gistfile1.clj

(let [gravity 4
      max-velocity 50]
  (defrule gravity-pulls-things-down
    :on :frame-entered
    (r/do entities <- (r/get-in-scene :entities)
          faller << (r/get-entities-with #{:velocity :gravity :hitbox})
          (r/when (not (e/collides-with? entities
                                         faller
                                         :below))
            (r/update-entity faller
                             (update-in faller [:velocity :y]
                                        #(-> %
                                             (+ gravity)
                                             (min max-velocity)))))))

(defrule the-keyboard-jumps
  :on :frame-entered
  (r/do input <- (r/get-in-app :input)
        (r/when (i/was-pressed? input :jump)
          entities <- (r/get-in-scene :entities)
          jumper << (r/get-entities-with #{:velocity :keyboard-jumper})
          (r/when (e/collides-with? entities
                                    jumper
                                    :below)
            (r/update-entity jumper
                             (update-in jumper [:velocity :y]
                                        - (-> jumper :keyboard-jumper :speed)))))))