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08:40 Haffe: Hi. How does references work in RACKET?
08:41 Haffe: I want to be able to refer to the same object at two different places.
08:41 bremner: Haffe: roughly speaking, references are like single element vectors
08:43 bremner: oh, and I guess you might want to search for "box"
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08:50 bremner: you have have to de-reference with "unbox" and update with "set-box!"
08:50 bremner: rudybot: init racket
08:50 rudybot: bremner: your sandbox is ready
08:51 bremner: rudybot: eval (define foo (box 1))
08:51 rudybot: bremner: Done.
08:51 bremner: rudybot: eval (set-box! foo 2)
08:51 rudybot: bremner: Done.
08:51 bremner: rudybot: eval (unbox foo)
08:51 rudybot: bremner: ; Value: 2
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09:20 Haffe: Ok.
09:22 Haffe: My original problem is that I have an instance of an object that I want to acsses in different functions.
09:22 Haffe: In this case a keyboard-handler and a canvas and a collision detector.
09:23 Haffe: If I use box and unbox. Will this work properley?
09:27 Haffe: Is there a more functional style of doing this?
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10:47 bremner: Haffe: do you need mutation?
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11:26 Haffe: I have to think about it.
11:26 Haffe: What I am doing is building a small game using object oriented development, and the player pieces are represented as objects.
11:27 Haffe: The player piece objects are stored in a draw-queue of a canvas. The player pieces also have to respon to keyboard interaction.
11:27 Haffe: So, I guess will have to be able to mutate.
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11:30 bremner: once you do mutation, I'm not sure what you would want as "more functional"; essentially the box notation just splits out the two kinds of de-references one usually has for references, as a value and an as an lvalue (assignment target)
11:31 Haffe: I read up on it on google and got the advice that often the software should be redisgned if it runs into these kind of problems.
11:32 bremner: Haffe: why not use one of the object systems supported by racket?
11:32 Haffe: bremner: I am using the build in.
11:32 Haffe: I.e. with send and such.
11:32 bremner: I guess I don't have enough practical experience with OO racket to really understand the problem.
11:33 bremner: hopefully somebody else will come along later who can give you more practical advice.
11:34 Haffe: I am building a pong clone. I have modeled it so the paddles are of one class, the ball is of another class and the canvas that does the drawing is of a third.
11:34 Haffe: Now the canvas hold the paddles and the ball in a draw-queue.
11:34 Haffe: But I need to be able to acces the actuall paddles from my key-handler.
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11:36 bremner: can't you also store them in some (semi)-global variable?
11:37 Haffe: I guess I could use a mixin in the canvas class to define some data members.
11:39 Haffe: Then I have one more question.
11:39 Haffe: What is the difference between an init, an init-field and a field?
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12:09 _gcr: Haffe: so init defines keywords that you may use in the rest of your class definition
12:09 _gcr: Haffe: so you can effectively set up parameters when you create the class
12:10 _gcr: Haffe: a "field" is a keyword that's accessible outside the class, like a "member" in other languages
12:10 _gcr: Haffe: the "init-field" form combines these two ideas, creating a field that both acts as a field/member, and is settable like an init keyword when you create instances of that class
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12:11 Haffe: Ok, thanks.
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12:26 yoklov: so, generics tend to be named gen:generic-name ?
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12:28 asumu: yoklov: yes, because the define-generics form automatically prepends the `gen:`.
12:28 yoklov: oh
12:28 yoklov: i see.
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13:16 poisonarms: This should technically work, yea? http://pastebin.com/2KLiXyg3 - Although, syntacitcally, it keeps complaining about my `if` structure.
13:17 mithos28: your if has 4 parts
13:17 poisonarms: Can I not perform a block of code as my second half of the if?
13:17 poisonarms: Can I only perform 1 instruction?
13:17 mithos28: you need a begin if you want that
13:18 mithos28: (if cond true-branch (begin false-branch1 false-branch2))
13:18 poisonarms: Thank you.
