add is_flow to specs

Author: leafo

spec/flow_spec.moon

@@ -1,5 +1,5 @@
 
-import Flow from require "lapis.flow"
+import Flow, is_flow from require "lapis.flow"
 
 describe "lapis.flow", ->
   local base_object
@@ -185,3 +185,40 @@ describe "lapis.flow", ->
 
       assert.same {}, obj
       assert.same {[multiple]: {1, 2}}, f[MEMO_KEY]
+
+  describe "is_flow", ->
+    it "returns false for nil", ->
+      assert.false is_flow nil
+
+    it "returns false for false", ->
+      assert.false is_flow false
+
+    it "returns true for Flow class", ->
+      assert.true is_flow Flow
+
+    it "returns true for class extending Flow", ->
+      class MyFlow extends Flow
+      assert.true is_flow MyFlow
+
+    it "returns true for deeply nested Flow subclass", ->
+      class FirstFlow extends Flow
+      class SecondFlow extends FirstFlow
+      class ThirdFlow extends SecondFlow
+
+      assert.true is_flow SecondFlow
+      assert.true is_flow ThirdFlow
+
+    it "returns false for regular table", ->
+      assert.false is_flow {}
+
+    it "returns false for regular class", ->
+      class BaseClass
+      class ChildClass extends BaseClass
+
+      assert.false is_flow BaseClass
+      assert.false is_flow ChildClass
+
+    it "returns false when passing a Flow instance directly", ->
+      class MyFlow extends Flow
+      f = MyFlow {}
+      assert.false is_flow f