data Piece = Pawn | Rook | Knight | Bishop | Queen | King

-- The type is 'Piece', and to construct an actual value of
-- that type, you use one of the six data constructors.

promotesTo :: Piece -> [Piece]
promotesTo Pawn = [ Rook, Knight, Bishop, Queen ]
promotesTo _ = []

-- then:
promotesTo Pawn -- evaluates to [Rook, Knight, Bishop, Queen]
promotesTo $ head $ promotesTo Pawn -- evaluates to []