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4 entries#jvm ×

2020

Clojure programming language

Clojure is a Lisp that runs on the Java Virtual Machine (JVM).

Learning resources

Clojure as a Lisp

Courseware

Presentations

Clojure like languages

IDE and infrastructure

  • Lightmod -- Lightmod is a tightly integrated tool for making full stack Clojure and ClojureScript web apps. It is fully self-contained -- no need to install the JDK or any build tool. It hot reloads both client and server code automatically with zero setup.

  • Nightlight is an editor you run inside your Clojure projects. Instead of being a separate application like traditional editors and IDEs, it's just a library that you add to your project. It spins up a web server and provides a completely browser-based interface to edit your code.

  • CIDER is the successor to nrepl.

  • Cursive Clojure IDE -- is an IDE built on IntelliJ

  • Parinfer

REPL

online playgrounds

Getting started with Lighttable

Via

Basic instructions. I use cmd for the shortcuts below, substitute ctrl on non-mac platforms.

  1. Install Leiningen http://leiningen.org/ you do not need to install clojure, lein will grab clojure for you.
  2. lein new playground from the command line to create a 'playground' project
  3. Open Light Table
  4. ctrl-space woto for "Toggle Workspace tree"
  5. Add the playground project folder you just created
  6. cmd-o core.clj to open the main clojure file for the project
  7. On an empty line type (+ 1 1) and hit cmd-enter

Things will grind away for a while as all the dependencies are downloaded and the JVM starts up. Get a cup of coffee or something. Eventually you'll see a green 2 show up.

You can then type anything in the tutorial and hit cmd-enter to evaluate it or cmd-shift-enter to evaluate the entire file. Think of the file as a shell with an editable history instead of something you'd want to commit to source control.

Documentation is searchable via ctrl-shift-d or moving your cursor on top of a function name and hitting ctrl-d. Autocomplete is available on tab, you can jump to definition with cmd-. and back with cmd-,

TODO: Explore CIDER + AC(autocomplete) + ritz.

Applications

Libraries

Spec

the biggest win from spec has been with parsing. This completely changes how you'd write a library that takes a data structure and parses it into something meaningful (for example, what hiccup does for html or what honeysql does for sql). In the past, this required a lot of very ugly parsing code and manual error-checking. With spec, you write specs and call s/conform. If it failed, you get a nice error, especially if you pair it with expound. If it succeeded, you get a destructured value that is really easy to pull data out of. I've done this in a half dozen different libraries and i'm pretty sure i wouldn't have even written them without spec.

Mallimalli and specmonstahspecmonstah are my favourite Clojure(Script) libraries built on top of Clojure Spec.

Clojure books

The first chapter, Names, explains why names define the structure of our software, and how to judge whether a name is any good.

The second chapter, Idioms, provides specific, syntactic advice for writing Clojure which is clean and readable.

The third chapter, Indirection, looks at how code can made simpler and more robust through separation.

The final chapter, Composition, explores how the constituent pieces of our code can be combined into an effective whole.

Clojurescript books

Experience reports and discussions

Articles

Talks and presentations

Tips and tricks

Cool projects / Read Code

Visualization / Notebook

Web development

Big data

Blogs

ClojureScript

Graal

  • This nextjournal notebook has a recipe for building Babashka "executing Clojure snippets or scripts in the same space where you would use Bash."

Web development

PCP

alekcz/pcp: PCP: Clojure Processor -- Like drugs but better

Just use it like PHP on a webserver (say nginx):

    #new pcp config => change .php to .clj
    index index.clj index.html index.htm;
    #new pcp config => change .php to .clj
    location ~ \.clj$ {
        #default nginx config on digital ocean lemp image
        #include snippets/fastcgi-php.conf;
        #fastcgi_pass unix:/run/php/php7.0-fpm.sock;

        include scgi_params;
        scgi_intercept_errors on;
        scgi_pass   127.0.0.1:9000;
    }

PCP has following libraries built in:

(require  '[cheshire.core :as json]
      '[clostache.parser :as clo]
      '[selmer.parser :as parser]
      '[selmer.filters :as filters]
      '[clj-http.lite.client :as client]
      '[next.jdbc :as jdbc]
      '[honeysql.core :as sql]
      '[honeysql.helpers :as helpers]
      '[postal.core :as email]
      '[clojurewerkz.scrypt.core :as sc])

Learning Journal

  • this is a good demo of how to use Clojure's -Sdeps at the command line to execute a script (even one hosted remotely on git):
clojure -Sdeps '{:deps
    {hello-clojure
        {:git/url "https://gist.github.com/athos/b68b15b08efedffaf14d8c020b125202"
        :sha "1c9a05106171f97f9b8e8ac8f58c7bd8dbe021f9"}}}'
    -m hello-clojure

See also: lisp, scheme.

notes#clojure#jvm#lisp#programming-language

2019

Ballerina programming language

"Ballerina" Could Become the Programming Language of Integration - By dhanushka

Ballerina is a static typed language.

