Aria Language

Aria is an expressive, interpreted, toy language built as an exercise on designing and interpreting a programming language. It has a noiseless syntax, free of useless semi colons, braces or parantheses, and treats everything as an expression. Technically, it's built with a hand written lexer and parser, a recursive decent one (Pratt), and a tree-walk interpreter. I have never set any goals for it to be either fast, nor bulletproof, so don't expect neither of them.

It features mutable and immutable values, if and switch conditionals, functions, type hinting, for loops, modules, the pipe operator, imports and many more. More importantly, it's getting expanded frequently with new features, more functions for the standard library and bug fixes. All of that while retaining it's expressiveness, clean syntax and easy of use.

var name = "aria language"
let expressive? = func (x: String) -> String
  if x != ""
    return "expressive " + x
  end
  "sorry, what?"
end

let pipe = name |> expressive?() |> String.capitalize()
println(pipe) // "Expressive Aria Language"

Usage
- Run a Source File
- REPL
Variables
- Constants
- Type Lock
Data Types
- String
- Atom
- Int
- Float
- Boolean
- Array
- Dictionary
- Nil
- Type Conversion
- Type Checking
Operators
- Shorthand Assignment
Functions
Conditionals
For Loop
Range Operator
Pipe Operator
Immutability
Modules
Imports
Comments
Standard Library

Usage

If you want to play with the language, but have no interest in toying with its code, you can download a built binary for your operating system. Just head to the latest release and download one of the archives.

The other option, where you get to play with the code and run your changes, is to go get github.com/fadion/aria and install it as a local binary with go install. Obviously, you'll need GOROOT in your path, but I guess you already know what you're doing.

Run a source file

To run an Aria source file, give it a path relative to the current directory.

aria run path/to/file.ari

REPL

As any serious language, Aria provides a REPL too:

aria repl

Variables

Variables in Aria start with the keyword var. Accessing an undeclared variable, in contrast with some languages, will not create it, but instead throw a runtime error.

var name = "John"
var married = false

var age = 40
age = 41

Names have to start with an alphabetic character and continue either with alphanumeric, underscores, questions marks or exclamation marks. When you see a question mark, don't confuse them with optionals like in some other languages. In here they have no special lexical meaning except that they allow for some nice variable names like is_empty? or do_it!.

Constants

Constants have the same traits as variables, except that they start with let and are immutable. Once declared, reassigning a constant will produce a runtime error. Even data structures are locked into immutability. Elements of an Array or Dictionary can't be added, updated or removed.

let name = "Ben"
name = "John" // runtime error

Type Lock

Type lock is a safety feature of mutable variables. Once they're declared with a certain data type, they can only be assigned to that same type. This makes for more predictable results, as an integer variable can't be assigned to a string or array. In this regard, Aria works as a strong typed language.

This will work:

var nr = 10
nr = 15

This won't:

var nr = 10
nr = "ten" // runtime error

Data Types

Aria supports 7 data types: String, Atom, Int, Float, Bool, Array, Dictionary and Nil.

String

Strings are UTF-8 encoded, meaning that you can stuff in there anything, even emojis.

let weather = "Hot"
let price = "円500"

String concatenation is handled with the + operator. Concats between a string and another data type will result in a runtime error.

let name = "Tony" + " " + "Stark"

Additionally, strings are treated as enumerables. They support subscripting and iteration in for in loops.

"howdy"[2] // "w"

Escape sequences are there too if you need them: \", \n, \t, \r, \a, \b, \f and \v. Nothing changes from other languages, so I'm sure you can figure out by yourself what every one of them does.

let code = "if(name == \"ben\"){\n\tprint(10)\n}"

Atom

Atoms, or symbols as some languages refer to them, are constants where the name is their value. Although they behave a lot like strings and can generally be interchanged, internally they are treated as their own type. As the language progresses, Atoms will be put to better use.

let eq = :dog == :cat
let arr = ["dog", :cat, :mouse]
let dict = [:name => "John", :age => 40]
let concat = "hello" + :world

They're interesting to use as control conditions, emulating enums as a fixed, already-known value:

let os = "linux"
switch os
case :linux
  println("FREE")
case :windows
  println("!FREE")
end

Int

Integers are whole numbers that support most of the arithmetic and bitwise operators, as you'll see later. They can be represented also as: binary with the 0b prefix, hexadecimal with the 0x prefix and octal with the 0o prefix.

let dec = 27
let oct = 0o33
let hex = 0x1B
let bin = 0b11011
let arch = 2 ** 32

A sugar feature both in Integer and Float is the underscore:

let big = 27_000_000

It has no special meaning, as it will be ignored in the lexing phase. Writing 1_000 and 1000 is the same thing to the interpreter.

