//! A lazy computation with no effects.
//!
//! This module provides [lazy](fn.lazy.html) and [pure](fn.pure.html) function.
//! `lazy` creates a computation with no effects that just executes the passed function.
//! `pure` converts the given value into a pure computation.

use super::{Context, Effectful, Poll};

use std::pin::Pin;

/// A lazy computation with no effects
pub struct Lazy<F>(Option<F>);

// Safe because we will not create Pin<&mut F>
impl<F> Unpin for Lazy<F> {}

impl<T, F> Effectful for Lazy<F>
where
    F: FnOnce() -> T,
{
    type Output = T;
    type Effect = !;

    /// Execute the computation
    ///
    /// # Panics
    /// Panics if the task is polled again after completion
    #[inline]
    fn poll(mut self: Pin<&mut Self>, _cx: &Context) -> Poll<Self::Output, Self::Effect> {
        Poll::complete(self.0.take().expect("poll after completion")())
    }
}

/// Converts a thunk into a one-shot effectful computation
#[inline]
pub fn lazy<T, F>(v: F) -> impl Effectful<Output = T, Effect = !>
where
    F: FnOnce() -> T,
{
    Lazy(Some(v))
}

/// Converts a value into a one-shot effectful computation that immediately resolves to said value
pub fn pure<T>(v: T) -> impl Effectful<Output = T, Effect = !> {
    lazy(move || v)
}