# PDL Language Definition

PDL allows defining a protocol using HOPP and TAPE.

## Data Types

| Syntax   | TN      | CN | Description
| -------- | ------- | -: | -----------
| I5       | SI      |    |   
| I8       | LI      |  0 | 
| I16      | LI      |  1 | 
| I32      | LI      |  3 | 
| I64      | LI      |  7 | 
| I128[^2] | LI      | 15 | 
| I256[^2] | LI      | 31 | 
| U5       | SI      |    | 
| U8       | LI      |  0 | 
| U16      | LI      |  1 | 
| U32      | LI      |  3 | 
| U64      | LI      |  7 | 
| U128[^2] | LI      | 15 | 
| U256[^2] | LI      | 31 | 
| F16      | FP      |  1 | 
| F32      | FP      |  3 | 
| F64      | FP      |  7 | 
| F128[^2] | FP      | 15 | 
| F256[^2] | FP      | 31 | 
| String   | SBA/LBA |  * | UTF-8 string
| Buffer   | SBA/LBA |  * | Byte array
| []<TYPE> | OTA     |  * | Array of any type[^1]
| Table    | KTV     |  * | 

[^1]: Excluding SI and SBA. I5 and U5 cannot be used in an array, but String and
Buffer are simply forced to use their "long" variant.

[^2]: Some systems may lack support for this.

## Tokens

| Name     | Syntax                |
| -------- | --------------------- |
| Method   | `M[0-9A-Fa-f]{4}`     |
| Key      | `[0-9A-Fa-f]{4}`      |
| Ident    | `[A-Za-z][A-Za-z0-9]` |
| Comma    | `,`                   |
| LBrace   | `{`                   |
| RBrace   | `}`                   |
| LBracket | `[`                   |
| RBracket | `]`                   |

## Syntax

Types are expressed with an Ident. A table can be used by either writing the
name of the type (Table), or by defining a schema with curly braces. Arrays must
be expressed using two matching square brackets before their element type.

A table schema contains comma-separated fields in-between its braces. Each field
has three parts: the key number (Key), the field name (Ident), and the field
type. Tables, Arrays, etc. can be nested.

Files directly contain messages and types, which start with a Method token and
an Ident token respectively. A message consists of the method code (Method), the
message name (Ident), and the message's root type. This is usually a table, but
can be anything.

Here is an example of all that:

```
M0000 Connect {
	0000 Name String,
	0001 Password String,
}

M0001 UserList {
	0000 Users []User,
}

User {
	0000 Name      String,
	0001 Bio       String,
	0002 Followers U32,
}
```