Float

Floating point numbers are used in a very similar way to Integers. In fact, they can be mixed and matched, like 3 + 0.2 or 5.0 + 2, where the result will always be a Float.

let pi = 3.14_159_265
let e = 2.71828182

Scientific notation is also supported via the e modifier:

let sci = 0.1e3
let negsci = 25e-5

Bool

It would be strange if this data type included anything else except true and false.

let mad = true
let genius = false

Expressions like the if/else, as you'll see later, will check for values that aren't necessarily boolean. Integers and Floats will be checked if they're equal to 0, and Strings, Arrays and Dictionaries if they're empty. These are called truthy expressions and internally, will be evaluated to boolean.

Array

Arrays are ordered collections of any data type. You can mix and match strings with integers, or floats with other arrays.

let multi = [5, "Hi", ["Hello", "World"]]
let names = ["John", "Ben", 1337]

let john = names[0]
let leet = names[-1]

Individual array elements can be accessed via subscripting with a 0-based index:

let names = ["Kirk", "Bones", "Spock"]
let first = names[0] // "Kirk"
let last = names[-1] // "Spock"

In the same style, an index can be used to check if it exists. It will return nil if it doesn't:

if names[10]
  // handle it
end

Individual elements can be reassigned on mutable arrays:

var numbers = [5, 8, 10, 15]
numbers[1] = 7

Appended with an empty or placeholder index:

numbers[] = 100
numbers[_] = 200 // Same.

Arrays can be compared with the == and != operators, which will check the position and value of every element of both arrays. Equal arrays should have the same exact values in the same position.

They can also be combined with the + operator, which adds the element of the right side to the array on the left side.

let concat = ["an", "array"] + ["and", "another"]
// ["an", "array", "and", "another"]

Oh and if you're that lazy, you can ommit commas too:

let nocomma = [5 7 9 "Hi"]

Dictionary

Dictionaries are hashes with a key and a value of any data type. They're good to hold unordered, structured data:

let user = ["name" => "Dr. Unusual", "proffesion" => "Illusionist", "age" => 150]

I'd argue that using Atoms for keys would make them look cleaner:

let user = [:name => "Dr. Unusual", :proffesion => "Illusionist", :age => 150]

Unlike arrays, internally their order is irrelevant, so you can't rely on index-based subscripting. They only support key-based subscripting:

user["name"] // "Dr. Unusual"

Values can be reassigned or inserted by key on mutable dictionaries:

var numbers = ["one" => 1, "two" => 2]
numbers["one"] = 5
numbers["three"] = 3 // new key:value

To check for a key's existence, you can access it as normal and check if it's nil or truthy:

if user["location"] == nil
  // do smth
end

Nil

Aria has a Nil type and yes, I'm totally aware of its problems. This was a choice for simplicity, at least for the time being. In the future, I plan to experiment with optionals and hopefully integrate them into the language.

let empty = nil

Type Conversion

Converting between types is handled in a few ways that produce exactly the same results. The as operator is probably the more convenient and more expressive of the bunch. Like all type conversion methods, it can convert to String, Int, Float and Array:

let nr = 10
nr as String
nr as Int
nr as Float
nr as Array

Provided by the runtime are the appropriately named functions: String(), Int(), Float() and Array().

let str = String(10)
let int = Int("10")
let fl = Float(10)
let arr = Array(10)

The Type module of the Standard Library provides interfaces to those same functions and even adds some more, like Type.of() and Type.isNumber().

let str = Type.toString(10)
let int = Type.toInt("10")
let fl = Type.toFloat(10)
let arr = Type.toArray(10)

Which method you choose to use is strictly preferential and depends on your background.

Type Checking

There will be more than one occassion where you'll need to type check a variable. Aria provides a few ways to achieve that.

