System Injection

Side effect management in Pen

@raviqqe

What is System Injection?

• Pen has a feature called System Injection.
• It manages side effects of functions.
• It injects system functions into whole applications.
  • System functions are passed as arguments of main functions to applications.
• No other functions can have side effects by themselves; side effects are always injected through function arguments.

What is it based on?

• Dependency injection
• Monads
• Effect systems
• Clean architecture

Benefits of system injection

It improves maintainability and portability of software.

• Unit tests are deterministic.
  • No more slow or flaky tests!
• Better software architecture
  • Application logic doesn't depend on implementation details directly.
  • They are decoupled and changeable independently.
  • It's easier to port applications isolated from platform-dependent codes.

Costs of System Injection

• Extra cognitive load
  • Enforcement of dependency injection
  • Context arguments
• In the short term, it doesn't pay for the cost...
  • More like investment
  • The same as unit tests, or any other software engineering methodologies
  • Pen is not for scripting, or software with short expectancy in general.

print = \(ctx Context, s string) none | error {
  File'Write(ctx, File'StdOut(), s)?

  none
}

Common questions

How do we run nondeterministic tests?

1. Replace nondeterministic codes with deterministic codes.
2. List up examples as separate tests.
   • In other words, choose the timelines.

Common questions

Why not effect systems (or monads)?

• Modern researches propose "statically provable" effect systems.
  • As extensions of type systems
• They incur extra cognitive costs for developers to understand and use it.
• System Injection is rather a dynamically typed effect system.
• Also, they do not work well with programming languages without generics.
  • Like Go
  • High-order functions always need to have two versions for pure and impure implementations.
• Functional programming | Clojure
  • Clojure is impure, in that it doesn’t force your program to be referentially transparent, and doesn’t strive for 'provable' programs.

Running integration tests

• Deterministic tests are good but that doesn't mean we don't need to test E2E.
• Currently, Pen has no way to run integration tests.
• pen test --integration?
• It's also possible to delegate them to third party tools.

Summary

• System Injection gives us:
  • Deterministic, fast, reliable unit tests.
  • Clean software architecture
• Integration tests are WIP.
• Feedback is welcome!