bring back FunctionalSequence

See example usage:
https://github.com/RustCrypto/AEADs/blob/d1a22983d2af3c6a1ac3644850351f0ab0a77e51/eax/src/online.rs#L352

src/functional.rs

@@ -0,0 +1,81 @@
+//! Functional programming with [`Array`]s.
+
+use crate::{Array, ArraySize};
+
+/// Defines functional programming methods for [`Array`]s.
+pub trait FunctionalSequence<T, U> {
+    /// Maps an `Array` to another `Array`
+    ///
+    /// If the mapping function panics, any already initialized elements in the new array will
+    /// be dropped, AND any unused elements in the source array will also be dropped.
+    fn map<O, F>(self, f: F) -> Array<O, U>
+    where
+        U: ArraySize,
+        F: FnMut(T) -> O;
+
+    /// Combines two `Array` instances and iterates through both of them, initialization a new
+    /// `Array` with the result of the zipped mapping function.
+    ///
+    /// If the mapping function panics, any already initialized elements in the new array will
+    /// be dropped, AND any unused elements in the source arrays will also be dropped.
+    fn zip<Rhs, F, O>(self, rhs: Array<Rhs, U>, f: F) -> Array<O, U>
+    where
+        U: ArraySize,
+        F: FnMut(T, Rhs) -> O;
+
+    /// Folds (or reduces) a sequence of data into a single value.
+    ///
+    /// If the fold function panics, any unused elements will be dropped.
+    fn fold<O, F>(self, init: O, f: F) -> O
+    where
+        F: FnMut(O, T) -> O;
+}
+
+impl<T, U> FunctionalSequence<T, U> for Array<T, U>
+where
+    U: ArraySize,
+{
+    #[inline(always)]
+    fn map<O, F>(self, mut f: F) -> Array<O, U>
+    where
+        F: FnMut(T) -> O,
+    {
+        let mut out = Array::uninit();
+
+        for (i, s) in self.into_iter().enumerate() {
+            out[i].write(f(s));
+        }
+
+        unsafe { out.assume_init() }
+    }
+
+    #[inline(always)]
+    fn zip<Rhs, F, O>(self, rhs: Array<Rhs, U>, mut f: F) -> Array<O, U>
+    where
+        U: ArraySize,
+        F: FnMut(T, Rhs) -> O,
+    {
+        let mut out = Array::uninit();
+
+        let mut s = self.into_iter().enumerate();
+        let mut r = rhs.into_iter();
+
+        while let (Some((i, s)), Some(r)) = (s.next(), r.next()) {
+            out[i].write(f(s, r));
+        }
+
+        unsafe { out.assume_init() }
+    }
+
+    #[inline(always)]
+    fn fold<O, F>(self, mut init: O, mut f: F) -> O
+    where
+        F: FnMut(O, T) -> O,
+    {
+        for t in self {
+            init = f(init, t)
+        }
+
+        init
+    }
+}

src/lib.rs

@@ -88,6 +88,7 @@
 extern crate std;
 
 mod from_fn;
+pub mod functional;
 mod iter;
 mod sizes;
 mod traits;