Function first 

pub fn first<A, B, Env, E1, E2>(
    e1: E1,
    e2: E2,
) -> BoxedEffect<A, AnalysisError, Env>
where
    A: Send + 'static,
    B: Send + 'static,
    Env: Clone + Send + Sync + 'static,
    E1: Effect<Output = A, Error = AnalysisError, Env = Env> + Send + 'static,
    E2: Effect<Output = B, Error = AnalysisError, Env = Env> + Send + 'static,

Run the first effect, ignoring the second’s value.

This is useful when you need side effects from the second but only care about the first’s result.

Example

let effect = first(compute_result(), log_completion());
// Returns compute_result's value, but log_completion runs too