but says nothing about higher-kinded types (HKTs). The two are different.
The rank of a type describes the depth at which universal quantifiers appear contravariantly. This is quite different from higher-kinded types, which allow type-level computation. For good monad support one uses HKTs. I wonder if Frege has HKTs and the description on the original page made a mistake?
Type inference for types of rank > 2 is undecidable, how does this go together with the claim that Frege has type inference?
I'm not a Haskell expert, but I think the
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.
