stillwater 1.0.1

Pragmatic effect composition and validation for Rust - pure core, imperative shell
Documentation 
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//! Map combinator - transforms the success value of an effect.

use crate::effect::trait_def::Effect;

/// Map combinator - transforms the success value.
///
/// Zero-cost: no heap allocation. The `Map` struct stores only
/// the inner effect and the transformation function.
///
/// # Example
///
/// ```rust,ignore
/// use stillwater::effect::prelude::*;
///
/// let effect = pure::<_, String, ()>(21).map(|x| x * 2);
/// assert_eq!(effect.execute(&()).await, Ok(42));
/// ```
pub struct Map<Inner, F> {
    pub(crate) inner: Inner,
    pub(crate) f: F,
}

impl<Inner, F> std::fmt::Debug for Map<Inner, F> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("Map")
            .field("inner", &"<effect>")
            .field("f", &"<function>")
            .finish()
    }
}

impl<Inner, F, U> Effect for Map<Inner, F>
where
    Inner: Effect,
    F: FnOnce(Inner::Output) -> U + Send,
    U: Send,
{
    type Output = U;
    type Error = Inner::Error;
    type Env = Inner::Env;

    async fn run(self, env: &Self::Env) -> Result<U, Self::Error> {
        let value = self.inner.run(env).await?;
        Ok((self.f)(value))
    }
}

// WriterEffect implementation for Map - passes writes through unchanged
impl<Inner, F, U> crate::effect::writer::WriterEffect for Map<Inner, F>
where
    Inner: crate::effect::writer::WriterEffect,
    F: FnOnce(Inner::Output) -> U + Send,
    U: Send,
{
    type Writes = Inner::Writes;

    async fn run_writer(
        self,
        env: &Self::Env,
    ) -> (Result<Self::Output, Self::Error>, Self::Writes) {
        let (result, writes) = self.inner.run_writer(env).await;
        (result.map(self.f), writes)
    }
}