1 files changed, 45 insertions, 0 deletions
diff --git a/stdlib/source/test/lux/control/writer.lux b/stdlib/source/test/lux/control/writer.lux
new file mode 100644
index 000000000..b5fb372d8
--- /dev/null
+++ b/stdlib/source/test/lux/control/writer.lux
@@ -0,0 +1,45 @@
+(.module:
+  [lux #*
+   ["." io]
+   [control
+    ["M" monad (#+ Monad do)]
+    pipe
+    ["&" writer]]
+   [data
+    ["." product]
+    ["." text ("text/." equivalence)]]]
+  lux/test)
+
+(context: "Writer."
+  (let [(^open "&/.") (&.monad text.monoid)
+        (^open "&/.") (&.apply text.monoid)]
+    ($_ seq
+        (test "Functor respects Writer."
+              (i/= +11 (product.right (&/map inc ["" +10]))))
+        
+        (test "Apply respects Writer."
+              (and (i/= +20 (product.right (&/wrap +20)))
+                   (i/= +30 (product.right (&/apply (&/wrap (i/+ +10)) (&/wrap +20))))))
+        
+        (test "Monad respects Writer."
+              (i/= +30 (product.right (do (&.monad text.monoid)
+                                        [f (wrap i/+)
+                                         a (wrap +10)
+                                         b (wrap +20)]
+                                        (wrap (f a b))))))
+        
+        (test "Can log any value."
+              (text/= "YOLO" (product.left (&.log "YOLO"))))
+        )))
+
+(context: "Monad transformer"
+  (let [lift (&.lift text.monoid io.monad)
+        (^open "io/.") io.monad]
+    (test "Can add writer functionality to any monad."
+          (|> (io.run (do (&.WriterT text.monoid io.monad)
+                        [a (lift (io/wrap +123))
+                         b (wrap +456)]
+                        (wrap (i/+ a b))))
+              (case> ["" +579] #1
+                     _         #0)))
+    ))