class-method-types.md

@@ -22,6 +22,21 @@ However this restriction is not implemented by Hugs, following a suggestion of M
 [ Typing Haskell in Haskell](http://www.cse.ogi.edu/~mpj/thih/), and can be turned off in GHC with `-fglasgow-exts`.
 
 
+## interaction with existentials
+
+
+
+If this were allowed than existential types of the form
+
+
+```wiki
+data SomeFoo = exists a . Foo a => SomeFoo a
+```
+
+
+would have to carry around a dictionary for Num as well as Foo in dictionary passing implementation of type classes.
+
+
 ## Cons