My Programming Principles in 2020

Abstract#

This is rough brain dump of several principles/ideas that I applied in 2020 while working at Uber.

The fact that the code runs and does what you want means very little#

• There are several other aspects of code that matter critically in a professional environment.

Correctness composes#

• It's easier to prove that a small thing is correct.
  • So write small classes; glue them together.
    • A small class is easy to test, change, reason about, delete, refactor.
    • The hard part is picking What it does and its Name, of course!
  • If the glue is correct, then everything is correct.
• To prove that something is correct:
  • Try all inputs
    • If there are too many combinations (N*M inputs)
    • Then use Types to constrain the domains! (smaller N * smaller M)
    • Or if you split the responsibility (if possible) you get N+M
  • Exercise all code paths
    • If there are too many combinations (N*M paths)
    • Then extract the inner part to a class. Now you have N+M.

Let tests tell you how complex your code is#

• If tests for a single class are:
  • Abundant (many test cases)
  • Require big preambles/setups
    • Like loading big, complex JSON fixtures
  • Hard to read
  • ...
• Then the code being tested is too complex!
• Note: do not try to refactor the tests!
  • They are only a side-effect of the logic being tested!

Try to not couple tests to logic that is not the subject of the test#

Some test inputs shall remain fully abstract#

There are kinds of responsibilities#

• For example:
  • Orchestrating other classes
  • Bridging domains
  • Making decisions
  • Bridging layers
  • …
• It is useful to understand which kind of responsibility you’re writing.
  • And make sure you’re doing only one!

There are kinds of data#

• For example:
  • Boundary types (aka DTOs)
  • …
• Again, it is useful to understand which kind of data type you’re writing.
  • And make sure you’re doing only one!

Arrows shall flow in a single direction#

• Arrows include:
  • Control flow
  • Data flow
  • Class relationships
  • Dependencies
  • Layers
  • …
• The ideal shape is a line.
  • But sometimes a shallow tree may be better than a very long line.
    • One problem with a long line is the burden of passing and accumulating context.
      • Either as data or just cognitively.
• The next best shape is a tree.
  • Remember: sibling tree branches never meet at any point (except for their parent).
  • Do note: I have a strong suspicion that shallow trees are easier to deal with than deep ones.
• If your arrows form a graph, you’re in trouble.
  • Think very very hard and try to make it a tree.
  • At least try to make it a DAG.
  • If all else fails, aim to reduce the size of your loops.

Immutability changes everything#

• Mutation introduces a boatload of problems:
  • ...
• That said, if you must, mutation is only allowed within the scope of a single method.
  • And only on data that was created within that scope.
  • Imagine constructing a list or some other structure.

Data or logic; not both#

• Only place logic in business classes.
• Business classes do not have data fields.
  • They only have dependencies on other business classes.
• There is one exception:
  • Base types (will try to think of a better name)
    • (Explanation)
  • But this comes at a cost: these are hard to mock and usually shouldn’t be mocked.
    • Hard to mock because of
      • A) coupling to constructor / static factory
      • B) repeated/complex interactions
        • Think how much fun you’d have mocking a Stream object!

Rich data reifies meaning, but is interpretation-free#

• This one is pretty abstract, but there is something to it. I need to think harder on how to articulate it.
• Some thoughts:
  • Computation flow/semantics can be reified into data
    • Think eDSL + interpreter

Plain data = pure interface#

• See: Scott encoding
• The moment you deviate from:
  • Immutable
  • Trivial constructor
  • Structural equality
• Then you are in a different territory.
  • Not bad.
  • Just be aware.

Think in terms of types and transformations#

Added meaning = changed type#

• For example:
  • If you validated that a String is a valid currency code,
  • Then you should wrap that String in a CurrencyCode type.
  • Otherwise, that String is still a String.
    • And any code that touches it must hold an implicit assumption (which implies accidental coupling) that the String has been validated at some point.
      • Haha! And sometimes that assumption becomes a hope!

Organize code in domain packages#

• What is the point of putting all Controllers in a controller folder when they’re all functionally unrelated to each other? Really.

