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<olivuser>
hello #racket. May I ask for help with a HTDP exercise which I simply cant wrap my head around?
<narimiran>
olivuser: don't ask a permision to ask. just ask. ;)
<olivuser>
@narimiran, thanks :) so I'm actually quite early in the book, Part I, exercise 45 (where you have to move the cat from left to right repeatedly)
<olivuser>
what I dont understand is this: with big-bang, you have a render which is re-rendered as the clock ticks, with a position that is altered through a tick-function
<olivuser>
I figured that when I want to have the cat re-appear to the left, I need to test whether the current position is smaller than the canvas width. As long as pos is smaller, proceed as normal (place-image CAT pos Y-CAT BACKGROUND). But, as soon as pos is bigger, re-set pos to 0 (place-image CAT 0 Y-CAT BACKGROUND)
<olivuser>
I did this with an "if"-expression (if smaller, then normal; else pos=0). However, when "else" is triggered, the cat is set to pos 0, but the animation stops - even though it appears as though the ticking continues.
<olivuser>
Why does the animation stop?
<olivuser>
Does the order of the "big-bang"-clauses matter?
<narimiran>
"You may wish to read up on the modulo function."
<olivuser>
@narimiran, yeah, I did read up on it. But for some reason I have no clue at all(!) how this would help me
<narimiran>
have in mind that i didn't solve those exercises nor do i exactly know how the remaining of your program works:
<narimiran>
position should always increase by 3 pixels, right?
<olivuser>
yes
<narimiran>
and when it reaches some value (right edge) it should reappear on the left?
<olivuser>
yes
<narimiran>
ok, so in an attempt to spoil as little as possible: how do you calculate a new position?
<olivuser>
adding 3 to t0 in order t receive the pos-value at t1
<olivuser>
rephrase: adding 3 to the pos-value at t0 in order to obtain the pos-value at t1
<narimiran>
ok
<narimiran>
so pos is 0, 3, 6, 9, 12, 15, ....
<narimiran>
let's say that right edge is at 10
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<narimiran>
how do you wrap around? can you use `modulo` for that?
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<olivuser>
if (modulo 10 3) = 1, then
<narimiran>
not quite....
<narimiran>
(the sequence should be: 0, 3, 6, 9, 2, 5, 8, 1, 4, 7, 0, 3, ....)
<olivuser>
so its like when it goes above 10, remove 10 and restart (9+3=12-10=2;8+3=11-10=1, ...)
<narimiran>
ok, we're getting closer :)
<narimiran>
once again: can you use `modulo` for that? :)
<olivuser>
actually I wouldnt know why modulo would help in that case, because a conditional could work just without modulo
<olivuser>
let me check wiki in parallel, maybe its a translation-related issue (I'm german reading the english book and sucking at math)
<narimiran>
what if "clock tick" is at value 63?
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<narimiran>
if you just do 63-10, you're way right of your right-limit at 10
<olivuser>
yeah, but modulo does simply tell me how much is left when I divide "number a" by "number b". And in the case of a "clock tick"-value of 63, the modulo value would be the same as in the case of a "clock tick"-value of 33.
<olivuser>
that is, I dont know how using modulo relates to setting a "rightmost" limit to how far a "pos"-value can go
<narimiran>
try `(modulo 63 10)`, it is not 33
<olivuser>
yeah I know, but (modulo 63 10) and (modulo 33 1) both yield 3
<narimiran>
yes
<narimiran>
so you can calculate your new position by always taking a modulo of it and right-limit)
<narimiran>
another example where modulo is useful: say you woke up at 10 o'clock, and then 45 hours pass. can you tell me what time is now? (and we both know it is not 55 o'clock :))
<olivuser>
okay I think I know where this is going. If I may I might come back to you in a moment
<narimiran>
:)
<olivuser>
I die. So this is the solution!? I literally experimented for like an hour with if and cond stuff wondering why it wouldnt work
<olivuser>
@narimiran, thanks a bunch. I didnt get that if x is smaller than y, it just outputs x, and that it, like, folds the result
<narimiran>
you're welcome :)
<olivuser>
man, theres a bunch of friendly people here on irc! I might come back when I have a stupid problem to solve the next time ;-)
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<jcowan>
Can someone explain the term "contracted values" to me? I see that they have something to do with contracts, but I don't understand the term itself.
<jcowan>
It seems to be used once per document, without definition or explanation
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<samth>
jcowan: it's a value that has had a contract applied to it
<jcowan>
You told me a while back that these were instances of identifer-syntax: is that because the contract is applied each time the variable holding the value is referenced?
