Contracts in source

Informative guide. This Book page explains Beskid but does not define the standard. For normative requirements, use the Beskid Standard.

A Beskid contract is a structural interface: required members, optional embeddings, checked at compile time when a type lists conformances.

Declaration

contract Disposable
{
    unit Dispose();
}

type Logger : Disposable
{
    unit Dispose() { /* ... */ }
}

Normative detail: Contracts.

Conformance lists

Types declare type Name : I, J { ... }. The compiler checks every required member (E1601–E1607). Conflicting embeddings from two contracts must error: no C#-style diamond denial as a lifestyle.

Embeddings

contract Readable
{
    i32 Read(ref u8 buffer);
}

contract ReadWrite : Readable
{
    i32 Write(ref u8 buffer);
}

Embedding flattens requirements; implementers satisfy the expanded surface once.

The corelib byte-stream contracts show the pattern. Stream lists the read, write and close members, and TcpStream in the network package conforms to it together with the single-purpose contracts.

classDiagram
  accTitle: Core.IO contracts and a conforming type
  accDescr: The Reader, Writer, Closer and Disposable contracts each declare one operation, Stream declares read, write and close, and TcpStream lists all of them as conformances.
  class Reader {
    <<contract>>
    Read(destination, offset, count) Result
  }
  class Writer {
    <<contract>>
    Write(source, offset, count) Result
  }
  class Closer {
    <<contract>>
    Close() Result
  }
  class Stream {
    <<contract>>
    Read(destination, offset, count) Result
    Write(source, offset, count) Result
    Close() Result
  }
  class Disposable {
    <<contract>>
    Dispose() Result
  }
  class TcpStream {
    word handle
  }
  TcpStream ..|> Stream
  TcpStream ..|> Reader
  TcpStream ..|> Writer
  TcpStream ..|> Closer
  TcpStream ..|> Disposable

Text equivalent: Reader, Writer, Closer and Disposable are one-operation contracts. Stream declares read, write and close. TcpStream lists Stream, Reader, Writer, Closer and Disposable in its conformance list.

Not Mod SDK contracts

Surface	Purpose
contract Foo { ... } in user code	Type conformance, static dispatch
Beskid.Compiler.* mod contracts	Compile-time plugins (Compiler Mod SDK)

Contracts vs classes

Contracts describe capabilities, not inheritance trees. Prefer small contracts composed with embeddings over one “god interface” copied from enterprise slides.

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Effects and purity