Make list.at return an error given sub-zero index

3 files changed, 12 insertions, 7 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d921c6f..83b76ed 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,8 @@
 - The `io.debug` function now prints to stderr instead of stdout when using
   the Erlang target or running in Node.js (but still uses `console.log`
   when running as JavaScript in a browser)
+- The `list.at` function now returns `Error(Nil)` if given index is smaller than
+  zero, instead of returning the first element.
 
 ## v0.25.0 - 2022-11-19
 
diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index 3f5f566..0fad104 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -1036,9 +1036,8 @@ pub fn intersperse(list: List(a), with elem: a) -> List(a) {
 /// Returns the element in the Nth position in the list, with 0 being the first
 /// position.
 ///
-/// `Error(Nil)` is returned if the list is not long enough for the given index.
-///
-/// For any `index` less than 0 this function behaves as if it was set to 0.
+/// `Error(Nil)` is returned if the list is not long enough for the given index
+/// or if the index is less than 0.
 ///
 /// ## Examples
 ///
@@ -1053,9 +1052,13 @@ pub fn intersperse(list: List(a), with elem: a) -> List(a) {
 /// ```
 ///
 pub fn at(in list: List(a), get index: Int) -> Result(a, Nil) {
-  list
-  |> drop(index)
-  |> first
+  case index >= 0 {
+    True ->
+      list
+      |> drop(index)
+      |> first
+    False -> Error(Nil)
+  }
 }
 
 /// Removes any duplicate elements from a given list.
diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam
index d09dfc0..ce865e4 100644
--- a/test/gleam/list_test.gleam
+++ b/test/gleam/list_test.gleam
@@ -537,7 +537,7 @@ pub fn at_test() {
   |> should.equal(Error(Nil))
 
   list.at([1, 2, 3, 4, 5, 6], -1)
-  |> should.equal(Ok(1))
+  |> should.equal(Error(Nil))
 }
 
 pub fn unique_test() {