EO (Elegant Objects)

Website | Source | Based on the phi-calculus

Example: Hello world:

import org.eolang.cli
import org.eolang.printed
import org.eolang.string
cli:
  printed:
    string:
      "Hello, world!"

Quick Start

Here is a simple program that gets a year from command line and tells you whether it's leap or not:

+alias org.eolang.io.stdout

[args...] > main
  [y] > leap
    or. > @
      and.
        eq. (mod. y 4) 0
        not. (eq. (mod. y 100) 0)
      eq. (mod. y 400) 0
  stdout > @
    sprintf
      "%d is %sa leap year!"
      (args.get 0).nextInt > year!
      if (leap year:y) "" "not "

In order to compile this program, put it into src/main/eo/main.eo and then create a file pom.xml... [Redacted]

Then, you just run mvn clean test (you will need Maven 3.3+) and the .eo file will be parsed to .xml files, transformed to .java files, and then compiled to .class files. You can see them all in the target directory. You will need Java 8+.

More examples are here.

Tutorial

Let's start with a simple EO program:

+alias stdout org.eolang.io.stdout

[] > app
  stdout > @
    "Hello, world!"

Here we create a new abstract object named app, which has got a single attribute named @. The object attached to the attribute @ is a copy of the object stdout with a single argument "Hello, world!". The object stdout is also abstract. It can't be used directly, a copy of it has to be created, with a few requirement arguments provided. This is how a copy of the object stdout is made:

stdout
  "Hello, world!"

The indentation in EO is important, just like in Python. There have to be two spaces in front of the line in order to go to the deeper level of nesting. This code can also be written in a "horizontal" notation:

stdout "Hello, world!"

Moreover, it's possible to use brackets in order to group arguments and avoid ambiguity. For example, instead of using a plain string "Hello, world!" we may want to create a copy of the object stdout with a more complex argument: a copy of the object sprintf:

+alias stdout org.eolang.io.stdout
+alias sprintf org.eolang.txt.sprintf

[] > app
  stdout > @
    sprintf
      "Hello, %s!"
      "Jeffrey"

Here, the object sprintf is also abstract. It is being copied with two arguments: "Hello, %s!" and "Jeffrey". This program can be written using horizontal notation:

+alias stdout org.eolang.io.stdout
+alias sprintf org.eolang.txt.sprintf

[] > app
  (stdout (sprintf "Hello, %s!" "Jeffrey")) > @

The special attribute @ denotes an object that is being decorated. In this example, the object app decorates the copy of the object stdout and through this starts to behave like
the object stdout: all attributes of stdout become the attributes of the app. The object app may have its own attributes. For example, it's possible to define a new abstract object inside app and use it to build the output string:

+alias stdout org.eolang.io.stdout
+alias sprintf org.eolang.txt.sprintf

[] > app
  stdout (msg "Jeffrey") > @
  [name] > msg
    sprintf "Hello, %s!" name > @

Now, the object app has two "bound" attributes: @ and msg. The attribute msg has an abstract object attached to it, with a single "free" attribute name.

This is how you iterate:

+package sandbox
+alias stdout org.eolang.io.stdout
+alias sprintf org.eolang.txt.sprintf

[args...] > app
  memory > x
  seq > @
    x.write 2
    while.
      x.less 6
      [i]
        seq > @
          stdout
            sprintf "%dx%d = %d\n" x x (x.pow 2)
          x.write (x.add 1)

This code will print this:

Tags: language object jvm

Last modified 05 February 2024