[iopq]

It's sorely lacking Haskell features, I have sometimes written functions to simplify my code to find out the function signature is several lines of code and MUST use references

    fn apply<A, B, C, G>(mut f: impl FnMut(B) -> G, a: A) -> impl FnMut(&B) -> C
    // must still be `for<'r> impl FnMut(&'r B) -> C`, because that’s what filter requires
    where
         G: FnMut(A) -> C,
         B: Copy, // for dereferencing
         A: Clone,
    {
         move |b| f(*b)(a.clone()) // this must do any bridging necessary to satisfy the requirements
    }

all this did so I could write in my code

    ...
         .filter(apply(second, i))
    ...