Introduction

mycl lets you modify a TypeScript function’s behavior without touching its declaration or its call sites. A capability is a wrapped function whose behavior a caller can re-bind per scope: swapping, wrapping or layering functionality transparently to the call site. Once declared, a capability becomes its own registry key, type contract and default implementation.

When you call a capability, it dispatches through its active scope and resolves its own binding there. The scope’s registries compose when you activate them through a mycl() factory call, bound to a private channel that encapsulates your capabilities.

What mycl is not

Not a DI container. DI containers wire a graph of objects at startup and hand you instances by type or token. mycl binds behavior to a scope on the call stack; there is no container, no lifetime, and no construction graph. You re-bind functions, not assemble services.

Not an effect system. It does not track effects in the type system, thread a runtime, or ask you to restructure programs around a monad. A capability is a plain function; calling it looks like calling any function.

Not monkey-patching. Monkey-patching mutates a shared module export for the whole process. A registry binding is immutable, and it only applies inside the scope you activate. The same capability keeps its default in every other scope.

Who it’s for

Library authors who want extension points without designing a plugin API. Wrap the functions you want consumers to be able to change in capable(), and the capability surface is the extension surface: consumers write bindings instead of asking you for new config knobs.

Application developers who need behavior that varies by context: tests that swap I/O without mocks, per-request flags on a server, or platform-specific implementations bound once at the entry point instead of branching throughout the code.

Where next

• Installation: add mycl and set up the ./capabilities convention.
• Quick Start: 60 seconds to your first override.
• Capabilities: the full story on defining and calling them.
• Prior Art: the deeper design rationale behind the mechanism.