1 files changed, 26 insertions, 0 deletions
diff --git a/third_party/bazel/rules_haskell/haskell/private/list.bzl b/third_party/bazel/rules_haskell/haskell/private/list.bzl
new file mode 100644
index 0000000000..14ffd5f068
--- /dev/null
+++ b/third_party/bazel/rules_haskell/haskell/private/list.bzl
@@ -0,0 +1,26 @@
+"""Helper functions on lists."""
+
+load(":private/set.bzl", "set")
+
+def _dedup_on(f, list_):
+    """deduplicate `list_` by comparing the result of applying
+    f to each element (e.g. comparing sub fields)
+
+    def compare_x(el):
+      return el.x
+
+    dedup_on([struct(x=3), struct(x=4), struct(x=3)], compare_x)
+    => [struct(x=3), struct(x=4)]
+    """
+    seen = set.empty()
+    deduped = []
+    for el in list_:
+        by = f(el)
+        if not set.is_member(seen, by):
+            set.mutable_insert(seen, by)
+            deduped.append(el)
+    return deduped
+
+list = struct(
+    dedup_on = _dedup_on,
+)