Notes on Clojure: Dipping My Toes In
Maybe because I’m such a late arrival to this programming pastime, I feel a certain inadequacy, a need to over compensate. Or maybe I just find it exciting learning new things, and I just want to get deeper and consume everything. Anyway, I’ve been getting more interested in functional programming, I don’t know why. I’m not so embedded in object-oriented, and I’ve been trying to understand JavaScript’s prototype-based programming, and if I’m honest, these concepts aren’t yet meaningful enough to me to matter so much outside of syntax and the practicalities of how to write code, yet functional programming seems to make a certain amount of sense. Perhaps this will become an endeavour in proving myself wrong. So, I listened to Carin Meier on The Changelog and bought her book, and am going to start learning Clojure.
Leiningen is the tool Carin recommends for getting started with Clojure, so let’s install that. Check Java is installed first:
$ java -version
java version "1.7.0_80"
Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)
Initially I used my package manager to install Leiningen, but this installed an old version.
$ sudo apt-get update
$ sudo apt-get leiningen
$ lein -v
Leiningen 1.7.1 on Java 1.7.0_80 Java HotSpot(TM) 64-Bit Server VM
So, I uninstalled that, and followed the instructions on the Leiningen site. Copy and Paste the contents of the lein file, into a file called lein. Move the file into my $PATH, and make it executable.
$ sudo apt-get remove leiningen # remove apt-get installed lein
$ vim ~/lein # copy and paste the contents of the lein file
$ echo $PATH # check path
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
:/usr/lib/jvm/java-7-oracle/bin:/usr/lib/jvm/java-7-oracle/db/bin:/usr/lib/jvm/java-7-or
acle/jre/bin:/usr/lib/jvm/java-7-oracle/bin:/usr/lib/jvm/java-7-oracle/db/bin:/usr/lib/j
vm/java-7-oracle/jre/bin
$ mv ~/lein /usr/bin/lein # move the lein file into my $PATH
$ chmod a+x /usr/bin/lein # make it executable
$ lein # aaaaaand run it!
Downloading Leiningen to /home/willem/.lein/self-installs/leiningen-2.5.2-standalone.jar now... [1050/1997]
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 406 0 406 0 0 653 0 --:--:-- --:--:-- --:--:-- 652
100 15.0M 100 15.0M 0 0 4160k 0 0:00:03 0:00:03 --:--:-- 6334k
Leiningen is a tool for working with Clojure projects.
Several tasks are available:
change Rewrite project.clj by applying a function.
check Check syntax and warn on reflection.
classpath Print the classpath of the current project.
clean Remove all files from project's target-path.
compile Compile Clojure source into .class files.
deploy Build and deploy jar to remote repository.
deps Download all dependencies.
do Higher-order task to perform other tasks in succession.
help Display a list of tasks or help for a given task.
install Install the current project to the local repository.
jar Package up all the project's files into a jar file.
javac Compile Java source files.
new Generate project scaffolding based on a template.
plugin DEPRECATED. Please use the :user profile instead.
pom Write a pom.xml file to disk for Maven interoperability.
release Perform :release-tasks.
repl Start a repl session either with the current project or standalone.
retest Run only the test namespaces which failed last time around.
run Run a -main function with optional command-line arguments.
search Search remote maven repositories for matching jars.
show-profiles List all available profiles or display one if given an argument.
test Run the project's tests.
trampoline Run a task without nesting the project's JVM inside Leiningen's.
uberjar Package up the project files and dependencies into a jar file.
update-in Perform arbitrary transformations on your project map.
upgrade Upgrade Leiningen to specified version or latest stable.
vcs Interact with the version control system.
version Print version for Leiningen and the current JVM.
with-profile Apply the given task with the profile(s) specified.
Run `lein help $TASK` for details.
Global Options:
-o Run a task offline.
-U Run a task after forcing update of snapshots.
-h, --help Print this help or help for a specific task.
-v, --version Print Leiningen's version.
See also: readme, faq, tutorial, news, sample, profiles, deploying, gpg,
mixed-source, templates, and copying.
$
$ lein -v # check the version again, 2.5.2 > 1.7.1, super.
