builtin(list): add List.chunksOf

crates/compiler/builtins/roc/List.roc

@@ -67,6 +67,7 @@ interface List
         releaseExcessCapacity,
         walkBackwardsUntil,
         countIf,
+        chunksOf,
     ]
     imports [
         Bool.{ Bool, Eq },
@@ -1197,6 +1198,26 @@ splitLast = \list, delimiter ->
 
         Err NotFound -> Err NotFound
 
+## Splits the list into many chunks, each of which is length of the given chunk
+## size. The last chunk will be shorter if the list does not evenly divide by the
+## chunk size. If the provided list is empty or if the chunk size is 0 then the
+## result is an empty list.
+chunksOf : List a, Nat -> List (List a)
+chunksOf = \list, chunkSize ->
+    if chunkSize == 0 || List.isEmpty list then
+        []
+    else
+        chunkCapacity = Num.divCeil (List.len list) chunkSize
+        chunksOfHelp list chunkSize (List.withCapacity chunkCapacity)
+
+chunksOfHelp : List a, Nat, List (List a) -> List (List a)
+chunksOfHelp = \listRest, chunkSize, chunks ->
+    if List.isEmpty listRest then
+        chunks
+    else
+        { before, others } = List.split listRest chunkSize
+        chunksOfHelp others chunkSize (List.append chunks before)
+
 ## Like [List.map], except the transformation function returns a [Result].
 ## If that function ever returns `Err`, [mapTry] immediately returns that `Err`.
 ## If it returns `Ok` for every element, [mapTry] returns `Ok` with the transformed list.

crates/compiler/module/src/symbol.rs

@@ -1432,6 +1432,7 @@ define_builtins! {
         81 LIST_RELEASE_EXCESS_CAPACITY: "releaseExcessCapacity"
         82 LIST_UPDATE: "update"
         83 LIST_WALK_WITH_INDEX: "walkWithIndex"
+        84 LIST_CHUNKS_OF: "chunksOf"
     }
     7 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" exposed_type=true // the Result.Result type alias

crates/compiler/test_gen/src/gen_list.rs

@@ -487,6 +487,38 @@ fn list_split_last() {
     );
 }
 
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_chunks_of() {
+    assert_evals_to!(
+        "List.chunksOf [1, 2, 3, 4, 5, 6, 7, 8] 3",
+        RocList::from_slice(&[
+            RocList::from_slice(&[1, 2, 3]),
+            RocList::from_slice(&[4, 5, 6]),
+            RocList::from_slice(&[7, 8]),
+        ]),
+        RocList<RocList<i64>>
+    );
+
+    assert_evals_to!(
+        "List.chunksOf [1, 2, 3, 4] 5",
+        RocList::from_slice(&[RocList::from_slice(&[1, 2, 3, 4]),]),
+        RocList<RocList<i64>>
+    );
+
+    assert_evals_to!(
+        "List.chunksOf [1, 2, 3] 0",
+        RocList::from_slice(&[]),
+        RocList<RocList<i64>>
+    );
+
+    assert_evals_to!(
+        "List.chunksOf [] 5",
+        RocList::from_slice(&[]),
+        RocList<RocList<i64>>
+    );
+}
+
 #[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-wasm", feature = "gen-dev"))]
 fn list_drop() {