The is operator is specialized in checking types and should be the one you'll want to use practically everywhere.

let nr = 10
if nr is Int
  println("Yes, an integer")
end

There's also the typeof() runtime function and Type.of() from the Standard Library. They essentially do the same thing, but not only they're longer to write, but return strings. The above would be equivalent to:

if Type.of(nr) == "Int"
  println("Yes, an integer")
end

Operators

You can't expect to run some calculations without a good batch of operators, right? Well, Aria has a range of arithmetic, boolean and bitwise operators to match your needs.

By order of precedence:

Boolean: && || (AND, OR)
Bitwise: & | ~ (Bitwise AND, OR, NOT)
Equality: == != (Equal, Not equal)
Comparison: < <= > >=
Bitshift: << >> (Bitshift left and right)
Arithmetic: + - * / % ** (addition, substraction, multiplication, division, modulo, power)

Arithmetic expressions can be safely used for Integers and Floats:

1 + 2 * 3 / 4.2
2 ** 8
3 % 2 * (5 - 3)

Addition can be used to concatenate Strings or combine Arrays and Dictionaries:

"obi" + " " + "wan"
[1, 2] + [3, 4]
["a" => 1, "b" => 2] + ["c" => 3]

Comparison operators can compare Integers and Float by exact value, Strings, Arrays and Dictionaries by length:

5 > 2
3.2 <= 4.5
"one" < "three"
[1, 2] > [5]
["a" => 1] < ["b" => 2, "c" => 3]

Equality and inequality can be used for most data types. Integers, Floats and Booleans will be compared by exact value, Strings by length, Arrays by the value and position of the elements, and Dictionaries by the the combination of key and value.

1 != 4
1.0 != 2.5
true == true
"one" == "three"
[1, 2, 3] != [1, 2]
["a" => 1, "b" => 2] != ["a" => 5, "b" => 6]

Boolean operators can only be used with Boolean values, namely true or false. Other data types will not be converted to truthy values.

true == true
false != true

Bitwise and bitshift operator apply only to Integers. Float values can't be used, even those that "look" as Integers, like 1.0 or 5.0.

10 >> 1
12 & 5 | 3
5 ~ 2

Shorthand Assignment

Operators like +, -, * and / support shorthand assignment to variables. Basically, statements like this:

count = count + 1

Can be expressed as:

count += 1

Functions

Aria treats functions as first class, like any sane language should. It checks all the boxes: they can be passed to variables, as arguments to other functions, and as elements to data structures. They also support recursion, closures, currying, variadic parameters, you name it.

let add = func x, y
  x + y
end

Parantheses are optional and for simple functions like the above, I'd omit them. Calling the function needs the parantheses though:

let sum = add(1335, 2)

Type Hinting

Like in strong typed languages, type hinting can be a very useful feature to validate function arguments and its return type. It's extra useful for library functions that have no assurance of the data types they're going to get.

This function call will produce output:

let add = func (x: Int, y: Int) -> Int
  x + y
end
println(add(5, 2))

This however, will cause a type missmatch runtime error:

println(add(5, "two"))

Aria is not a strong typed language, so type hinting is completely optional. Generally, it's a good idea to use it as a validation measure. Once you enforce a certain type, you'll be sure of how the function executes.

Default Parameters

Function parameters can have default values, used when the parameters are omitted from function calls.

let architecture = func bits = 6
  2 ** bits
end

architecture() // 64
architecture(4) // 16

They can be combined with type hinting and, obviously, need to be of the same declared type.

let architecture = func bits: Int = 6
  2 ** bits
end

Return Statement

Until now we haven't seen a single return statement. Functions are expressions, so the last line is considered its return value. In most cases, especially with small functions, you don't have to bother. However, there are scenarios with multiple return points that need to explicitly tell the interpreter.

let even = func n
  if n % 2 == 0
    return true
  end
  false
end

The last statement doesn't need a return, as it's the last line and will be automatically inferred. With the if on the other hand, the interpreter can't understand the intention, as it's just another expression. It needs the explicit return to stop the other statements from being interpreted.

In the case of multiple return points, I'd advise to always use return, no matter if it's the first or last statement. It will make for clearer intentions.

