erg 0.1.1

# The Erg Programming Language


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    <img width="500" src="./assets/erg_logo_with_slogan.svg">
</div>

<br>[Erg](https://mtshiba.github.io/TheErgBook) is a statically typed language that is Python compatible.

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    <img alt="Build status" src="https://github.com/erg-lang/erg/actions/workflows/rust.yml/badge.svg">
    <a href="https://opensource.org/licenses/Apache-2.0"><img alt="License: MIT & APACHE 2.0" src="https://img.shields.io/badge/license-Apache--2.0_OR_MIT-blue"></a><br>
    English | <a href='./README_JA.md'>日本語</a>
</p>

## Erg can be recommended to a person that&colon;


* uses Python, but want 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](./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 = import "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 10th 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()` in Python
    immut_arr = [1, 3, 2]
    assert immut_arr.sort() == [1, 2, 3]
    # Object-oriented style (mutable)
    mut_arr = ![1, 3, 2]
    mut_arr.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

    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: in order 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](https://www.python.org/) 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
```

## Contribution


Contributions are always welcome!
If you have any questions, please feel free to ask them on the [Discord channel](https://discord.gg/zfAAUbgGr4).

## License


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