Learn functional programming with me - more idiomatic Clojure

Published on 2013-6-27

I went through my code in the last entry and did a bit of re-factoring in an effort to get my state under control, I've also had some feedback that some of my Clojure could be a bit more idiomatic so let's sort that out.

assoc-in vs update-in

Here is some code I wrote

(defn enemiesLogic [state]
  (let [direction (:direction state)
        enemies (:enemies state)
        func (if(= direction 1) inc dec)
       ]
    (assoc state :enemies
      (for [enemy enemies]
        (assoc enemy :x (func (:x enemy)))
      )
    )
  )
)

What I'm essentially doing is

Well, why bother with this? We can do

(defn enemiesLogic [state]
  (let [direction (:direction state)
        enemies (:enemies state)
        func (if(= direction 1) inc dec)
       ]
    (assoc state :enemies
      (for [enemy enemies]
        (update-in enemy [:x] func)
      )
    )
  )
)

update-in takes in a func to enact on the value, rather than the new value and saves us precious reading time. Thinking about it, we can actually save some effort here too.

This makes a big difference in cases where I've been lazy and done this

(assoc bullet :y (dec (:y bullet)))

Looks better like

(update-in bullet [:y] dec)

Passing around functions seems pretty functional.

State doesn't define an object

Why do I have this?

(defn initEnemy [x y w h]
 {
  :x (* x 30)
  :y (* y 30)
  :w w
  :h h
 }
)

(defn initPlayer [x y w h]
 {
  :x x
  :y y
  :w w
  :h h
 }
)

(defn initBullet [x y w h]
 {
  :x x
  :y y
  :w w
  :h h
 }
)

Seems to me that everything in my game is a rect, and what changes between these everything is the behaviour that is performed over that state.

So, death to these things and in with

(defn initRect [x y w h]
 {
  :x x
  :y y
  :w w
  :h h
 }
)

Actually, I'm pulling this structure apart in a number of places, such as my drawing functions

(let [{:keys [x y w h]} bullet]
  (drawSquare ctx x y w h "#000")
)

A solution to this would be to make drawSquare take in this map and de-construct it there. I've established that this is a primitive I want in my application and I'll use it as such.

I could further solidify this by turning it into a Record but I'm not yet feeling any pain from not having it as one so I won't bother..

(defn drawSquare [[ctx width height] rect c]
  (set! (. ctx -fillStyle) c)
  (let [{:keys [x y w h]} rect]
    (.fillRect ctx x y w h) 
  )
)

(defn enemiesRender [ctx state]
  (let [enemies (:enemies state)]
    (doseq [enemy enemies] 
      (drawSquare ctx enemy "#FF0")
    )
  )
)

(defn bulletsRender [ctx state]
  (doseq [bullet (:active (:bullets state))] 
    (drawSquare ctx bullet "#000")
  )
)

(defn playerRender [ctx state]
  (let [player (:player state)]
    (drawSquare ctx player "#F00")
  )
)

In other thoughts, these functions are all the same apart from the colour of the rect, it seems a bit daft to me, how about

(defn renderRects [ctx rects colour]
  (doseq [rect rects] 
    (drawSquare ctx enemy colour)
  )
)

(defn enemiesRender [ctx state]
  (renderRects ctx (:enemies state) "#FF0")
)

(defn bulletsRender [ctx state]
  (renderRects ctx (get-in state [:bullets :active]) "#000")
)

(defn playerRender [ctx state]
  (drawSquare ctx (:player state) "#F00")
)

Keeping the number of data structures to a minimum and re-using tiny little functions seems to be a thing in these parts.

Naming clojure constructs

I've been a naughy boy and stuck to my JavaScript ways of doing things with the camelCase all up in the show.

Turns out that Clojure fiends prefer hyphenation-on-their-names and I can kinda get behind that.

I'll also take the opportunity to formally start using 'rect' everywhere instead a combination of 'square' and 'rect' etc, and do a general tidy up of names.

I'll not show them all, but in essence

(defn bulletsRender [ctx state]
  (renderRects ctx (get-in state [:bullets :active]) "#000")
)

Will become

(defn render-bullets [ctx state]
  (render-rects ctx (get-in state [:bullets :active]) "#000")
)

And so on (so don't get confused in latter entries where the names of things have changed!)

The only downside to this arrangement is ctrl-p is now broken in vim for method names, this is a chance for the Emacs people to point and laugh at me.

Parenthesis white space

I'm getting little value from new-lining my parentheses.

(defn update-state [state]
  (update-bullets
    (update-player
      (update-enemies
        (update-direction state)
      )
    )
  )
)

When re-factoring or moving code around, I'm using the vim shortcuts

And so on, and actually it would be easier if I could just use left and right to move to the brace I want to grab the contents of.

So, I'm going to sort that out too and go with

(defn update-state [state]
  (update-bullets
    (update-player
      (update-enemies
        (update-direction state)))))

Gasp. Radical. This will reduce my vertical space usage and allow me to see more context on my screen (I'm a k+r person in C#/JS etc so I'm totally up in this way of doing things)

The fully tidied up source code can be found at this snapshot on Github if you want to catch up with where I am so far.

With this, I can go and look at collision detection!

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