I'm a bit disappointed by the double colon; it's my least favorite part of Haskell syntax (I probably shouldn't care, but there you go). It also bugs me in PureScript. However, Agda, Idris, and Elm, all of which have syntax heavily inspired by Haskell, all do it the "right way", so there's no reason a Haskell-inspired language can't. Plus, this doesn't look that much like Haskell, certainly not to the point of being derivative.