| Yeah, here's a program which causes GHC to hang on compilation, but | causes no problem for hugs. Does this qualify as higher-order unification? | | newtype X a = X (X a -> a) | | selfapp :: X a -> a | selfapp self@(X f) = f self It's a documented bug in GHC. http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs.html Nothing to do with h-o unification Simon