Actually it could be PureScript, Curry, Clean, or Miranda. Even more swap the double/single colon. It’s a common syntax :)

EDIT: Or Plutus (blockchain smart contract dialect), or Frege (Haskell dialect for the JVM), or Eta (also JVM), or Discus (formerly Disciple, strict Haskell dialect), or this interpreted Haskell dialect which I forget the name of.