09:53 <haakonhr> Good morning! I'm trying to learn Racket by doing some image processing, but I'm struggling a bit with figuring out how to access single pixels and also which image library is recommended for these kinds of things. For example, if I want to compute the distance between two images according to some metric, for example just the squared difference of the mean pixel or mirror an image by reversing every "row" of pixels,

09:53 <haakonhr> which module and #lang is suitable to access this?

09:55 <haakonhr> I also haven't been able to read in an image from file as monochrome using (make-monochrome-bitmap "path/to/file") and variations on this. Any clue what I am missing? I am using 2htdp/image and #lang racket/base currently.

12:49 <samlamamma> What exactly determines if an identifier which is define:d is mutable or constant?

12:51 <bremner> in the simple case whether it's in the same module? there are probably syntax/macro tricks as well that makes const identifiers

12:53 <samlamamma> Right, so to give some context: In DrRacket I can have a file consisting of only (define foo 1). If I then run that file and try to run (set! foo 2) then Racket will signal an error, saying that it's constant. If I however put the set! line in the code then it runs with no error being signalled. What's up with that?

13:05 <bremner> I _guess_ that counts as a different module

13:06 <bremner> (i.e repl is not in-module, definitions is)

13:07 <bremner> but I suspect others can state this more precisely

13:17 <samth> bremner: samlamamma: yes, any uses of `set!` on a module-defined variable must be syntactically within that module

13:23 <samlamamma> Ahaa, okay. But if I `set!` in the file and run it, then I can `set!` it in the REPL. What happens there?

18:03 <MustardCheese> I have a function wrapper that accepts a procedure, some other arguments and then varargs. The varargs are used to take any arguments that are to to be applied to the wrapped function. When I put a keyword in the varargs, the keyword is interpreted by the wrapper function before it can be passed to the wrapped function. It seems as though keywords are not first class. What alternatives do I have?

18:05 <samth> MustardCheese: you probably need to use keyword-procedure and keyword-apply.

18:11 <MustardCheese> Aha so these automatically convert the keywords into datums to be fed into the child procedure? This would seem to be what I'm looking for, yes. I'll experiment with them for a while and get back to you, thank you!

18:51 <MustardCheese> It's pretty mind bending stuff this, though I've managed to knock together some implementations that work. Thank you again!