codec.js

interface Format<a> {
  format:Format;
  defaultHashAlgorithm: string;
  encode(a):Promise<Bytes<a>>;
  decode(Bytes<a>):Promise<a>;
}

opaque type Bytes<a>:Uint8Array = Uint8Array
type BlockData<a> = Concat<Format, Bytes<a>>
opaque type Concat<a:Uint8Array, b:Uint8Array>:Uint8Array = Uint8Array

class Codec<a> {
  provider:Format<a>;
  constructor(provider:Format<a>) {
    this.provider = provider
  }
  async toBlock(data:a):Promise<BlockData<a>> {
    const bytes = await this.provider.encode(data)
    return multicodec.addPrefix(this.format, bytes)
  }
  fromBlock(block:BlockData<a>):Promise<a> {
    const bytes = multicodec.rmPrefix(block)
    return this.provider.decode(bytes)
  }
}