From a7decf5f049ffa246445da785d4c16c320cf1b79 Mon Sep 17 00:00:00 2001
From: Takumi Shotoku <sinsoku.listy@gmail.com>
Date: Sat, 5 Sep 2026 17:48:09 +0900
Subject: [PATCH] Support a lambda pattern

`in ->(x) { ... }` matches by `===`, but AST.create_pattern_node had no
branch for :lambda_node and raised "unknown pattern node type:
lambda_node". Route it through the same install_pattern0 path as the
other value patterns; CaseMatchNode#install0 discards the pattern's
return value, so returning a proc instead of the subject is harmless.
The subject is not passed to the lambda, so its parameter stays untyped
and the body is not checked against the matched value.
---
 lib/typeprof/core/ast.rb        |  1 +
 scenario/patterns/lambda_pat.rb | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)
 create mode 100644 scenario/patterns/lambda_pat.rb

diff --git a/lib/typeprof/core/ast.rb b/lib/typeprof/core/ast.rb
index 8420c3881..f13e34b6b 100644
--- a/lib/typeprof/core/ast.rb
+++ b/lib/typeprof/core/ast.rb
@@ -396,6 +396,7 @@ def self.create_pattern_node(raw_node, lenv)
 
       when :array_node then ArrayNode.new(raw_node, lenv) # for %w[foo bar]
       when :range_node then RangeNode.new(raw_node, lenv) # TODO: support range pattern correctly
+      when :lambda_node then LambdaNode.new(raw_node, lenv) # TODO: match by `===` instead of ignoring the subject
 
       else
         raise "unknown pattern node type: #{ raw_node.type }"
diff --git a/scenario/patterns/lambda_pat.rb b/scenario/patterns/lambda_pat.rb
new file mode 100644
index 000000000..dba64ca74
--- /dev/null
+++ b/scenario/patterns/lambda_pat.rb
@@ -0,0 +1,16 @@
+## update: test.rb
+def check(x)
+  case x
+  in ->(i) { i == 0 }
+    :zero
+  in Integer
+    :int
+  end
+end
+
+check(0)
+
+## assert
+class Object
+  def check: (Integer) -> (:int | :zero)
+end