Type inference for higher ranks is in fact undecidable, but not type checking. Hence, exactly like in Haskell with RankNTypes, you need to annotate your higher

Type inference for higher ranks is in fact undecidable, but not type checking. Hence, exactly like in Haskell with RankNTypes, you need to annotate your higher rank functions. Actually, the Frege compiler employs an algorithm described in Simon Peyton-Jones paper "Practical Type Inference for Higher Ranked Types". Ordinary HM types are inferred, and higher ranked types checked.