The Erg Programming Language

Erg is a statically typed language that is Python-compatible.

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Erg can be recommended to a person that:

• uses Python, but wants Rust-like robustness and comfortable compiler support.
• and yet, doesn't need the verbose type specifications & memory management model like Rust.
• wants a simple and consistent language like ML.
• wants a practical general-purpose language with dependent/refinement types.
• wants a language like Scala that can be used both object-oriented and functional.

Features

Some features are not yet implemented. Please see TODO.md for implementation status.

1. Robustness

   Erg has a smart & powerful type system. For example, Erg can do null checking (Option type), division by zero, and out-of-range addresses in arrays at compile time.

   ```python rand = pyimport "random"

   l = [1, 2, 3] assert l in [Nat; 3] # type checking assert l in [1..3; 3] # more detailed l2 = l.push(rand.choice! 0..10) assert l2 in [0..10; 4] assert l2 + [3, 5, 7] in [0..10; 7]

   This causes an IndexError, Erg can detect it at compile time

   l2[10] # IndexError: l2 has 7 elements but was accessed the 11th element

   2.times! do!: print! "hello, ", end: ""

   => hello, hello,

   -2.times! do!: print! "hello, ", end: ""

   TypeError: .times! is a method of Nat (0 or more Int), not Int

   {Meter; Sec; meter; yard; sec; ...} = import "unit"

   velocity x: Meter, t: Sec = x / t

   v = velocity 3yard, 2sec # TypeError: the type of x was mismatched: expect Meter, found Yard v = velocity 3meter, 2sec # v == 1.5 m/s ```

2. Simplicity

   Erg consists of a very simple syntax, which can significantly reduce the amount of code compared to other languages. However, its functionality is not inferior to them.

   Since the type inference system is powerful, you can code like a dynamically typed language.

   python fib 0 = 0 fib 1 = 1 fib n = fib(n - 1) + fib(n - 2) assert fib(10) == 55

   In Erg, there are very few things that are treated as special; there are no reserved words. even for and while expressions are just one of the subroutines, so this is possible.

   ```python loop! block = while! True, block

   equals to while! True, do! print! "hello"

   loop! do!: print! "hello" ```

3. Functional & Object-oriented

   Erg is a pure object-oriented language. Everything is an object; types, functions, and operators are all objects. On the other hand, Erg is also a functional language. Erg requires some kinds of markers to be placed on code that causes side effects or changes internal state, which can localize the complexity of code. This will greatly improve the maintainability of your code.

   ```python

   Functional style (immutable), same as sorted(list) in Python

   immutarr = [1, 3, 2] assert immutarr.sort() == [1, 2, 3]

   Object-oriented style (mutable)

   mutarr = ![1, 3, 2] mutarr.sort!() assert mut_arr == [1, 2, 3] i = !1 i.update! old -> old + 1 assert i == 2

   Functions cannot cause side effects

   inc i: Int! = i.update! old -> old + 1

   SyntaxError: cannot call a procedural method in a function

   hint: only methods of mutable types can change the state of objects

   Code that uses a lot of side effects is redundant, so you will naturally write pure code

   Counter! = Inherit Int! Counter!. new i: Int = Self!::new !i inc! ref! self = self.update! old -> old + 1

   c = Counter!.new 1 c.inc!() assert c == 2 ```

4. Interoperability

   Erg is internally compatible with Python and can import the Python API at zero cost.

   ```python

   using built-in Python modules

   math, time = pyimport "math", "time" {sin; pi; ...} = math

   using an external Python module

   Tqdm! = pyimport("tqdm").'tqdm'

   print! sin pi # 1.2246467991473532e-16 for! Tqdm!.'call'(0..99), i => time.sleep! 0.01 * i ```

5. Readable Error Messages

   Erg emphasizes the readability of error messages; Erg is a programmer-friendly language, ~~unlike C++.~~

   python proc! x = l = [1, 2, 3] l.push!(x) l

   console Error[#12]: File example.er, line 3, in <module>::proc! 2│ l = [1, 2, 3] 3│ l.push!(x) ^^^^^ AttributeError: Array object has no attribute `.push!` hint: to update the internal state of an object, make it mutable by using `!` operator hint: `Array` has `push`, see https://erg-lang.github.io/docs/prelude/Array/##push for more information hint: `Array!` has `push!`, see https://erg-lang.github.io/docs/prelude/Array!/##push! for more information

Requirements

A Python3 interpreter is required. If it is already installed on your machine, no setup is required.

Installation

Installing by cargo (Rust package manager)

sh cargo install erg

Building from source

Building from source code requires the Rust toolchain.

sh git clone https://github.com/erg-lang/erg.git cd erg cargo build --release

Building by Nix

If you've been installed Nix, the following command will be generate binary into result/bin/erg under the project.

sh git clone https://github.com/erg-lang/erg.git cd erg nix-build

If you've been enabled Nix Flakes.

sh git clone https://github.com/erg-lang/erg.git cd erg nix build

Contribution

Contributions are always welcome! If you have any questions, please feel free to ask them on the Discord channel.

License

Erg is distributed under the terms of both the MIT license and the Apache License (Version 2.0). See LICENSE-APACHE, LICENSE-MIT for details.