Learn functional programming with me - Adding lots more state

Published on 2013-6-4

I've got a single moving square and a small pile of Clojure written so far, but now what I want to do is have a collection of moving squares because no game of Space Invaders is complete without aliens, and last I checked there was more than one of them.

Well, if I want lots of squares I'm going to have to look at some means of representing collections in Clojure and my searches take me to Sequences

To create a sequence, I can do something like this

(defn createSeq []
  (for [x [0 1 2 3]]
    x
  )
)

Which returns me a sequence containing 0 1 2 3, an important point here is (at least I think), is that this sequence is lazy, it has not been executed yet.

Now, my collection of invaders will actually be 2D, so what I can actually do is

(defn createSeq []
  (for [x [0 1 2 3] y [0 1 2 3]]
    [x y]
  )
)

Which returns me a sequence containing lots of [x y] pairs, so [0 0] [0 1] [0 2] etc.

Of course, there are functions for me here, so I can make a range

(defn createSeq []
  (for [x (range 0 100 10) 
        y (range 0 100 10)]
    [x y]
  )
)

Pretty nifty if I do say so myself, so what I can start with if I want to draw a grid of yellow squares is something like

(defn initState []
 (for [x (range 0 100 10)
       y (range 0 100 10)]
   [x y]
 )
)

And I can call my initial tick method, passing in this state

(defn ^:export init []
  (tick (initState)) 
)

And then loop through this

(defn tick [enemies]
  (let [ctx (context)] 
    (clearScreen ctx) 
    (doseq [[x y] enemies] (drawSquare ctx x y 5 5))
    (js/setTimeout (fn []
      (tick enemies) 
    ) 33  )
  )
)

Note that I'm able to expand the content of this sequence as part of the doseq call

  (doseq [[x y] enemies] (drawSquare ctx x y 5 5))

Gotta like that at least a little bit!

About this doseq, this doseq is explicitly created to allow for side effects, I could have done

  (for [[x y] enemies] (drawSquare ctx x y 5 5))

But nothing would happen because this is lazy too - had my head scratching for a while on that one...

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