"order of calls" inside another function

Question

Trying to teach myself some clojure, am using play-clj. I dont understand why this works:

(defn add-shape
  [x y entities]
  (write-state [x y])
  (conj entities (o-shape x y)))

While this doesnt:

(defn add-shape
  [x y entities]
  (conj entities (o-shape x y)
  (write-state [x y]))

Exception in thread "LWJGL Application" java.lang.IllegalArgumentException:No implementation of method: :draw-entity! of protocol: #'play-clj.entities/Entity found for class: java.lang.Long

These are the two related functions:

(defn x-shape
  [x y] 
  (shape :line 
         :line (- x 100) (- 600 100 y) (+ x 100) (- (+ 600 100) y)
         :line (- x 100) (- (+ 600 100) y) (+ x 100) (- 600 100 y)))

(defn write-state
  [arg]
  (dosync (alter state conj arg)))

The `o-shape`s should be `x-shape`s, I think. – Thumbnail Aug 11 '14 at 18:07 — Thumbnail, Aug 11 '14 at 18:07

YosemiteMark · Accepted Answer · 2014-08-11T17:47:22.227

2

Pretty sure you have a misplaced paren in the second. Try this:

(defn add-shape
  [x y entities]
  (conj entities (o-shape x y)) ;; This is effectively a no-op, results are not used.
  (write-state [x y]))  ;; returns the results of write-state, 
                        ;; ie the result of conj-ing [x y] onto the value in ref state

However, I now think the root of your problem is the two versions have different return values. Here's what the first returns:

(defn add-shape
  [x y entities]
  (write-state [x y])
  (conj entities (o-shape x y))) ;; returns the results of conj-ing the results of (o-shape x y)
                                 ;; onto the passed-in value of entities

TLDR: the functions return different values, your program probably only works with the results of your first.

edited Aug 11 '14 at 17:47

answered Aug 11 '14 at 16:59

YosemiteMark

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Do you still get the exception with my version? If so, it's the different return values that are causing the problem, elsewhere in your app. In any case, can you explain more about what you want the function to do, in terms of updating state and returning values? – YosemiteMark Aug 11 '14 at 17:27
I've expanded on my answer, I'm now guessing the different return values are the root of your problem – YosemiteMark Aug 11 '14 at 17:48
It does work yeah. It was my mistake. I wasnt counting all them parens right, so the conj was failing. And also mistyped the parens here while posting it. These functions are updating a ref and drawing a shape on the screen. And yeah the original issue is the two differend return values. – jaakkoj Aug 11 '14 at 17:53

"order of calls" inside another function

1 Answers1