add optional_index for use by optionally_at

Signed-off-by: James Hillyerd <[email protected]>

Author: jhillyerd

Diff for: src/gleam/dynamic/decode.gleam

@@ -423,6 +423,46 @@ fn index(
   }
 }
 
+// Indexes into a path similar to `index`, but treats Nil values as a miss
+// instead of an error.
+fn optional_index(
+  path: List(a),
+  position: List(a),
+  inner: fn(Dynamic) -> #(b, List(DecodeError)),
+  data: Dynamic,
+  handle_miss: fn(Dynamic, List(a)) -> #(b, List(DecodeError)),
+) -> #(b, List(DecodeError)) {
+  case path {
+    [] -> {
+      inner(data)
+      |> push_path(list.reverse(position))
+    }
+
+    [key, ..path] -> {
+      case bare_index(data, key) {
+        Ok(Some(data)) -> {
+          optional_index(path, [key, ..position], inner, data, handle_miss)
+        }
+        Ok(None) -> {
+          handle_miss(data, [key, ..position])
+        }
+        Error(kind) -> {
+          case is_null(data) {
+            True -> {
+              handle_miss(data, [key, ..position])
+            }
+            False -> {
+              let #(default, _) = inner(data)
+              #(default, [DecodeError(kind, dynamic.classify(data), [])])
+              |> push_path(list.reverse(position))
+            }
+          }
+        }
+      }
+    }
+  }
+}
+
 @external(erlang, "gleam_stdlib_decode_ffi", "index")
 @external(javascript, "../../gleam_stdlib_decode_ffi.mjs", "index")
 fn bare_index(data: Dynamic, key: anything) -> Result(Option(Dynamic), String)
@@ -592,7 +632,7 @@ pub fn optionally_at(
   inner: Decoder(a),
 ) -> Decoder(a) {
   Decoder(function: fn(data) {
-    index(path, [], inner.function, data, fn(_, _) { #(default, []) })
+    optional_index(path, [], inner.function, data, fn(_, _) { #(default, []) })
   })
 }
 

Diff for: test/gleam/dynamic/decode_test.gleam

@@ -923,6 +923,24 @@ pub fn optionally_at_nil_in_path_error_test() {
   |> should.equal(100)
 }
 
+pub fn optionally_at_nil_target_error_test() {
+  dynamic.from(
+    dict.from_list([
+      #(
+        "first",
+        dict.from_list([#("second", dict.from_list([#("third", Nil)]))]),
+      ),
+    ]),
+  )
+  |> decode.run(decode.optionally_at(
+    ["first", "second", "third"],
+    100,
+    decode.int,
+  ))
+  |> should.be_error
+  |> should.equal([DecodeError("Int", "Nil", ["first", "second", "third"])])
+}
+
 @external(erlang, "maps", "from_list")
 @external(javascript, "../../gleam_stdlib_test_ffi.mjs", "object")
 fn make_object(items: List(#(String, t))) -> Dynamic