generators...?

[Christopher Howard]

Hi. I'm still fairly new to Clojure. I was wondering: What's the
easiest way to make a generator (or something that would be as useful).
In one application, I need something that will return a different color
each time I call it - following a predefined list of colors, and
starting over again at the beginning when colors are exhausted.

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working with "multiple" return values

[Christopher Howard]

In Clojure, what is the easiest (cleanest) way to return "multiple"
values from a function, in order to work with them immediately
afterwards? In Haskell you can return a tuple, and pattern match the
components into variables.

Prelude> let (a, b) = (1 + 4, 2 - 1)
Prelude> a
5
Prelude> b
1

IIRC, Python has something similar.

I know how to return a list or a vector, of course, but I'm imagining
having to put in a bunch of cludgy code to extract the components into
variables, which is not appealing.

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using contrib functions

[Christopher Howard]

Hi. --Insert here the usual caveats about being new to Clojure and
Java.--

I wanted to try out this Contrib function describe on this
page
.
 How
do I get that in my project? Trying to follow documentation, I did
"lein search":
  
code:

my-host:~/my-project$ lein search contrib
Warning: couldn't download index for http://repo1.maven.org/maven2
 == Results from clojars - Showing page 1 / 1 total
[webnf.deps/contrib "0.0.1"] The uber dependency to get a full set of popular
  dependencies. For development or when you have room in your .m2 repo.

my-host:~/my-project$ lein search clojure.contrib.types
Warning: couldn't download index for http://repo1.maven.org/maven2
my-host:~/my-project$ lein search defadt
Warning: couldn't download index for http://repo1.maven.org/maven2


Is that "webnf" package the thing I am supposed to put in my
"dependencies" list? Version 0.0.1 doesn't sound right. Or am I
supposed to download something myself and throw it in my lib
directory?

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Re: using contrib functions

[Christopher Howard]

On Fri, 28 Mar 2014 20:06:59 -0700
Sean Corfield  wrote:

> BTW, where did you find the references to defadt and
> clojure.contrib.types? Perhaps we can get the original references
> updated so people aren't misled in future...
> 
> Sean
> 

I think it was this old StackOverflow post:

http://stackoverflow.com/questions/5430673/clojure-algebraic-data-types

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REPL: viewing data structures containing infinite lists

[Christopher Howard]

Is there some kind of "safe" function for printing representations of
lazy, infinite data structures? I'm finding I like using them inside
other data structures here and there. However, when I go to play
around with things in the REPL, sooner or later my workflow is
interrupted by 3 million characters streaming across the console.

I don't imagine there would be any way for the REPL to detect that a
lazy sequence was infinite. However, if it would simply refuse to
evaluate lazy sequence (say, represent them by some special identifier) that
would be good enough for me.

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remove first?

[Christopher Howard]

Is there some like "remove", but only removing the first item found?
Not exactly an incredibly hard problem, but with all this clojure
stuff about collections and sequences, I'm not quite sure what the
appropriate way is to implement it.

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Re: remove first?

[Christopher Howard]

On Wed, 2 Apr 2014 14:07:47 -0600
Timothy Baldridge  wrote:

> I don't know of anything built-in, but this should do the trick:
> 
> (defn remove-first [f [head & tail]]
>   (if (f head)
>   tail
>   (cons head (lazy-seq (remove-first f tail)
> 
> Timothy
> 
> 

Thanks. This seems to work for my purposes, although I modified it to
handle the empty vector:

(defn remove-first [f [head & tail]]
  (if (not head) []
  (if (f head)
  tail
  (cons head (lazy-seq (remove-first f tail))

This implementation doesn't seem to work on everything that "remove"
works on:

com.example.myapp=> (remove #(= [1 2] %) {1 2 3 4})
([3 4])
com.example.myapp=> (remove-first #(= [1 2] %) {1 2 3 4})
UnsupportedOperationException nth not supported on this type: 
PersistentArrayMap  clojure.lang.RT.nthFrom (RT.java:835)

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list all functions in namespace?

[Christopher Howard]

Is there some trick Clojure command to list all functions defined in a
namespace?

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Elementary question about displaying images in an app

[Christopher Howard]

This is, I think, as much of a Java question as a Clojure question
(I'm learning both at the same time). I'm writing a small desktop
application, and I need to paint a few images in one part of the
window (contained in PNGs). I'm looking through AWT/Swing/Graphics2D
documentation, but I am a little unclear about the correct/modern way
of doing this.

Is Canvas the component I need? If so, how do I subclass it and
override the paint method (from within Clojure). Or is there a better
way to approach this?

