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pub struct Reader<'a, E, A> {
pub run_reader: Box< dyn 'a + Fn(E) -> A>,
}
impl<'a, E: 'a + Clone, A: 'static + Clone> Reader<'a, E, A> {
pub fn pure(x: A) -> Self {
Reader { run_reader: Box::new( move |_| x.clone())}
}
pub fn initial_env(self, e: E) -> A {
(* self.run_reader) (e)
}
pub fn bind<B: 'static, F: 'static>(self, f: F) -> Reader<'a, E, B>
where
F: Fn(A) -> Reader<'a, E, B>,
B: 'static,
{
Reader { run_reader:
Box::new( move |e: E| { (* f( (* self.run_reader)( e.clone()) ).run_reader)( e) })
}
}
pub fn local<F: 'a>(self, f: F) -> Reader<'a, E, A>
where
F: Fn(E) -> E,
{
Reader { run_reader:
Box::new(move |e: E| { (*self.run_reader) (f(e)) })
}
}
}
pub fn ask<'a, E: Clone>() -> Reader<'a, E, E> {
Reader { run_reader: Box::new(|e: E| e.clone())}
}
pub fn local_do<'a, E, A, F>(f: F, rdr: Reader<'a, E, A>) -> Reader<'a, E, A>
where
F: 'a + Fn(E) -> E,
E: 'a + Clone,
A: 'static + Clone,
{
Reader { run_reader:
Box::new(move |e: E| { (*rdr.run_reader) (f(e)) })
}
}
#[macro_export]
macro_rules! rdrdo {
(pure $e:expr ) => [Reader::pure($e)];
(let $v:ident = $e:expr ; $($rest:tt)*) => [Reader::pure($e).bind( move |$v| { rdrdo!($($rest)*)} )];
(_ <- $monad:expr ; $($rest:tt)* ) => [Reader::bind(($monad), move |_| { rdrdo!($($rest)*)} )];
($v:ident <- pure $e:expr ; $($rest:tt)* ) => [Reader::bind( Reader::pure($e), move |$v| { rdrdo!($($rest)*)} )];
($v:ident <- $monad:expr ; $($rest:tt)* ) => [Reader::bind(($monad), move |$v| { rdrdo!($($rest)*)} )];
($monad:expr ) => [$monad];
}