Remove list.at

exercises/practice/matrix/.meta/example.gleam

@@ -6,7 +6,7 @@ import gleam/string
 pub fn row(index: Int, matrix: String) -> Result(List(Int), Nil) {
   matrix
   |> parse_rows()
-  |> result.then(fn(rows) { list.at(rows, index - 1) })
+  |> result.then(fn(rows) { index_into(rows, index - 1) })
 }
 
 pub fn column(index: Int, matrix: String) -> Result(List(Int), Nil) {
@@ -15,7 +15,7 @@ pub fn column(index: Int, matrix: String) -> Result(List(Int), Nil) {
   |> result.then(fn(rows) {
     rows
     |> list.transpose()
-    |> list.at(index - 1)
+    |> index_into(index - 1)
   })
 }
 
@@ -28,3 +28,11 @@ fn parse_rows(input: String) -> Result(List(List(Int)), Nil) {
     |> list.try_map(int.parse)
   })
 }
+
+fn index_into(list: List(a), index: Int) -> Result(a, Nil) {
+  case list {
+    [] -> Error(Nil)
+    [first, ..] if index == 0 -> Ok(first)
+    [_, ..rest] -> index_into(list, index - 1)
+  }
+}