Noja/docs/language.md

# The Noja language

## Table of contents
1. [Introduction](#introduction)
2. [Implementation overview](#implementation-overview)
3. [The first program](#the-first-program)
4. [Expressions](#expressions)
5. [Branches](#branches)
6. [Loops](#loops)
7. [Functions](#functions)

## Introduction

  This language was written as a personal study of how interpreters
and compilers work. For this reason, the language is very basic.
  One of the main inspirations was the CPython's source code since
it's extremely readable and has a very simple and clean architecture.

  This file was intended for people who already program in other 
high level languages (such as Python, Javascript, Ruby) and don't 
need to be introduced to basic programming concepts (variables, 
expressions and branches). This way, there is more space for the 
comparison of the language's features with the mainstream languages.

## Implementation overview

  The interpreter works by compiling the provided source to a bytecode 
format and executing it. The bytecode is very high level since it
does things like:

  - explicitly referring to variables by name.

  - treating values as atomic things: from the perspective of the 
    bytecode, a list and an integer occupy the same space on the 
    stack, which is 1.

  - referring to instructions by their index.

For example, by compiling the following snippet

```py
define = true;

if define:
        a = 33;

print(a, '\n');
```

one would obtain the following bytecode:

```
	 0: PUSHTRU 
	 1: ASS     "define" 
	 2: POP     1 
	 3: PUSHVAR "define" 
	 4: JUMPIFNOTANDPOP 8 
	 5: PUSHINT 33 
	 6: ASS     "a" 
	 7: POP     1 
	 8: PUSHSTR "\n"
	 9: PUSHVAR "a" 
	10: PUSHVAR "print" 
	11: CALL    2 
	12: POP     1 
	13: RETURN

```

as you can see, there are instructions like `ASS` and `PUSHVAR` that
assign to and read from variables by specifying names, and jumps
that refer to other points of the "executable" by specifying indices
(like `JUMPIFNOTANDPOP`) instead of raw addresses.

  All values (objects) are allocated on a garbage-collected heap. 
For this reason all variables are simply references to these objects.
The garbage collection algorithm is a copy-and-compact one. It 
behaves as a bump-pointer allocator until there is space left, 
and when space runs out, it creates a new heap, copies all of the 
alive object into it, calls the destructors of the dead objects 
and frees the old one.

## The first program

The sintax is similar to Python's but is more C-like. A Noja script 
is a list of statements that can be of multiple kinds:

  - function declaractions
  - expressions
  - if-else branches
  - while loops
  - do-while loops
  - return statements
  - composit statements

In general, unless it's inside strings, whitespace is ignored and 
comments start with the `#` character. 

The most basic yet interesting program is:

```py
print('Hello, world!\n'); 
```

as in other languages, this kind of statement is an expression.
Expression statements require a ';' to determine their end.

The print function can take any number of arguments of any type
and doesn't add any spaces or newlines to the output. 

```py
print(1, 2, 3, true, '\n');
```

## Expressions

You can set variables without declaring them first by using the
assignment operator:
```py
a = 5;
```
which is similar to Python's assignment, but is a little different.
In this language, assignments are considered as expressions, in fact 
you can do things like
```py
a = (b = 1) + 1;

# The value resulting from an assignment is the assigned value.
# After this expression, b's value is 1 and a's value is 2. 

print('b = ', b, '\n'); # b = 1
print('a = ', a, '\n'); # a = 2
```
all of the basic arithmetic operators are available:
```py
x = 1 + 1;
y = 1 - 2;
z = 3 * 2;
w = 10 / 3;

print('x = ', x, '\n'); # x = 2
print('y = ', y, '\n'); # y = -1
print('z = ', z, '\n'); # z = 6
print('w = ', w, '\n'); # w = 3
```
Note how the division returns the rounded down version of the result.
This is because the division was performed on integers. By making one
of the operands a floating point value, also a floating point result
is returned:
```py
w = 10 / 3.0;

print('w = ', w, '\n'); 
```
Arithmetic operators are only available for numeric types of objects.
If you try to apply them on other kinds of types, you get a runtime
error.

Relational operators are also available:
```py
print(1 < 2, '\n');  # true
print(1 > 2, '\n');  # false

print(1 >= 0, '\n'); # true
print(1 <= 0, '\n'); # false

print(1 == 5, '\n'); # false
print(6 == 6, '\n'); # true

print(1 != 5, '\n'); # true
print(6 != 6, '\n'); # false
```
The equal and not equal operators are available on every type of object,
while the others are only available for numeric types.

