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    //! Functional programming with generic sequences
    //!
    //! Please see `tests/generics.rs` for examples of how to best use these in your generic functions.

    use super::ArrayLength;
    use core::iter::FromIterator;

    use crate::sequence::*;

    /// Defines the relationship between one generic sequence and another,
    /// for operations such as `map` and `zip`.
    pub unsafe trait MappedGenericSequence<T, U>: GenericSequence<T>
    where
        Self::Length: ArrayLength<U>,
    {
        /// Mapped sequence type
        type Mapped: GenericSequence<U, Length = Self::Length>;
    }

    unsafe impl<'a, T, U, S: MappedGenericSequence<T, U>> MappedGenericSequence<T, U> for &'a S
    where
        &'a S: GenericSequence<T>,
        S: GenericSequence<T, Length = <&'a S as GenericSequence<T>>::Length>,
        <S as GenericSequence<T>>::Length: ArrayLength<U>,
    {
        type Mapped = <S as MappedGenericSequence<T, U>>::Mapped;
    }

    unsafe impl<'a, T, U, S: MappedGenericSequence<T, U>> MappedGenericSequence<T, U> for &'a mut S
    where
        &'a mut S: GenericSequence<T>,
        S: GenericSequence<T, Length = <&'a mut S as GenericSequence<T>>::Length>,
        <S as GenericSequence<T>>::Length: ArrayLength<U>,
    {
        type Mapped = <S as MappedGenericSequence<T, U>>::Mapped;
    }

    /// Accessor type for a mapped generic sequence
    pub type MappedSequence<S, T, U> =
        <<S as MappedGenericSequence<T, U>>::Mapped as GenericSequence<U>>::Sequence;

    /// Defines functional programming methods for generic sequences
    pub unsafe trait FunctionalSequence<T>: GenericSequence<T> {
        /// Maps a `GenericSequence` to another `GenericSequence`.
        ///
        /// If the mapping function panics, any already initialized elements in the new sequence
        /// will be dropped, AND any unused elements in the source sequence will also be dropped.
        fn map<U, F>(self, f: F) -> MappedSequence<Self, T, U>
        where
            Self: MappedGenericSequence<T, U>,
            Self::Length: ArrayLength<U>,
            F: FnMut(Self::Item) -> U,
        {
            FromIterator::from_iter(self.into_iter().map(f))
        }

        /// Combines two `GenericSequence` instances and iterates through both of them,
        /// initializing a new `GenericSequence` with the result of the zipped mapping function.
        ///
        /// If the mapping function panics, any already initialized elements in the new sequence
        /// will be dropped, AND any unused elements in the source sequences will also be dropped.
        #[inline]
        fn zip<B, Rhs, U, F>(self, rhs: Rhs, f: F) -> MappedSequence<Self, T, U>
        where
            Self: MappedGenericSequence<T, U>,
            Rhs: MappedGenericSequence<B, U, Mapped = MappedSequence<Self, T, U>>,
            Self::Length: ArrayLength<B> + ArrayLength<U>,
            Rhs: GenericSequence<B, Length = Self::Length>,
            F: FnMut(Self::Item, Rhs::Item) -> U,
        {
            rhs.inverted_zip2(self, f)
        }

        /// Folds (or reduces) a sequence of data into a single value.
        ///
        /// If the fold function panics, any unused elements will be dropped.
        fn fold<U, F>(self, init: U, f: F) -> U
        where
            F: FnMut(U, Self::Item) -> U,
        {
            self.into_iter().fold(init, f)
        }
    }

    unsafe impl<'a, T, S: GenericSequence<T>> FunctionalSequence<T> for &'a S
    where
        &'a S: GenericSequence<T>,
    {
    }

    unsafe impl<'a, T, S: GenericSequence<T>> FunctionalSequence<T> for &'a mut S
    where
        &'a mut S: GenericSequence<T>,
    {
    }