Variadic

Variadic functions take an indefinite number of parameters and merge them all into a single, Array argument. Their first use would be as a sugar:

let add = func ...nums
  var count = 0
  for n in nums
    count = count + n
  end
  count
end

add(1, 2, 3, 4, 5) // 10

Even better, they can be used for functions that respond differently based on the number of arguments:

let structure = func ...args
  if Enum.size(args) == 2
    let key = args[0]
    let val = args[1]
    return [key: val]
  end
  if Enum.size(args) > 2
    return args
  end
  args[0]
end

structure("name", "John") // dictionary
structure(1, 2, 3) // array
structure(5) // integer

Functions may have as many parameters as needed, as long the variadic argument is the last parameter:

let calc = func mult, ...nums
  mult * Enum.reduce(nums, 0, func x, acc do x + acc end)
end
calc(10, 1, 2, 3, 4) // 100

Variadic arguments can even have default values:

let join = func (glue: String, ...words = ["hello", "there"])
  String.join(words, glue)
end

join(" ") // "hello there"

Arrow Functions

Very useful when passing short functions as arguments, arrow functions provide a very clean syntax. They're handled internally exactly like normal functions. The only difference is that they're meant as a single line of code, while normal functions can handle blocks.

This normal function:

let sub = func x
  x - 5
end

Is equivalent to:

let sub = (x) -> x - 5

They're not that useful to just spare a couple lines of code. They shine when passed as arguments:

Enum.map([1, 2, 3, 4], (x) -> x * 2)
Enum.reduce(1..10, 0, (x, acc) -> x + acc)

Closures

Closures are functions inside functions that hold on to values from the parent and "close" them when executed. This allows for some interesting side effects, like currying:

let add = func x
  func y
    x + y
  end
end

add(5)(7) // 12

Some would prefer a more explicit way of calling:

let add_5 = add(5) // returns a function
let add_5_7 = add_5(7) // 12

You could nest a virtually unlimited amount of functions inside other functions, and all of them will have the scope of the parents.

Recursion

Recursive functions calculate results by calling themselves. Although loops are probably easier to mentally visualize, recursion provides for some highly expressive and clean code. Technically, they build an intermediate stack and rewind it with the correct values in place when a finishing, non-recursive result is met. It's easier to understand them if you think of how they're executed. Let's see the classic factorial example:

let fac = func n
  if n == 0
    return 1
  end
  
  n * fac(n - 1)
end

Keep in mind that Aria doesn't provide tail call optimization, as Go still doesn't support it. That would allow for more memory efficient recursion, especially when creating large stacks.

Tricks

As first class, functions have their share of tricks. First, they can self-execute and return their result immediately:

let pow_2 = func x
  x ** 2
end(2)

Not sure how useful, but they can be passed as elements to data structures, like arrays and dictionaries:

let add = func x, y do x + y end
let list = [1, 2, add]
list[2](5, 7)

Finally, like you may have guessed from previous examples, they can be passed as parameters to other functions:

let add = func x, factor
  x + factor(x)
end
add(5, (x) -> x * 2)

Conditionals

Aria provides two types of conditional statements. The if/else is limited to just an if and/or else block, without support for multiple else if blocks. That's because it advocates the use of the much better looking and flexible switch statement.

If

An if/else block looks pretty familiar:

if 1 == 2
  println("Not calling me.")
else
  println("1 isn't equal to 2. Duh!")
end

Sometimes it's useful to inline it for simple checks:

let married = true
let free_time = if married then 0 else 100_000_000 end

Ternary Operator

The ternary operator ?: is a short-hand if/else, mostly useful when declaring variables based on a condition or when passing function parameters. It's behaviour is exactly as that of an if/else.

let price = 100
let offer = 120
let status = offer > price ? "sold" : "bidding"

Although multiple ternary operators can be nested, I wouldn't say that would be the most readable code. Actually, except for simple checks, it generally makes for unreadable code.

