platform-abstraction

Platform Abstraction

Demonstrates how to write portable code using abstract extern declarations with platform-specific implementations.

Key Concepts

Abstract Declarations

Declare an interface with kind="extern-abstract":

snippet id="console.println" kind="extern-abstract"

effects
  effect console
end

signature
  fn name="println"
    param name="msg" type="String"
    returns type="Unit"
  end
end

end

Platform Implementations

Provide platform-specific implementations with kind="extern-impl":

snippet id="console.println.browser" kind="extern-impl"
  implements="console.println" platform="browser"

metadata
  contract="@browser/console.log"
end

end

Supported platforms: browser, node, wasi

Compiler Selection

User code calls the abstract ID - the compiler selects the appropriate implementation based on the compilation target:

step id="s1" kind="call"
  fn="console.println"    // Always use abstract ID
  arg name="msg" from="name"
  as="_"
end

Compile with: covenant compile --target=browser ...

When to Use

• Console/logging across platforms
• File system access (Node.js vs WASI)
• HTTP clients (fetch vs Node http)
• Any capability that differs by runtime

See Also

• using-bindings/ - Basic extern usage patterns
• extern-bindings/ - Extern declaration syntax