
}
fn deduct(d: i32) -> State<'static, Account, i32> {
State::new(move |a: Account| (Account { balance: a.balance - d }, 0))
}
fn contribute(d: i32) -> State<'static, Account, i32> {
State::new(move |a: Account| (Account { balance: a.balance + d }, 0))
}
#[test]
fn it_works() {
let account = Account { balance: 0 };
let x = contribute(10).flat_map(move |_: i32| deduct(5));
let (account2, fee) = (x.run)(account);
println!("____ Balance: {:?} Fee {:?}", account2.balance, fee);
assert!(true);
}
}