Leiningen 2.5.2 on Java 1.7.0_80 Java HotSpot(TM) 64-Bit Server VM
$
$ lein repl # Fire up the REPL, first time this did some more stuff, see below
# but subsequent times it didn't, no worries.
Retrieving org/clojure/tools.nrepl/0.2.10/tools.nrepl-0.2.10.pom from central
Retrieving org/clojure/pom.contrib/0.1.2/pom.contrib-0.1.2.pom from central
Retrieving org/sonatype/oss/oss-parent/7/oss-parent-7.pom from central
Retrieving clojure-complete/clojure-complete/0.2.3/clojure-complete-0.2.3.pom from clojars
Retrieving org/clojure/clojure/1.7.0/clojure-1.7.0.pom from central
Retrieving org/clojure/clojure/1.7.0/clojure-1.7.0.jar from central
Retrieving org/clojure/tools.nrepl/0.2.10/tools.nrepl-0.2.10.jar from central
Retrieving clojure-complete/clojure-complete/0.2.3/clojure-complete-0.2.3.jar from clojars
nREPL server started on port 33732 on host 127.0.0.1 - nrepl://127.0.0.1:33732
REPL-y 0.3.7, nREPL 0.2.10
Clojure 1.7.0
Java HotSpot(TM) 64-Bit Server VM 1.7.0_80-b15
Docs: (doc function-name-here)
(find-doc "part-of-name-here")
Source: (source function-name-here)
Javadoc: (javadoc java-object-or-class-here)
Exit: Control+D or (exit) or (quit)
Results: Stored in vars *1, *2, *3, an exception in *e
user=>
user=> quit
Bye for now!
$
So the following is basically just me following along with Carin’s book, where she starts to introduce Clojure. So all credit goes to her, I’m just a messenger.
$ lein new wonderland # start a new Clojure project
Generating a project called wonderland based on the 'default' template.
The default template is intended for library projects, not applications.
To see other templates (app, plugin, etc), try `lein help new`.
$ cd wonderland/ # change into it
/wonderland$ lein repl # and get the REPL going again
nREPL server started on port 50609 on host 127.0.0.1 - nrepl://127.0.0.1:50609
REPL-y 0.3.7, nREPL 0.2.10
Clojure 1.7.0
Java HotSpot(TM) 64-Bit Server VM 1.7.0_80-b15
Docs: (doc function-name-here)
(find-doc "part-of-name-here")
Source: (source function-name-here)
Javadoc: (javadoc java-object-or-class-here)
Exit: Control+D or (exit) or (quit)
Results: Stored in vars *1, *2, *3, an exception in *e
user=>
g ; Some basic number stuff. Clojure is littered with parenthesis, you get used to ; it. Clojure structures things differently to what I’m used to. The operator ; goes first, then the parameters it takes. I really quite like this, it’s a ; nice way of organising things, though I can see it might annoy some people. user=> (+ 1 1) 2 user=> 42 42 user=> 6⁄3 ; Clojure will reduce a ratio if it can. ; This is different to division 2 user=> 8⁄3 ; And won’t if it can’t, but will leave it as a ratio, rather than ; change it to a decimal 8⁄3 user=> 6.0/3 ; you can’t use decimals in a ratio. NumberFormatException Invalid number: 6.0/3 clojure.lang.LispReader.readNumber (LispReader.java:330)
user=> 6⁄3.0 NumberFormatException Invalid number: 6⁄3.0 clojure.lang.LispReader.readNumber (LispReader.java:330)
user=> 6.0/3.0
NumberFormatException Invalid number: 6.0/3.0 clojure.lang.LispReader.readNumber (LispReader.java:330) user=> (/ 6 3) 2 user=> (/ 3 6) ; Dividing whole numbers will result in a ratio 1⁄2 user=> (/ 3.0 6.0) ; To get a decimal result, use a decimal in the equation 0.5 user=> (/ 3.0 6) 0.5 user=> “blues” ; yep, that’s a string “blues” user=> blues CompilerException java.lang.RuntimeException: Unable to resolve symbol: blues in this context, compiling:(/tmp/form-init7599042722842248080.clj:1:1010)
user=> true ; Ain’t nothing going on but a boolean, yo. true user=> nil ; nil is the absence of a value, like None in Python nil user=> (+ 5 (- 4 2)) ; Here’s the beginning of the growth of the parens 7 user=> ‘(5 “blues” :music) ; A list is denoted with a ‘ (5 “blues” :music) user=> (first ‘(5, “blues”, :music)) ; first gives us the first list item 5 user=> (rest ‘(5, “blues”, :music)) ; and rest, the rest (“blues” :music) user=> (first (rest ‘(5, “blues”, :music))) ; we can nest these fuctions “blues” user=> (cons “down” ‘(5, “blues”, :music)) ; and use cons to add to our list (“down” 5 “blues” :music) user=> (cons “down” (cons 7 ‘(5, “blues”, :music))) ; and nest our cons (“down” 7 5 “blues” :music) ```
That’s as far as I got last night. I know it’s not far, but having kids makes me tired. And I just want to reiterate again, this is basically just regurgitating Carin’s work, go buy her book.
Interestingly, the name Leiningen comes from the short story Leiningen versus the Ants. A tale of plantation owner Leiningen’s battle against an invading army of flesh-eating ants. It’s a really good read, triumph over adversity, the importance of intelligence and resourcefulness, I highly recommend you read it.