Add trait to flatten a heterogenous list of heterogenous lists

src/lib.rs

@@ -143,14 +143,18 @@ where
     Tail: HList,
 {
     const LEN: usize = Tail::LEN + 1;
+
+    fn is_empty(&self) -> bool {
+        false
+    }
 }
 
 mod sealed {
     pub trait Sealed {}
 
     impl Sealed for crate::Nil {}
 
-    impl<Head, Tail> Sealed for crate::Cons<Head, Tail> {}
+    impl<Head, Tail> Sealed for crate::Cons<Head, Tail> where Tail: Sealed {}
 }
 
 /// Macro creating heterogenous list values from list of expressions.

src/ops/flatten.rs

@@ -0,0 +1,70 @@
+use crate::{Cons, HList, Nil};
+
+use super::Extend;
+
+/// Flattens one level of nesting in a heterogenous list of heterogenous lists.
+///
+/// This is useful when you have a heterogenous list of heterogenous lists
+/// and you want to remove one level of indirection.
+pub trait Flatten: HList {
+    /// Flattened heterogenous list.
+    type Output: HList;
+
+    /// Flattens a heterogenous list of heterogenous lists, removing one level of indirection.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// use hlist2::{hlist, ops::Flatten};
+    ///
+    /// let data = hlist![hlist![1, 2.0, true, "hello world"], hlist![5, 6.0]];
+    /// let flattened = data.flatten();
+    /// assert_eq!(flattened, hlist![1, 2.0, true, "hello world", 5, 6.0]);
+    /// ```
+    ///
+    /// Flattening only removes one level of nesting at a time:
+    ///
+    /// ```
+    /// use hlist2::{hlist, ops::Flatten};
+    ///
+    /// let d3 = hlist![
+    ///     hlist![hlist![1, 2], hlist![3, 4]],
+    ///     hlist![hlist![5, 6], hlist![7, 8]],
+    /// ];
+    ///
+    /// let d2 = d3.flatten();
+    /// assert_eq!(d2, hlist![hlist![1, 2], hlist![3, 4], hlist![5, 6], hlist![7, 8]]);
+    ///
+    /// let d1 = d3.flatten().flatten();
+    /// assert_eq!(d1, hlist![1, 2, 3, 4, 5, 6, 7, 8]);
+    /// ```
+    ///
+    /// Here we see that `flatten()` does not perform a “deep” flatten. Instead, only one level of nesting is removed.
+    /// That is, if you `flatten()` a three-dimensional array, the result will be two-dimensional and not one-dimensional.
+    /// To get a one-dimensional structure, you have to `flatten()` again.
+    fn flatten(self) -> Self::Output;
+}
+
+impl Flatten for Nil {
+    type Output = Self;
+
+    fn flatten(self) -> Self::Output {
+        self
+    }
+}
+
+impl<Head, Tail> Flatten for Cons<Head, Tail>
+where
+    Head: Extend,
+    Tail: Flatten,
+{
+    type Output = Head::Output<Tail::Output>;
+
+    fn flatten(self) -> Self::Output {
+        let Cons(head, tail) = self;
+        let tail = tail.flatten();
+        head.extend(tail)
+    }
+}

src/ops/mod.rs

@@ -3,6 +3,7 @@
 pub use self::{
     append::Append,
     extend::Extend,
+    flatten::Flatten,
     fold::{Fold, FoldFn, Folder, RFold},
     get::Get,
     index::{Here, There},
@@ -19,6 +20,7 @@ pub use self::{
 
 mod append;
 mod extend;
+mod flatten;
 mod fold;
 mod get;
 mod index;