Answer will be revealed in a day, put guesses below
Haskell
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.
Haskell! One of the most beautiful looking languages IMO.
Looks like Haskell to me
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The answer is haskell! There are some other languages that could fit the syntax but haskell is generally the most known out of all of them. Sorry for the late post with this answer, been having some issues with my laptop recently. All the answers got it right