Index 8

Author: lpil

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+- The decode API can now index into the first 8 elements of lists.
+
 ## v0.55.0 - 2025-02-21
 
 - The performance of `dict.is_empty` has been improved.

src/gleam/dynamic/decode.gleam

@@ -292,7 +292,7 @@ pub opaque type Decoder(t) {
 ///
 /// This function will index into dictionaries with any key type, and if the key is
 /// an int then it'll also index into Erlang tuples and JavaScript arrays, and
-/// the first two elements of Gleam lists.
+/// the first eight elements of Gleam lists.
 ///
 /// # Examples
 ///
@@ -358,7 +358,7 @@ pub fn run(data: Dynamic, decoder: Decoder(t)) -> Result(t, List(DecodeError)) {
 ///
 /// This function will index into dictionaries with any key type, and if the key is
 /// an int then it'll also index into Erlang tuples and JavaScript arrays, and
-/// the first two elements of Gleam lists.
+/// the first eight elements of Gleam lists.
 ///
 /// # Examples
 ///
@@ -486,7 +486,7 @@ pub fn decode_error(
 ///
 /// This function will index into dictionaries with any key type, and if the key is
 /// an int then it'll also index into Erlang tuples and JavaScript arrays, and
-/// the first two elements of Gleam lists.
+/// the first eight elements of Gleam lists.
 ///
 /// # Examples
 ///
@@ -526,7 +526,7 @@ pub fn field(
 ///
 /// This function will index into dictionaries with any key type, and if the key is
 /// an int then it'll also index into Erlang tuples and JavaScript arrays, and
-/// the first two elements of Gleam lists.
+/// the first eight elements of Gleam lists.
 ///
 /// # Examples
 ///
@@ -570,7 +570,7 @@ pub fn optional_field(
 ///
 /// This function will index into dictionaries with any key type, and if the key is
 /// an int then it'll also index into Erlang tuples and JavaScript arrays, and
-/// the first two elements of Gleam lists.
+/// the first eight elements of Gleam lists.
 ///
 /// # Examples
 ///

src/gleam_stdlib_decode_ffi.erl

@@ -6,6 +6,18 @@ index([X | _], 0) ->
     {ok, {some, X}};
 index([_, X | _], 1) ->
     {ok, {some, X}};
+index([_, _, X | _], 2) ->
+    {ok, {some, X}};
+index([_, _, _, X | _], 3) ->
+    {ok, {some, X}};
+index([_, _, _, _, X | _], 4) ->
+    {ok, {some, X}};
+index([_, _, _, _, _, X | _], 5) ->
+    {ok, {some, X}};
+index([_, _, _, _, _, _, X | _], 6) ->
+    {ok, {some, X}};
+index([_, _, _, _, _, _, _, X | _], 7) ->
+    {ok, {some, X}};
 index(Tuple, Index) when is_tuple(Tuple) andalso is_integer(Index) ->
     {ok, try
         {some, element(Index + 1, Tuple)}

src/gleam_stdlib_decode_ffi.mjs

@@ -16,7 +16,7 @@ export function index(data, key) {
   }
 
   // The first 3 elements of lists can be indexed
-  if ((key === 0 || key === 1 || key === 2) && data instanceof List) {
+  if (Number.isInteger(key) && key < 8 && data instanceof List) {
     let i = 0;
     for (const value of data) {
       if (i === key) return new Ok(new Some(value));

test/gleam/dynamic/decode_test.gleam

@@ -104,6 +104,19 @@ pub fn field_int_index_list_ok_test() {
   |> should.equal(#("one", "two"))
 }
 
+pub fn field_int_index_big_list_ok_test() {
+  let decoder = {
+    use x <- decode.field(6, decode.string)
+    use y <- decode.field(7, decode.string)
+    decode.success(#(x, y))
+  }
+
+  dynamic.from(["one", "two", "three", "four", "five", "six", "seven", "eight"])
+  |> decode.run(decoder)
+  |> should.be_ok
+  |> should.equal(#("seven", "eight"))
+}
+
 pub fn subfield_not_found_error_test() {
   let decoder = {
     use name <- decode.subfield(["name"], decode.string)