Switch

Switch expressions on the other hand are way more interesting. They can have multiple cases with multiple conditions that break automatically on each successful case, act as generic if/else, and match array elements.

let a = 5
switch a
case 2, 3
  println("Is it 2 or 3?")
case 5
  println("It is 5. Magic!")
default
  println("No idea, sorry.")
end

Not only that, but a switch can behave as a typical if/else when no control condition is provided. It basically becomes a switch true.

let a = "John"
switch
case a == "John"
  println("John")
case a == "Ben"
  println("Ben") 
default
  println("Nobody")
end

Pattern Matching

When fed arrays as the control condition, the switch can pattern match its elements. Every argument to the switch case is compared to the respective element of the array. Off course, for a match, the number of arguments should match the size of the array.

switch ["game", "of", "thrones"]
case "game", "thrones"
  println("no match")
case "game", "of", "thrones"
  println("yep!")
end

That's probably useful from time to time, but it's totally achievable with array cases. The switch can do much better than that.

switch ["John", "Lick", 2]
case "John", _, _
  println("John Something")
case _, _ 2
  println("Something 2")
default
  println("Lame movie pun not found")
end

The _ is a placeholder that will match any type and value. That makes it powerful to compare arrays where you don't need to know every element. You can mix and match values with placeholders in any position, as long as they match the size of the array.

For Loop

Aria takes a modern approach to the for loop, evading from the traditional, 3-parts for we've been using for decades. Instead, it focuses on a flexible for in loop that iterates arrays, dictionaries, and as you'll see later, ranges.

for v in [1, 2, 3, 4]
  println(v)
end

Obviously, the result of the loop can be passed to a variable, and that's what makes them interesting to manipulate enumerables.

let plus_one = for v in [1, 2, 3, 4]
  v + 1
end
println(plus_one) // [2, 3, 4, 5]

Passing two arguments for arrays or strings will return the current index and value. For dictionaries, the first argument will be the key.

for i, v in "abcd"
  println(i + "=>" + v)
end

for k, v in ["name" => "John", "age" => 40]
  println(k)
  println(v)
end

With that power, you could build a function like map in no time:

let map = func x, f
  for v in x
    f(v)
  end
end

let plus_one = map([1, 2, 3, 4], (x) -> x + 1)

println(plus_one) // [2, 3, 4, 5]

Without arguments, the for loop can behave as an infite loop, much like a traditional while. Although there's not too many usecases, it does its job when needed. An example would be prompting the user for input and only breaking the infinite loop on a specific text.

for
  let pass = prompt("Enter the password: ")
  if pass == "123"
    println("Good, strong password!")
    break
  end
end

The break and continue keywords, well break or skip the iteration. They function exactly like you're used to.

for i in 1..10
  if i == 5
    continue
  end
end

The for loop is currently naively parsed. It works for most cases, but still, it's not robust enough. I'm working to find a better solution.

Range Operator

The range operator is a special type of sugar to quickly generate an array of integers or strings.

let numbers = 0..9
let huge = 999..100
let alphabet = "a".."z"

As it creates an enumerable, it can be put into a for in loop or any other function that expects an array.

for v in 10..20
  println(v)
end

Although its bounds are inclusive, meaning that the left and right expressions are included in the generated array, nothing stops you from doing calculations. This is completely valid:

let numbers = [1, 2, 3, 4]
for i in 0..Enum.size(numbers) - 1
  println(i)
end

Pipe Operator

The pipe operator, inspired by Elixir, is a very expressive way of chaining functions calls. Instead of ugly code like the one below, where the order of operations is from the inner function to the outers ones:

subtract(pow(add(2, 1)))

You'll be writing beauties like this one:

add(2, 1) |> pow() |> substract()

The pipe starts from left to right, evaluating each left expression and passing it automatically as the first parameter to the function on the right side. Basically, the result of add is passed to pow, and finally the result of pow to substract.

It gets even more interesting when combined with standard library functions:

["hello", "world"] |> String.join(" ") |> String.capitalize()

Enumerable functions too:

Enum.map([1, 2, 3], (x) -> x + 1) |> Enum.filter((x) -> x % 2 == 1)

// or even nicer

[1, 2, 3] |> Enum.map((x) -> x + 1) |> Enum.filter((x) -> x % 2 == 1)

Such a simple operator hides so much power and flexibility into making more readable code. Almost always, if you have a chain of functions, think that they could be put into a pipe.

Immutability

Now that you've seen most of the language constructs, it's time to fight the dragon. Immutability is something you may not agree with immediately, but it makes a lot of sense the more you think about it. What you'll earn is increased clarity and programs that are easier to reason about.