13:18 mithos28: otherwise how does if know that you want 2 expressions in the false branch and not in the true branch
13:19 poisonarms: I don't know...magic? Ha. Never schemed before.
13:20 mithos28: Well we are very willing to help here. When you run into more issues just ask again.
13:21 poisonarms: Thanks.
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15:22 asumu: poisonarms: you could also just use `cond` if you want more than one expression.
15:22 asumu: `cond` is also nice because you can use `define` in each branch.
15:23 poisonarms: I see
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15:55 mithos28: Does TR not support higher kinded polymorphism? (define-type (App F y) (F y))
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17:22 korkyra: Are there any conservative, rigid right-winger out there (beside M. Felleisen)? (For an explanation of this "stupid" question, see the bottom of his home page: http://www.ccs.neu.edu/home/matthias/ ). I had previously thought that the computer scientist are somewhat smarter than the others, but apparently it is not so ... lol
17:27 neilv: the romney/ryan campaign button?
17:28 neilv: that realm of racket silhouette boy looks depressed
17:28 neilv: or possibly urinating and watching his aim
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18:24 asumu: Let's keep politics out of #racket.
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18:31 neilv: s/he already left
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20:05 poisonarms: Whelp...It KIND of works, hahaha: http://ideone.com/WfWba
20:07 mithos28: You want snoc, not cons there. But since its not a standard library function, you'll have to implement it yourself
20:07 mithos28: snoc is cons backwards, and adds to the end of the list
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20:22 poisonarms: How terrible is embedding recursion within recursion?
20:22 mithos28: Its fine
20:23 mithos28: The issue is with the running time, but don't worry about that at first
20:23 mithos28: I'm assuming you are still working on reverse
20:23 poisonarms: Yes.
20:24 mithos28: The first solution you come up with, will probably be O(n^2) where n is the length of the list
20:25 poisonarms: The mindset of working from the tail to the head is hard to get into.
20:27 poisonarms: But so is recursion.
20:27 poisonarms: Headache inducing.
20:29 poisonarms: http://ideone.com/IWIyj
20:30 mithos28: what does last-element do?
20:30 poisonarms: Returns the last element of a list.
20:31 mithos28: ok, what type of value does it return
20:31 poisonarms: (car ls)
20:31 mithos28: Is that a list?
20:31 poisonarms: Guess not.
20:32 mithos28: ok, what type does reverse require its argument to be
20:32 poisonarms: List.
20:32 poisonarms: So if I return (list (car ls))
20:33 poisonarms: But then I get "Expected: pair?, given: '()"
20:35 mithos28: I think replacing your ifs with conds might help, because then you will see your errors closer to where you are making them
20:35 mithos28: so instead of (if (null? ls) x y), (cond ((null? ls) x) ((pair? ls) y))
20:37 poisonarms: Well, it's my first delve into Scheme, functional programming and recursion all at once, over the course of the past few days, ha. That also might have something to do with it. Usually program in C like languages.
20:37 poisonarms: But I'll give it a try.
20:37 mithos28: Are you following a book or tutorial?
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20:38 poisonarms: Mmmm...yes/no? I'm reading the Racket docs and watching the SICP lectures in between fooling around.
20:38 mithos28: http://www.ccs.neu.edu/home/matthias/HtDP2e/
20:41 poisonarms: So last-element becomes: http://pastebin.com/uXmP7WA8 ?
20:42 mithos28: what is the last-element of null
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20:44 poisonarms: I guess it SHOULD be null, because null is null, it doesn't have one.
20:45 mithos28: I would say it is an error not null. Null is the empty list not the null from java or C.
20:45 mithos28: Usually one uses the names empty and empty? instead of null and null?
20:46 poisonarms: I've been using null and null? for the sake of using ideone.com for pasting :P
20:46 mithos28: oh, because it will evaluate it?
20:46 poisonarms: Yea
20:47 bremner: rudybot: init racket
20:47 rudybot: bremner: your sandbox is ready
20:47 bremner: rudybot: eval null
20:47 rudybot: bremner: ; Value: ()
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20:51 poisonarms: Oi, I think I'll take a break.
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