Another special type of variable is that “anydata” data type. This type is a union of the ()|boolean|int|float|decimal|string|(anydata|error)[]|map<anydata|error>|xml|table data types. anydata variables can be used in places where you expect pure values.

Since Ballerina specifically designed to construct apps operating on networks, it natively supports JSON and XML.

json user = {
         fname: "Peter",
         lname: "Stallone",
         "age": age,
         address: {
             line: "20 Palm Grove",
             city: "Colombo 03",
             country: "Sri Lanka"
         }
    };
io:println(user.address.country);

xml x1 = xml `<book>The Lost World</book>`;
io:println(x1);

News

Articles

James Clark's blog:

notes#ballerina#jvm#programming-language

Scala programming language

See Also: Scala for Datascience, ScalaJS, coursier, select-scala

July, 2015 -- Scala book reviews Feb, 2018 -- Recursion and Trampolines in Scala

Random Notes

scala-cli—power repl -S 2.13 -A —ammonite-ver 2.5.11

Notes from Programming in Scala

Chapter 7 - built-in control structures.

  • has very few control structures
  • if while for try match and function calls.
  • scala accumulates control structs in libraries.
  • use val instead of var wherever possible. makes it easy to refactor. "equational reasoning".
  • no = sign while defining a function => returns a Unit value ()
  • while and do-while are NOT expressions. they return Unit.
  • prefer NOT to use while.
  • for
  • "generator" syntax -- file <- files. works on all collections.
  • file is a val.
  • Ranges -- 1 to 4 (yields 1,2,3,4)
  • 1 until 4 -- exlcude the upper bound (yields 1,2,3)
  • generators can also include filters -- for (f <- files if f.getName.endsWith('.py'))
  • nested iteration.
  • mid-stream variable bindings
  • new collection:
def pyFiles =
  for {
    file <- files
    if file.getName.endsWith('.py')
  } yield file
  • IMP: don't put yield inside the curlies.

Exceptions:

try {


} catch {
  case ex: FileNotFoundException => //handle
  case ex: IOException => //handle
}
  • like other control structures, try-catch-finally also results in a value. (this is unlike Java) //imp

Match:


val fruit =
    someArg match {
      case "foo" => "bar"
      case "banana" => "pepper"
      case _ => "what?"
}

Chapter 8- Functions and Closures

First class functions

(x: Int) => x+1

named function:

var increase = (x: Int) => x + 1
  • target typing: someNumbersList.filter((x) => x > 0). the type is inferred from the the list of integers.
  • alternatively: someNumbersList.filter(x => x > 0)

Placeholder syntax:

someNumbersList.filter( _ > 0)

val f = (_: Int) + (_: Int)

Chapter 12 -- Traits

Traits are like java interfaces with concrete methods + declare fields and maintain state.

Traits can do everything classes can do, except --

  1. trait cannot have any clas parameters (ie., those passed to primary constructors).
  2. super calls are not statically bound.

Unlike Java, adding a concrete method to scala trait is an one-time effort.

Chapter 15 -- Case classes and pattern matching

Notes from "Scala in Action"

Chapter 2 -- Getting started

// working with arrays and Lists
@ val nList = 1 :: 2 :: 3 :: 4 :: Nil
nList: List[Int] = List(1, 2, 3, 4)
@ nList.fil
filter     filterNot
@ nList.filter( _ > 2)
res17: List[Int] = List(3, 4)

@ // for comprehensions
@ oldList
res19: List[Int] = List(1, 2)
@ newList
res20: List[Int] = List(1, 2, 3)
@ for { a <- oldList; b <- newList } println (a+b)
2
3
4
3
4
5
@ // above is imperative form
@ // The functional form for comprehension is:
@ // also called the sequence composition
@ for { a <- oldList; b <- newList } yield a+b
res22: List[Int] = List(2, 3, 4, 3, 4, 5)

@ // Pattern Matching
@ val x = 3
x: Int = 3
@ def ordinal(number: Int):  Unit = number match {
    case 0 => println("One")
    case 1 => println("Two")
    case 2 => println("Three")
    case 3 => println("Four")
    case _ => println("what is this?")
  }
defined function ordinal
@ ordinal(x)
Four

@ // Pattern Matching Types
@ dBef printType(obj: AnyRef) = obj match {
    case s: String => println("This is a string")
    case i: java.util.Date => println("This is a Date")
    case l: List[_] => println("An array eh?")
    case _ => println("dunno who ye are")
  }
defined function printType
@ printType(List(1,1,2))
An array eh?


