fix: hash is not invalidate when GHC internal change

Close tweag#202

- GHC internal fingerprint computation may change, leading to an
  invalidation of (some) of the hash computed. Instead of using the type
  fingerprint, we only use the `show $ typeRef x` (which is roughly a
  string representation of the type). It will be more stable and should
  discriminate enough.

cas/hashable/app/Main.hs

@@ -34,5 +34,5 @@ main = do
 
   -- This hash should not change, unless the hashing algorithm change or the order of the walk in the structure changes.
   let 
-      expected = "ContentHash \"7f953cc9aaf79bac4d02e70dca91da1ee630b4a7894aeca9ed61942cc3a20d8a\""
+      expected = "ContentHash \"da33d38419fc15489a5435694a5107f07fd62b4659ddd799720f62b9e423e3a0\""
   when (show rawHash /= expected) $ error $ "Raw hash is different than expected: " <> show rawHash

cas/hashable/src/Data/CAS/ContentHashable.hs

@@ -478,11 +478,15 @@ instance (Constructor c, GContentHashable m f) => GContentHashable m (C1 c f) wh
       nameCtx = hashUpdate ctx0 $ C8.pack (conName x)
 
 instance (Datatype d, GContentHashable m f) => GContentHashable m (D1 d f) where
-  gContentHashUpdate ctx0 x = packageCtx `seq` gContentHashUpdate packageCtx (unM1 x)
+  gContentHashUpdate ctx0 x = datatypeCtx `seq` gContentHashUpdate datatypeCtx (unM1 x)
     where
+      -- Only add the datatype context in the hash
+      -- Adding the module and package would lead to hash invalidation when
+      -- moving the type to another module
+      -- See https://github.com/tweag/funflow/issues/202
       datatypeCtx = hashUpdate ctx0 $ C8.pack (datatypeName x)
-      moduleCtx = hashUpdate datatypeCtx $ C8.pack (datatypeName x)
-      packageCtx = hashUpdate moduleCtx $ C8.pack (datatypeName x)
+      -- moduleCtx = hashUpdate datatypeCtx $ C8.pack (datatypeName x)
+      -- packageCtx = hashUpdate moduleCtx $ C8.pack (datatypeName x)
 
 instance GContentHashable m f => GContentHashable m (S1 s f) where
   gContentHashUpdate ctx x = gContentHashUpdate ctx (unM1 x)