From 2c97c840fc454d8272083aaac8ac852dbd5b630a Mon Sep 17 00:00:00 2001
From: Folkert <folkert@folkertdev.nl>
Date: Fri, 31 Dec 2021 14:26:23 +0100
Subject: [PATCH] walk the chain till we find a tag union to make recursive

---
 compiler/test_gen/src/gen_primitives.rs | 46 +++++++++++++++++++++++++
 compiler/unify/src/unify.rs             | 13 ++++++-
 2 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/compiler/test_gen/src/gen_primitives.rs b/compiler/test_gen/src/gen_primitives.rs
index d339961dc5..0381b0dd70 100644
--- a/compiler/test_gen/src/gen_primitives.rs
+++ b/compiler/test_gen/src/gen_primitives.rs
@@ -3138,3 +3138,49 @@ fn alias_defined_out_of_order() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm"))]
+fn recursively_build_effect() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [ main ] to "./platform"
+
+            greeting =
+                hi = "Hello"
+                name = "World"
+
+                "\(hi), \(name)!"
+
+            main =
+                when nestHelp 4 is
+                    _ -> greeting
+
+            nestHelp : I64 -> XEffect {}
+            nestHelp = \m ->
+                when m is
+                    0 ->
+                        always {}
+
+                    _ ->
+                        always {} |> after \_ -> nestHelp (m - 1)
+
+
+            XEffect a : [ @XEffect ({} -> a) ]
+
+            always : a -> XEffect a
+            always = \x -> @XEffect (\{} -> x)
+
+            after : XEffect a, (a -> XEffect b) -> XEffect b
+            after = \(@XEffect e), toB ->
+                @XEffect \{} ->
+                    when toB (e {}) is
+                        @XEffect e2 ->
+                            e2 {}
+            "#
+        ),
+        RocStr::from_slice(b"Hello, World!"),
+        RocStr
+    );
+}
diff --git a/compiler/unify/src/unify.rs b/compiler/unify/src/unify.rs
index e9ee4d42db..0a426b4a56 100644
--- a/compiler/unify/src/unify.rs
+++ b/compiler/unify/src/unify.rs
@@ -833,10 +833,21 @@ enum OtherTags2 {
 }
 
 fn maybe_mark_tag_union_recursive(subs: &mut Subs, tag_union_var: Variable) {
-    while let Err((recursive, _chain)) = subs.occurs(tag_union_var) {
+    'outer: while let Err((recursive, chain)) = subs.occurs(tag_union_var) {
         let description = subs.get(recursive);
         if let Content::Structure(FlatType::TagUnion(tags, ext_var)) = description.content {
             subs.mark_tag_union_recursive(recursive, tags, ext_var);
+        } else {
+            // walk the chain till we find a tag union
+            for v in &chain[..chain.len() - 1] {
+                let description = subs.get(*v);
+                if let Content::Structure(FlatType::TagUnion(tags, ext_var)) = description.content {
+                    subs.mark_tag_union_recursive(*v, tags, ext_var);
+                    continue 'outer;
+                }
+            }
+
+            panic!("recursive loop does not contain a tag union")
         }
     }
 }