# Overview

TODO

This is an example of a program:

```
def fib(n: u16): u16 {
    if n < 2 {
        return n;
    } else {
        return fib(n - 1) + fib(n - 2);
    }
}

fib(20); # 6765
```

## Programs

A program is a sequence of:

 * function declarations
 * statements
 * TODO: if-else, for, for-in, etc; with no semicolon

Statements are delimited with semicolons (`;`), and are one of:

 * expression (e.g. `1+2`)
 * function call
 * lambda definition
 * variable
 * variable declaration
 * return statement

XXX: do we really want expressions without effects?

TODO: entry point; `main` to be C compatible?

## Modules

A `micro2` file must start with the module name.
```
# our main module (this is a comment)
module main
```

The module name is used by the linker.

TODO: "import" and use modules.

## Variables

Variable declaration:
```
var a: u8 = 123;
```

Group declaration:
```
var (
    a: u8 = 123,
    b: u16 = 1234
);
```

Variables must be initialized, there are not default values, with the exception of structures that is optional.

Variables can refer to a memory address with `@` operator and the data on that address will be used as initialization:
```
var p: u8 = @0x8000;

p; # whatever byte is in address 0x8000 (peek)
p = 0; # byte at 0x8000 is now 0 (poke)
```

### Constants

Constant are immutable values and no memory is allocated for them:
```
const K: u8 = 10;
```

Group declaration:
```
const (
    A: u8 = 128,
    B: u16 = 4096
);
```

Must be resolved at compilation time.
```
var a: u8 = 1;

const A: u8 = a + 1; # error: unresolved value
```

## Built-in types

### Integers

| Type | Description | Samples |
| --- | --- | --- |
| u8 | unsigned 8-bit | `123; 0xce; 0b10000;` |
| s8 | signed 8-bit | `-123;` |
| u16 | unsigned 16-bit | `65535; 0xffff;` |
| s16 | signed 16-bit | `-4096;` |

Built-in functions:

| Function | Description | Samples |
| --- | --- | --- |
| hi | Get the MSB on a 16-bit number | `hi(0xaabb); # 0xaa`|
| lo | Get the LSB on a 16-bit number | `lo(0xaabb); # 0xbb`|

`hi` and `lo` can also be used with references (functions, structures and arrays):
```
def fn() { return; }

hi(fn); # MSB of fn address
```

Type conversion is explicit:
```
var a: u8 = 10;
var b: u16 = 10;

a + b; # error: type mismatch
u16(a) + b; # 20: u16
```

### Booleans

Logic operators result on a boolean type.

| Type | Description | Samples |
| --- | --- | --- |
| bool | boolean | `true; false; 1 == 1;` |

### Functions

| Type | Description | Samples |
| --- | --- | --- |
| ([parameters]) -> [return] | functions | `(a: u8) -> u8 { return a + 1; }` |
| ([parameters]) | functions (no return value) | `() { return; }` |

Functions can be declared with `def` when they have a name, or as anonymous using the lambda syntax.

```
def add(a: u8, b: u8): u8 {
    return a + b;
}

add(2, 8); # 10

```

Anonymous function:
```
# increment the value passed as argument
(a: u8): u8 {
    return a + 1;
}(10); # 11

```

Function are higher order functions:
```
def apply(fn: (u8) -> u8, a: u8, b: u8): u8 {
    return fn(a, b);
}

apply(add, 2, 10); # 8

# using a lambda
apply((a: u8, b: u8): u8 {
        return a + b;
    },
    2, 8); # 10

# lambdas can be assigned to variables
var double: (u8) -> u8 = (a: u8): u8 {
    return a + a;
};

double(10); # 20
```

Anonymous functions can only access local variables (closures aren't supported):
```
def closure(a: u8): () -> u8 {
    return (): u8 {
        return a; # invalid return, undefined a
    };
}
```

Functions are exported by default, unless they are defined as private:
```
# not exported (exported is the default)
private def dec(a: u8): u8 {
    return a - 1;
}
```

Function variables use references:
```
def fn() { return; }

# fn and fn2 refer to the same function
var fn2: () = fn;
```

### Special value: nil

`nil` is a reference not pointing to a value, used for example on variables with type function or structure.

Using a *nil* reference will result in a runtime error.

```
# fn doesn't hold a reference
var fn: (u8) -> u8 = nil;

fn(10); # runtime error
```

### Structures

Structures can be used to group data and functions.

Structure can be declared with `def` and provide values for the grouped data. Those values will be used when allocating an instance.

Any variable or function declared in the structure can be accessed like a local variable inside the structure, and in the instances using the dot (`.`) operator.

```
def A {
    # constants can be part of an structure as well
    const INC: u8 = 1;

    var (
        n: u8 = 100,
        m: bool = false,
        dec: (u8) -> u8 = nil
    );

    # it is possible to define functions local
    # to a structure
    def inc(): u8 {
        n = n + INC;
        return n;
    }
}

# instantiate structure A
var a: A;

a.n; # 100
a.m; # false
a.dec; # nil

a.inc(); # 101
a.inc(); # 102

a.n; # 102
```

Variables of type structure handle a reference:

```
var b: A = a;

# b points to the same data as a
b.inc(); # 103
a.n; # 103

# c doesn't hold a reference to an instance of A
var c: A = nil;
```

Recursive structures are not supported and local structures can't be used as return value in a function.

### Arrays

Arrays are zero based and are supported for all types.

XXX: including arrays? e.g. `[10][10]u8`.

Array size is a numeric literal or a constant expression (must be known at compilation type), and all the elements on an array must be of the same type.

Arrays are initialised to literals with `[` and `]` providing a list of values.
```
# array of 5 u8
var arr: [5]u8 = [0, 0, 0, 0, 0];
```

It is possible so initialize the array providing one single value that will be used for all the elements:
```
# this is equivalent to the previous example using [ and ]
var arr: [5]u8 = 0;
```

The array size is optional when initializing using literals:
```
var xs: []bool = [true, true, false];

len(xs); # 3

var xs2: []bool = true; # error: missing array size
```

Array elements can be accessed using `[]`:
```
var arr: [5]u8 = 0;

arr[0]; # 0
arr[0] = 100;
arr[0]; # 100
```

Variables of type array handle a reference.
```
# arr and arr2 refer to the same array
var arr2: [5]u8 = arr;
```

Local arrays can't be used as return value in a function.
```
def fn(): [5]u8 {
    var local [5]u8 = 0;
    return local; # error: returning a local value of type array
}
```

Built-in functions:

| Function | Description | Samples |
| --- | --- | --- |
| len | get the length of an array as u16 | `len(arr); # 5`|

### Strings

TODO: a zero ended array of u8 with special initializers

## Operators

TODO

## Flow control

TODO

* if-else
* loops (while-like, infinite, break, continue)
* for-in

## External functions

TODO

* is this "import"?
* calling conventions?
* namespace support?

## In-line ASM

TODO