# PDL Compiler Specification

Given one or more PDL files representing a protocol, the compiler shall generate
a Go package named "protocol", which shall contain types for message and type
definitions, as well as encoding and decoding methods.

## Static Section

The compiler shall write a static section alongside the generated code. It
shall contain this text:

```go
// Table is a KTV table with an undefined schema.
type Table map[uint16] any

// Message is any message that can be sent along this protocol.
type Message interface {
	codec.Encodable
	codec.Decodable

	// Method returns the method code of the message.
	Method() uint16
}
```

## Preamble

At the start of each file but after the package name, the compiler shall emit
this text:

```go
/* # Do not edit this package by hand!
 * 
 * This file was automatically generated by the Holanet PDL compiler. The
 * source file is located at <path>
 * Please edit that file instead, and re-compile it to this location.
 *
 * HOPP, TAPE, METADAPT, PDL/0 (c) 2025 holanet.xyz
 */
```

Where `<path>` is the path of the protocol definition file relative to the
generated file.

## Message Definitions

For each defined message, the compiler shall generate a Go type named
`MessageName`, where `Name` is the name of the message as written in its
definition. The message shall be encodable, and shall have `Encode` and `Decode`
methods as described below.

All messages shall satisfy a `Message` interface, which is defined in the
static section.

## Type Definitions

For each defined type, the compiler shall generate a Go type with the same name
as written in its definition. The Go type shall be encodable, and shall have
`Encode` and `Decode` methods as described below.

## Encoding and Decoding Methods

Each encodable type shall be given an `Encode` method and a `Decode` method,
which will take in a `codec.Encoder` and a `codec.Decoder` respectively. Both
return an `(int, error)` pair describing the amount of bytes written and an
error if the write stopped early. `Encode` will encode the data within the
message to the given encoder, and `Decode` will decode data from the given
decoder and place it in the type's value. The methods shall not retain or close
any encoders or decoders they are given. Both methods shall have pointer
receivers. In effect, these methods will satisfy `codec.Encodable` and
`codec.Decodable`.

## Connection

The compiler shall generate a `Conn` struct which embeds a `hopp.Conn`, which
is the real "porcelain" of the generated code. It shall provide methods to
create and accept transactions. Each transaction shall be a struct which embeds
a `hopp.Trans`, and shall have methods for sending and receiving messages.

### Sending

To send a message along a transaction, the program shall:

1. Obtain the method code from the message
3. Obtain a writer from the connection using the method code
4. Wrap the writer in a `codec.Encoder`
5. Use the encoder to encode the message
6. Close the writer

### Receiving

To receiving a message from a transaction, the program shall:

1. Obtain a method code and reader from the connection
2. Wrap the reader in a `codec.Decoder`
3. Switch on the method code, and decode the correct message using the decoder
4. Return the message to the caller as a value

The recieve function must return the message as a value instead of a pointer in
order to avoid making an allocation. Because of this, the return value must be
`any` instead of `Message`. The caller must then use a type switch to figure out
what message was sent.