Array fold

4 files changed, 25 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 04012f9..07d70c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,4 +3,4 @@
 ## Unreleased
 
 - The `array` module was created with the `Array(element)` type and `to_list`,
-  `from_list`, `map`, and `length` functions.
+  `from_list`, `map`, `fold`, and `length` functions.
diff --git a/src/ffi.js b/src/ffi.js
index 06303fc..91a2f73 100644
--- a/src/ffi.js
+++ b/src/ffi.js
@@ -9,3 +9,7 @@ export function map(thing, fn) {
 export function length(thing) {
   return thing.length;
 }
+
+export function reduce(thing, acc, fn) {
+  return thing.reduce(fn, acc);
+}
diff --git a/src/gleam/javascript/array.gleam b/src/gleam/javascript/array.gleam
index 6c4819e..20d12d1 100644
--- a/src/gleam/javascript/array.gleam
+++ b/src/gleam/javascript/array.gleam
@@ -11,3 +11,6 @@ pub external fn length(Array(element)) -> Int =
 
 pub external fn map(Array(a), fn(a) -> b) -> Array(b) =
   "../../ffi.js" "map"
+
+pub external fn fold(Array(e), a, fn(a, e) -> a) -> a =
+  "../../ffi.js" "reduce"
diff --git a/test/gleam/javascript/array_test.gleam b/test/gleam/javascript/array_test.gleam
index 2f149a5..5bf52db 100644
--- a/test/gleam/javascript/array_test.gleam
+++ b/test/gleam/javascript/array_test.gleam
@@ -31,3 +31,20 @@ pub fn map_test() {
     |> array.map(fn(a) { a + 1 })
     |> array.to_list
 }
+
+pub fn fold_test() {
+  assert [] =
+    []
+    |> array.from_list
+    |> array.fold([], fn(a, e) { [e, ..a] })
+
+  assert [4, 3, 2, 1] =
+    [1, 2, 3, 4]
+    |> array.from_list
+    |> array.fold([], fn(a, e) { [e, ..a] })
+
+  assert [1, 2, 3, 4] =
+    [4, 3, 2, 1]
+    |> array.from_list
+    |> array.fold([], fn(a, e) { [e, ..a] })
+}