<samth>
for functions with contracts on them, when they are used the source location for when they're used is used as part of the contract for the error message
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<jcowan>
Ah, thanks.
<jcowan>
I'm having a discussion about whether identifier-syntax should go into R7RS-large
<jcowan>
so use cases like the ones you gave me are helpful
<ecraven>
identifier-syntax is syntax is non-call-position, right?
<ecraven>
somehow this still feels weird to me ;)
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<jcowan>
To me as well. Of course it would be tasteless to use this for something complex or hard to evaluate: it should be strictly an abbreviation. But for example it can be used to implement immutable variables, something Scheme doesn't have explicitly.
<samth>
jcowan: it's necessary to make macro-based implementation truly transparent
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<samth>
another example -- keyword-argument functions in Racket are implemented as macros, so that when you use them directly with the correct keywords it calls the underlying function directly without keyword parsing overhead
<samth>
but you can't do that transparently without identifier syntax
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<samth>
because obviously functions can be used in both first-order and higher-order ways
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<jcowan>
I know about keyword-apply, but how do procedure-with-keywords arguments work?
<jcowan>
that is, higher-order?
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<samth>
what do you mean
<samth>
you can write (lambda (f) (f #:x 1))
<jcowan>
That's a direct call. I mean, in CL you can pass a keyword function as an argument and invoke it with FUNCALL.
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<samth>
that's not a direct call
<samth>
( is funcall
<samth>
jcowan: this quote "Note that Racket converts all variable references and function calls to use magic macros, so the syntax is truly uniform. " doesn't seem right to me
<samth>
also, "since R7RS-small was never intended to be a successor of R6RS" is ridiculous
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<lavaflow>
what's the difference between (kill-thread th) and (break-thread th 'terminate) ?
<lavaflow>
both end the execution of that thread, right? kill-thread doesn't seem to send a break:terminate, but is there other differences?
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<lavaflow>
the documentation on this doesn't seem clear to me
<samth>
lavaflow: threads can react to breaks, but not to being killed
<lavaflow>
ah okay, so I can't catch a kill-thread
<lavaflow>
I didn't see a way but I wasn't sure
<samth>
right
<lavaflow>
cool, thanks!
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<jcowan>
samth: What's ridiculous about it?
<samth>
jcowan: you can tell it's a successor because of the 7 in the name
<erkin>
haha
<jcowan>
Edward VIII was not a successor of Edward VII, though he postdated him
<samth>
he was A successor, however, and had be been the immediately-following king he would have been THE succesor
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<samth>
I realize that many people involved in the r7rs process would like to pretend various things about it, but the rest of us don't have to go along
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<jcowan>
On the other hand, you are not in a position to say what our intentions were.
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<aeth>
they should have gone with the name r{5+i}rs instead.
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<jcowan>
It's true that R7RS is not the Revised R6RS, and indeed I argued against this name in favor of designing our own (but retaining "Scheme" in it), but tradition prevailed.
<jcowan>
Names of the form "X Scheme" are common for implementations, so it would have to have some other form. Date-based names are common for other languages, but given our slow progress they would be an embarrassment.
<aeth>
R7RS could have been Scheme 13 and R7RS-large would be Scheme 13B
<aeth>
You'd be setting yourself up for a potential problem with the year 2048, though
<jcowan>
We had no way of knowing in advance that 13 would be relevant, though.
<jcowan>
As Riastradh says, R7RS is done when someone puts in a toothpick and it comes out clean.
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<bremner>
sounds like a lou reed song
<bremner>
not really, sounds like cooking metaphor, but reminds me a of a lou reed song
<jcowan>
Sounds like my mother to me
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<selimcan>
Programming language names should be designed not by piling a qualifier on top of qualifier, but by removing the weaknesses and restrictions that make additional qualifiers appear necessary.
<lavaflow>
e.g. "perl 6" -> "raku" ?
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<tlcu>
yeah the raku thing just baffles me
<winny>
i've heard from peers they don't want to learn raku because it's not perl 6 anymore (it's that trivial to some prospective learners)
<tlcu>
I did some testing of it last weekend and it was very slow, and in that respect killed one of the positives of perl for me personally.
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<FreeFull>
Give your programming language a really appealing name, like Rust or Cobol
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<tlcu>
Scheme and Racket certainly did improve on Lisp in at least this regard
<FreeFull>
"Scheme" and "Racket" sound criminal
<FreeFull>
Especially Racket
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<aeth>
Ugh, the worst thing about "Perl 6" -> "Raku" is now that name's taken so they can't do "R7RS" -> "Raku"
<aeth>
The Racket implementation, of course, would be Racku
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