extract `drive` function which can go forward and backward

rover.fnl

@@ -13,6 +13,13 @@
                               :actual actual#
                               })))))
 
+(fn drive [{: x : y : direction} distance]
+  (match direction
+    :n {:y (+ y distance)}
+    :s {:y (- y distance)}
+    :w {:x (- x distance)}
+    :e {:x (+ x distance)}))
+
 (fn turn-left [direction]
   (-> {
        :n :w
@@ -34,16 +41,8 @@
 (fn command [rover string]
   (merge rover
          (match string
-           :f (match rover.direction
-                :n {:y (+ rover.y 1)}
-                :s {:y (- rover.y 1)}
-                :w {:x (- rover.x 1)}
-                :e {:x (+ rover.x 1)})
-           :b (match rover.direction
-                :n {:y (- rover.y 1)}
-                :s {:y (+ rover.y 1)}
-                :w {:x (+ rover.x 1)}
-                :e {:x (- rover.x 1)})
+           :f (drive rover 1)
+           :b (drive rover -1)
            :r {:direction (turn-right rover.direction)}
            :l {:direction (turn-left rover.direction)}
            _ (assert false (. "unrecognised command " string)))))