Hugs will produce funny looking results when using forall quantified fields in records. For instance newtype Foo = Foo { foo :: forall a. a -> a } Main> foo (Foo {foo = id}) 42 Foo_Foo id 42 Main> $$ + 1 INTERNAL ERROR: Bignum expected Main> foo (Foo {foo = id}) 42 + (1 :: Int) 1 Main> foo (Foo {foo = id}) True Program error: {instShow_v81_v1123 0 (Foo_Foo id Bool_True)} Everything works fine though, if you don't use the record update syntax: Main> foo (Foo id) True True / Ulf Norell