## Branches

It's possible to make the execution of a statement optional, based on the
result of an expression. Like in other languages, you do this using if-else
statements:

```py
if 1 < 2:
	print('Took the branch!\n'); # This is executed!

if 1 > 2:
	print('Didn\'t take the branch\n'); # This isn't!
```
..or you can specify an alternative branch, which is executed when the 
condition isn't true:

```py
if 1 > 2:
	print('Not executed..\n');
else
	print('Executed!\n');
```
You can have multiple statements inside a branch by having them inside a 
compound statement. Compound statements are statement lists wrapped inside
curly brackets, like this:

```py
{ print('Hello from a '); print('compound statement!\n'); }
```
This way they count as one statement.

```py
if 1 == 1:
	{
		print('Executed\n');
		print('Also executed\n');
	}
```

Variables defined inside an if-else statement's branch are defined
in the parent's context. This implies that variables may or may not
be defined when you access them, based on which branch is taken.

```py
a = 1;

if a < 2:
	x = 100;

# Now x is defined, but if "a" were to be higher or equal to 2, it 
# wouldn't be defined and the runtime would return an error.
```

## Loops

Looping constructs are available in the form of while and do-while 
statements. The while statement checks the condition before each
iteration:

```py
i = 0;
while i < 10:
	i = i + 1;
```

This loop runs for 10 times. As for the if-else statement, a single
statement is expected as the body of the while statement. You can
provide it a compound statement tho.

```py
i = 0;
while i < 10:
	{
		print('While iteration no. ', i, '\n');
		i = i + 1;
	}
```

The do-while statement checks the condition at the end of each
iteration. This means that at least one iteration is performed!

```py
i = 0;
do
	{
		print('Do-while iteration no. ', i, '\n');
		i = i + 1;
	}
while i < 10;
```

Like for if-else statements, variables defined inside the loop
body are shared with the parent's context.

## Functions

Functions can be defined using the following syntax:

```py
# Define it
fun say_hello_to(name)
	print('Hello, ', name, '!\n\n');

# .. and then call it.
say_hello_to('Francesco');
```

Functions can have an arbitrary amount of arguments. If the function is
called with more arguments than it expected, the extra values are thrown
away. If the function is called with less arguments than it expected,
the argument set if filled up with none values.

```py
fun test_func(a, b, c)
	{
		print('a = ', a, '\n');
		print('b = ', b, '\n');
		print('c = ', c, '\n\n');
	}

test_func();
# a = none
# b = none
# c = none

test_func(1, 2);
# a = 1
# b = 2
# c = none

test_func(1, 2, 3);
# a = 1
# b = 2
# c = 3

test_func(1, 2, 3, 4);
# a = 1
# b = 2
# c = 3
```

Functions are actually variables like the ones that are be defined using
the assignment operator. In fact, you can reassign them new values if you
want.

```py
test_func = 5;

# The following line, if executed, returns an error because the test_func
# identifier is now associated to 5, which is not a function. 

test_func(); # Error!!
```

Functions can return values exactly like in other languages:

```py
fun multiply(x, y)
	return x * y;

p = 4;
q = 7;
r = multiply(p, q);

print(p, ' * ', q, ' = ', r, '\n');
```

If the function doesn't return any values, then the `none` value is returned.
As an example, the `print` function always returns `none`

```py
print(print()); # none
```

Functions are always "pure", in the sense that the only values that the
function body can access are the ones provided as arguments. Usually in
other languages, functions can access the global scope and the parent
scope (closures). There's no such mechanism in this language (at the 
moment).

The only exception is made for the "built in" variables, which are 
provided by the runtime of the language and can't be modified by the
user. The print function is one of these variables. One may override
these variables but the effect only lasts for the lifetame of the
context local to the assignment.

```py
# Overwrite the print variable inside the global 
# scope..
print = 5;

# The reference to the print function is lost 
# withing this scope.

fun test()
	{
		# If the previous assignment were to overwrite the 
		# print function globally, the next statement would 
		# fail because the value 5 isn't a function. But
		# it doesn't fail!
		print('Not overwritten here!\n');
	}

test();

# We can take the reference to the print function 
# by taking it from a function!

fun get_print_back()
	return print;

print = get_print_back();

print('Hei! Print is back!\n');
```