// Guards
@ def numCatcher(num: Int): Unit = num match {
    case x if x < 10 => println("Number less than 10")
    case y if (y%2 == 0) => println("Is an even number")
  }
defined function numCatcher
@ numCatcher(5)
Number less than 10

@ numCatcher(12)
Is an even number

Miscellaneous notes

sbt plays an important role in the day to day programming of scala. I believe 'sbt' stood for "scala build tool" at one time, but is no longer.

To start working on a scala program, you can start with a simple text editor, however using something like IntelliJ IDEA is probably far more productive.

Creating a "project structure" can be automated. Essentially this boilBs down to creating the build.sbt file, project directory and optional plugins.sbt file inside it, a scala file inside the proper src/main/scala/com/example/projectname/ directory etc., There is little reason to do all this manually, except when you are trying to understand the reason behind such a recommended directory layout.

giter8 ie., the g8 is a tool that can create a project structure based on pre-defined templates. Think of this as paster tool for Python.

When you have g8 installed, creating a new project (say, a basic scala project using this template ), all you have to do is:

$ g8 fayimora/basic-scala-project.g8

A basic scala project

name [Basic Project]: basic-scala
organization [com.example]: com.btbytes
version [0.0.1]:

Template applied in ./basic-scala

$ cd basic-scala
$ ls
build-sbt src

$ sbt run will download the dependencies defined in the build.sbt file,B compile source and run the program.

sbt can also create the files necessary to make this an "IntelliJ IDEA project". For this, we add a dependency to the plugins.sbt file:

#project/plugins.sbt -- add this line
addSbtPlugin("com.github.mpeltonen" % "sbt-idea" % "1.6.0")

Running sbt gen-idea will generate the IDEA project files (.idea, some .iml).

Launch up IDEA and "open project" and open the root directory of the project. You are now set to work on the code.

Packaging the binary to a .jar:

$ sbt packageBin

Will produce target/scala-2.10/basic-scala_2.10-0.0.1.jar (where basic-scala is the name of the project).

Why not Scala

Concepts

Covariance, Contravariance and Invariance

Reading:

In general, covariant type parameter can be used as immutable field type, method return type and also as method argument type if the method argument type has a lower bound. Because of those restrictions, covariance is most commonly used in producers (types that return something) and immutable types.

Adhoc polymorphism

In this article we discussed the motivation for introducing ad hoc polymorphism and implementing it using the Type Class Pattern in Scala. It allows us to build abstractions which are completely decoupled from the underlying data types on which they operate. Hence we can implement polymorphic functions which operate on types we have no control over, such as types defined in the standard library or some other third party library, without sacrificing the static type safety.

Concurrency (Akka etc.,)

StBreams

Libraries

Tools

  • lihaoyi/Ammonite: Scala Scripting. Ammonite enables shell-like scripting in the Scala programming language.
  • Conscript -- is a tool for installing and updating Scala software programs.
  • giter8 -- is a command line tool to generate files and directories from templates published on github or any other git repository.

SBT

Ensime -- Enhanced Scala

Interaction Mode for Emacs.

Talks

Articles

Blogs

Libraries

  • kantan.csv has good documentation/tutorials.
  • Quill Quoted Domain Specific Language (QDSL) to express queries in Scala and execute them in a target language. The library’s core is designed to support multiple target languages, currently featuring specializations for Structured Query Language (SQL) and Cassandra Query Language (CQL).

REPL

Ammonite REPL is much more featureful than the default REPL and comes with a lot of ergonomic improvements and configurability that may be familiar to people coming from IDEs or other REPLs such as IPython or Zsh.

Links

Scalajs

Scala Native

Scala CLI

With scala-cli, it is quite easy to package a scala program into a standalone program:

scala-cli --power package --native-image Echo.scala -o echo -- --no-fallback

where Echo.scala is:

object Echo {
def main(args: Array[String]): Unit =
	println(args.mkString(" "))
}

$ ls -alrt echo
-rwxr-xr-x@ 1 pradeep  staff  14120984 Aug 23 17:15 echo
$ du -sh echo
 13M    echo

Books

Tutorials

Indyscala notes

Feb 6, 2017 -- Resource management in Scala

Presenter -- Ross A. Baker

May 1, 2017

notes#jvm#programming-language#scala