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seesaw >> canvas >> paint

[Christopher Howard]

Suppose one is using seesaw, and wants to have a canvas with lots of
images and whatnot drawn inside it. The github examples pretty well
cover that. However, what if you want the paint function to behave
differently depending on some data elsewhere? (For example, the canvas
is supposed to be a visual representation of model data.) Should you
just have the paint function capture the (modifiable) data variable,
and use that each time? Or...?

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setting environment variables for all repls?

[Christopher Howard]

Hey all, I want

(set! *print-length* 40)

to be run each time I open "lein repl" (regardless of the project,
preferably). I thought that is what the ~/.lein/init.clj file was for,
but apparently not.

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seesaw: drawing text

[Christopher Howard]

Hi. In seesaw, how do you draw text to a graphics 2d object? I see in
seesaw.graphics how to draw circles and lines and such, but want to
draw a line of text. (I'm drawing on a canvas.)

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arguments passed to a macro

[Christopher Howard]

Is there a trick for "pre-expanding" some of the arguments passed to a
macro? Say you wanted to do this

code:

(def swing-imports '(javax.swing JFrame JButton ImageIcon JPanel))

(ns example.myprogram
  (:import swing-imports))
  ...


ns will complain that it can't find the "swing-imports" class.

I've been trying various things I thought might work (like backtick
evaluation and evals) but haven't made progress.

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using "hidden" parameters?

[Christopher Howard]

This might be kind of perverse, but I was wondering if it was possible
to write a function or macro that takes "hidden parameters", i.e.,
uses symbols defined in the scope of use, without passing them in
explicitly.

For example, function "next-number" takes hidden parameter "x", so

=> (let [x 12] (next-number))

Would return 13.

With the whole "code is data" paradigm it seems like this should be
possible, but I haven't figured out how to do this yet without getting
an error.

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macro question

[Christopher Howard]

Disclosure: I'm rather fascinated with macros (the things they are
supposed to allow us to do), but I'm still rather mystified about how
they actually work. It seems like every time I try to write one, it
rarely behaves anything like I expect.

Question: Is there any obvious problems with this seemingly innocent
macro?:

(defmacro if-zero [test-expr true-expr false-expr]
  (if (= test-expr 0) true-expr false-expr))

It doesn't seem to work the way I would expect when used inside other
functions.

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restarts

[Christopher Howard]

Does Clojure have restarts or continuable-errors like in CL? Or
something similiar? If not, is that something that could be
implemented some how?

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Beginner: let with Java object

[Christopher Howard]

When I run

tutorial.core> (let [img (new-image 32 32)] (set-pixel img 10 10 cyan)
(show img))

from the REPL, this returns a javax.swing.JFrame object, and displays
the frame and image as expected. However, if put the same in

(defn -main
  "Generates image."
  [& args]
  (let [img (new-image 32 32)]
(set-pixel img 10 10 cyan)
(show img)))

in more core.clj, and run (-main) from the REPL, I get error

ClassCastException java.awt.image.BufferedImage cannot be cast to
clojure.lang.IFn  tutorial.core/-main (core.clj:11)

Why the difference?

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Re: Beginner: let with Java object

[Christopher Howard]

I restarted the Emacs CIDER REPL, and now it seems to work fine. Maybe I
just don't understand how to use this CIDER REPL properly.

On 05/26/2017 07:58 AM, Gary Trakhman wrote:
> There shouldn't be a difference, I would suspect a typo in your file.
> 
> The error 'ClassCastException java.awt.image.BufferedImage cannot be cast to
> clojure.lang.IFn  tutorial.core/-main (core.clj:11)' would lead me to
> look for things like '(img)' where the object ends up in call position
> of the s-expression.
> 
> On Fri, May 26, 2017 at 11:54 AM Christopher Howard
> mailto:christopher.how...@qlfiles.net>>
> wrote:
> 
> When I run
> 
> tutorial.core> (let [img (new-image 32 32)] (set-pixel img 10 10 cyan)
> (show img))
> 
> from the REPL, this returns a javax.swing.JFrame object, and displays
> the frame and image as expected. However, if put the same in
> 
> (defn -main
>   "Generates image."
>   [& args]
>   (let [img (new-image 32 32)]
> (set-pixel img 10 10 cyan)
> (show img)))
> 
> in more core.clj, and run (-main) from the REPL, I get error
> 
> ClassCastException java.awt.image.BufferedImage cannot be cast to
> clojure.lang.IFn  tutorial.core/-main (core.clj:11)
> 
> Why the difference?
> 
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