Yes, ML is adding new constructors to exn type at runtime, but this is different from GHC implementation of dynamic typing which uses typeclasses, because GHC does not add typeclasses at runtime. Take a look at ezyang(who is a GHC developer)'s comments in the post.
I repeat that this is extremely obscure and probably only interesting to people like ezyang. He is engaged in the discussion, so I guess the post was not pointless.
Comments
Yes, ML is adding new constructors to exn type at runtime, but this is different from GHC implementation of dynamic typing which uses typeclasses, because GHC does not add typeclasses at runtime. Take a look at ezyang(who is a GHC developer)'s comments in the post.
I repeat that this is extremely obscure and probably only interesting to people like ezyang. He is engaged in the discussion, so I guess the post was not pointless.