2 files changed, 154 insertions, 0 deletions
diff --git a/src/gleam/iterator.gleam b/src/gleam/iterator.gleam
new file mode 100644
index 0000000..53b80d3
--- /dev/null
+++ b/src/gleam/iterator.gleam
@@ -0,0 +1,140 @@
+import gleam/list
+
+// Internal private representation of an Iterator
+
+enum Action(element) {
+  // Improper dancing in the middle of the street
+  // Improper dancing in the middle of the street
+  // Improper dancing in the middle of the street
+  // Somebody better notify the chief of police
+  Stop
+  Continue(element, fn() -> Action(element))
+  // Yes!
+}
+
+// Wrapper to hide the internal representation
+
+pub external type Iterator(element);
+
+// TODO: remove once we have opaque type wrappers
+external fn opaque(fn() -> Action(element)) -> Iterator(element)
+  = "gleam@iterator" "identity"
+
+external fn unopaque(Iterator(element)) -> fn() -> Action(element)
+  = "gleam@iterator" "identity"
+
+pub fn identity(x) {
+  x
+}
+
+// Public API for iteration
+
+pub enum Step(element, acc) {
+  Next(element, acc)
+  Done
+}
+
+// Creating Iterators
+
+fn do_unfold(initial, f) {
+  fn() {
+    case f(initial) {
+      Next(x, acc) -> Continue(x, do_unfold(acc, f))
+      Done -> Stop
+    }
+  }
+}
+
+// TODO: test
+// TODO: document
+pub fn unfold(from initial: acc, with f: fn(acc) -> Step(element, acc)) -> Iterator(element) {
+  opaque(do_unfold(initial, f))
+}
+
+// TODO: test
+// TODO: document
+pub fn repeatedly(f: fn() -> element) -> Iterator(element) {
+  unfold(Nil, fn(acc) { Next(f(), acc) })
+}
+
+// TODO: test
+// TODO: document
+pub fn repeat(x: element) -> Iterator(element) {
+  repeatedly(fn() { x })
+}
+
+// TODO: test
+// TODO: document
+pub fn from_list(list: List(element)) -> Iterator(element) {
+  unfold(list, fn(acc) {
+    case acc {
+      [] -> Done
+      [head | tail] -> Next(head, tail)
+    }
+  })
+}
+
+// Consuming Iterators
+
+fn do_fold(iterator, initial, f) {
+  case iterator() {
+    Continue(element, iterator) -> do_fold(iterator, f(element, initial), f)
+    Stop -> initial
+  }
+}
+
+// TODO: test
+// TODO: document
+pub fn fold(over iterator: Iterator(e), from initial: acc, with f: fn(e, acc) -> acc) -> acc {
+  do_fold(unopaque(iterator), initial, f)
+}
+
+// TODO: test
+// TODO: document
+// For side effects
+pub fn run(iterator) -> Nil {
+  fold(iterator, Nil, fn(_, acc) { acc })
+}
+
+// TODO: test
+// TODO: document
+pub fn to_list(iterator: Iterator(element)) -> List(element) {
+  iterator
+  |> fold(_, [], fn(e, acc) { [e | acc] })
+  |> list.reverse
+}
+
+// Transforming Iterators
+
+fn do_map(iterator, f) {
+  fn() {
+    case iterator() {
+      Continue(e, iterator) -> Continue(f(e), do_map(iterator, f))
+      Stop -> Stop
+    }
+  }
+}
+
+// TODO: test
+// TODO: document
+pub fn map(over iterator: Iterator(a), with f: fn(a) -> b) -> Iterator(b) {
+  opaque(do_map(unopaque(iterator), f))
+}
+
+fn do_filter(iterator, predicate) {
+  fn() {
+    case iterator() {
+      Continue(e, iterator) -> case predicate(e) {
+        True -> Continue(e, iterator)
+        False -> do_filter(iterator, predicate)()
+      }
+      Stop -> Stop
+    }
+  }
+}
+
+// TODO: test
+// TODO: document
+pub fn filter(iterator: Iterator(a), for predicate: fn(a) -> Bool) -> Iterator(a) {
+  opaque(do_filter(unopaque(iterator), predicate))
+}
diff --git a/src/gleam_iterator.app.src b/src/gleam_iterator.app.src
new file mode 100644
index 0000000..98defca
--- /dev/null
+++ b/src/gleam_iterator.app.src
@@ -0,0 +1,14 @@
+{application, gleam_iterator,
+ [{description, "A Gleam program"},
+  {vsn, "1.0.0"},
+  {registered, []},
+  {applications,
+   [kernel,
+    stdlib
+   ]},
+  {env,[]},
+  {modules, []},
+
+  {licenses, ["Apache 2.0"]},
+  {links, []}
+]}.