Nex — Write code that keeps its promises

// why nex

Small on purpose.

Big in expressive power: contracts, readable syntax, objects, functions and the JVM, all in one compact design. Pick a tab to see each in action.

State what must be true. Nex checks it, and names the exact promise that was broken.

no contractsilent

spend(amount: Real)
do
  this.money := money - amount
end

REPL

> w.spend(20.0)
> w.money
-9.8   -- negative balance… and nobody noticed

with requireguarded

spend(amount: Real)
require
  non_negative_amount: amount >= 0.0
  enough: amount <= money
do
  this.money := money - amount
ensure
  decreased: money = old money - amount
end

REPL

> w.spend(20.0)
Error: Precondition violation: enough

class invariantalways true

invariant
  never_negative: money >= 0.0
end

REPL

> create Wallet.with_balance(-10.0)
Error: Class invariant violation: never_negative

Words instead of punctuation. Blocks open and close with plain keywords, so code reads close to how you would say it.

decide.nex

let age := 12

if age >= 18 then
  print("adult")
elseif age >= 13 then
  print("teen")
else
  print("child")
end

output

child

repeat.nex

across [10, 20, 30] as x do
  print(x)
end

repeat 3 do
  print("tick")
end

output

10
20
30
tick
tick
tick

choose.nex

let category := when age >= 18
  then "adult"
  else "minor"
end

case age of
  0, 1, 2 then print("small")
  3, 4, 5 then print("medium")
  else print("large")
end

REPL

> category
"minor"
large
-- when is an expression; case picks a branch

Classes and inheritance sit beside first-class functions, so you can pick the right tool for each problem.

counter.nex

class Counter
create
  make(start: Integer) do
    this.value := start
  end
feature
  value: Integer
  inc() do
    this.value := this.value + 1
  end
  current(): Integer do
    result := value
  end
end

REPL

> let c: Counter := create Counter.make(10)
> c.inc
> print(c.current)
11

animals.nex

class Animal
feature
  name: String
  speak do print(name) end
create
  named(name: String) do
    this.name := name
  end
end

class Dog
  inherit Animal
feature
  speak do
    print(name + " says woof")
  end
end

REPL

> let a: Animal := create Animal.named("Ko")
> a.speak
Ko
> let d: Animal := create Dog.named("Ki")
> d.speak
Ki says woof

functions.nex

function double(n: Integer): Integer
do
  result := n * 2
end

-- anonymous functions
let inc := fn (n: Integer): Integer do
  result := n + 1
end

REPL

> print(double(5))
10
> print(inc(10))
11

Nex compiles to JVM bytecode and can call any Java library. Ship one jar that runs anywhere Java does.

line_buffer.nex

import java.lang.StringBuilder

class Line_Buffer
create
  make() do
    with "java" do
      this.builder := create StringBuilder
    end
  end
feature
  builder: Any
  append_line(s: String) do
    with "java" do
      builder.append(s)
      builder.append("\n")
    end
  end
end

notes

-- import names a Java class
-- with "java" do … end runs host calls
--
-- keep host types behind a small Nex class,
-- so the rest of the program depends on a
-- clear contract, not on Java details.

script.nex

print("Hello from the JVM")

terminalstandalone jar

$ nex compile jvm script.nex ./build
# writes ./build/script.jar

terminal

$ java -jar ./build/script.jar
Hello from the JVM

notes

-- copy the jar to any machine with a JVM.
-- no Nex runtime needed there.

// get started

Install in one line.

Linux and macOS (on Windows, use WSL with Ubuntu). Then run nex to open the REPL.

Quick install

Installs the nex command system-wide, with REPL, formatter, docs and compiler tools. If they're missing, it also installs Java and the Clojure CLI, so it asks for your administrator password. It compiles Nex as it goes, which takes a minute or two.

bash <(curl -fsSL https://raw.githubusercontent.com/vijaymathew/nex/main/bootstrap-install.sh) jvm --install-deps

Without sudo

User-local install under ~/.local. Needs Java 17 or later already installed (ask an administrator); the installer fetches everything else. Open a new terminal afterwards so nex is on your PATH.

bash <(curl -fsSL https://raw.githubusercontent.com/vijaymathew/nex/main/bootstrap-install.sh) jvm --prefix "$HOME/.local" --install-deps

Editor support

An Emacs major mode ships in editor/emacs/nex-mode.el, with highlighting, indentation, Imenu and REPL integration (setup guide ↗). Support for other editors is planned.

Stuck, or have a question?

Ask in GitHub Discussions ↗. No question is too basic: installing, an error you don't understand, or an exercise from one of the books. The full install guide ↗ covers supported systems, uninstalling and troubleshooting.

Write code that keeps its promises with Nex

Small on purpose.

From "hello" to real systems.

Teaching programming or software engineering?

Install in one line.