Iterators are typical examples where mutability is seeked for. The dreaded i variable shows itself in almost every language's for loop. Aria keeps it simple with the for in loop that tracks the index and value. Even if it looks like it, the index and value aren't mutable, but instead arguments to each iteration of the loop.

let numbers = [10, 5, 9]
for k, v in numbers
  println(v) 
  println(numbers[k]) // same thing
end

But there may be more complicated scenarios, like wanting to modify an array's values. Sure, you can do it with the for in loop as we've seen earlier, but higher order functions play even better:

let plus_one = Enum.map([1, 2, 3], (x) -> x + 1)
println(plus_one) // [2, 3, 4]

What about accumulators? Let's say you want the product of all the integer elements of an array (factorial) and obviously, you'll need a mutable variable to hold it. Fortunately we have reduce:

let product = Enum.reduce(1..5, 1, (x, acc) -> x * acc)
println(product)

Think first of how you would write the problem with immutable values and only move to mutable ones when it's impossible, hard or counter-intuitive. In most cases, immutability is the better choice.

Modules

Modules are very simple containers of data and nothing more. They're not an imitation of classes, as they can't be initialized, don't have any type of access control, inheritance or whatever. If you need to think in Object Oriented terms, they're like a class with only static properties and methods. They're good to give some structure to a program, but not to represent cars, trees and cats.

module Color
  let white = "#fff"
  let grey = "#666"
  let hexToRGB = func hex
    // some calculations
  end
end

let background = Color.white
let font_color = Color.hexToRGB(Color.grey)

Because modules are interpreted and cached before-hand, properties and functions have access to each other. In contrast to modules, everything else in Aria is single pass and as such, it will only recognize calls to a module that has already been declared.

Imports

Source file imports are a good way of breaking down projects into smaller, easily digestible files. There's no special syntax or rules to imported files. They're included in the caller's scope and treated as if they were originally there. Imports are cached, so in multiple imports, only the first one is actually interpreted.

// cat.ari
let name = "Bella"
let hi = func x
  "moew " + x
end

// main.ari
import "cat"

let phrase = name + " " + hi("John")
println(phrase) // "Bella moew John"

The file is relatively referenced from the caller and in this case, both main.ari and dog.ari reside in the same folder. As the long as the extension is .ari, there's no need to write it in the import statement. Even the quotes can be omited and the file written as an identifier, as long as it doesn't include a dot (as in cat.ari) and isn't a reserved keyword.

A more useful pattern would be to wrap imported files into a module. That would make for a more intuitive system and prevent scope leakage. The cat case above could be written simply into:

// cat.ari
module Cat
  let name = "Bella"
  let hi = func x
    "moew " + x
  end
end

// main.ari
import cat

let phrase = Cat.name + " " + Cat.hi("John")

Imports are expressions too! Technically, they can be used anywhere else an Integer or String can, even though it probably wouldn't make for the classiest code ever.

// exp.ari
let x = 10
let y = 15
x + y

// main.ari
let value = import exp
println(value) // 25

if import exp == 25
  println("Yay")
end

Comments

Nothing ground breaking in here. You can write either single line or multi line comments:

// an inline comment
/*
  I'm spanning multiple
  lines.
*/

Standard Library

The Standard Library is fully written in Aria with the help of a few essential functions provided by the runtime. That is currently the best source to check out some "production" Aria code and see what it's capable of. Read the documentation.

Future Plans

Although this is a language made purely for fun and experimentation, it doesn't mean I will abandon it in it's first release. Adding other features means I'll learn even more!

In the near future, hopefully, I plan to:

Improve the Standard Library with more functions.
~~Support closures and recursion~~.
~~Add a short syntax for functions in the form of: (x) -> x~~.
~~Add importing of other files~~.
~~Add the pipe operator!~~
Support optional values for null returns.
Write more tests!
Write some useful benchmarks with non-trivial programs.

Credits

Aria was developed by Fadion Dashi, a freelance web and mobile developer from Tirana.

The implementation is based on the fantastic Writing an Interpreter in Go. If you're even vaguely interested in interpreters, with Golang or not, I highly suggest that book.

The reader.Buffer package is a "fork" of Golang's bytes.Buffer, in which I had to add a method that reads a rune without moving the internal cursor. I hate doing that, but unfortunately couldn't find a way out of it. That package has its own BSD-style license.

Cheap and reliable Node.js hosting starts at $3/month, and $1/month static HTML hosting

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