‘Code reuse’ considered harmful#

• Writing libraries is extremely hard.
  • And even then, experts constantly fail.
  • I’m sure you’ve come across more than a few hideous libraries and frameworks.
• And the cost of failure is, by definition, high.
  • The code is being used in many places. So it will be wrong in many places.
• Another point:
  • Things that are conceptually different should be different.
    • TODO make this a principle?
  • ...

‘Everything is an X’ considered harmful#

• Everything is a Controller. Everything is a Service. Everything is an Object.
• If you like, sure.
• But what distinguishes controller A from controller B?
• And how do you make that distinction evident to others?

‘Mappers’ considered harmful#

• ‘Mapping’ means nothing.
  • Any function is a mapping. Literally.
• Because the name means nothing and imposes no restrictions
  • Then the code will inevitably do anything
  • This is a ‘Pit of Despair’
• Important: if they are not declared dependencies (e.g. in Spring) they will be retested and add more complexity to the overall tests.
  • See: "Unmockable + reused = double-testing & test breakage"
• Instead consider:
  • If the input is some structured object and the output is a String
    • Call this a Format or Codec
  • If the input is a String and the output is a structured object
    • Call this a Parser
  • If the input is a Thrift/Proto object and the output is a structured object
    • Call this a Parser
    • Yes, that’s right.
      • Because the code is taking barely-structured data (structs and strings, basically) into well-typed, well-structured data in your internal model
    • Bonus: parsers compose (shameless plug).
  • And more
    • The point is to consider what are the layers or domains being bridged.

Java Builders considered harmful#

• Automatically-generated Builders (eg via Lombok) have issues:
  • It is possible to forget setting a field, so it will be null and NullAway wont pick it up
    • Source of bugs
    • Pit of Despair
  • If you use @NonNull, then you just turned a compiler-detectable error into a runtime error
    • Read the point below
    • Also: "Static > Dynamic"
• Instead, use straight up constructors or factory methods
  • Then whenever you add a new field to your data type, all callers will break. And that is exactly what you want. The compiler will not let you move on unless you fix this.
    • Builders will silently compile and fail at runtime.

null considered harmful#

‘Automappers’ considered harmful#

‘Anti-boilerplate’ sometimes considered harmful#

Mind your layers#

Mind your public API surface#

Unmockable + reused = double-testing & test breakage#

If error handling is not required, it will be skipped#

• Yet another ‘Pit of Despair’.
• Consider using ADTs
  • Where success and failure cases are represented by the same type
  • Then the observer must pattern match on the ADT to be able to proceed
    • And it must handle all the cases

Algebraic Data Types are not great; they’re necessary#

• What are ADTs? Shameless plug.
  • Think of them as “enums with data inside”.
• A very useful feature is that an ADT disjunction (case A, or case B) can be used not only to model decisions but also to separate the decision maker from the decision observer.
  • The decision maker returns the decision as one of the cases of an ADT.
  • The decision observer uses pattern matching to exercise/interpret the decision.
  • This helps with:
    • Separation of concerns
    • Sticking to a single responsibility
    • Avoiding deep class dependencies
      • Because instead of calling the next step in the flow as a decision, you return the decision as data and let your caller do that.

(Tip) Use ADTs in your public API to remain open to extension#

Build ‘Pits of Success’#

• If you are designing an API that others will use (e.g. Thrift)
  • Then you want the correct thing to be the easiest and simplest thing to do
• If it is easy to make a mistake (misinterpret, misuse, misunderstand)
  • Then the wrong thing will inevitably happen!
  • aka Murphy’s Law
• In contrast, a ‘Pit of Despair/Doom’ is an API that is so hard to use that misuse is the default.
• Some tips:
  • Leverage types!
    • Make incorrect inputs or states impossible to represent!
  • If all else fails, use documentation.
• Credits to Scott Hanselman for this idea.

Incorrect code should not compile#

• Contrast these signatures:
  • sadMethod(String, String, String)
  • happyMethod(Currency, Locale, UserName)
• I think it is clear which one evokes the ‘Pit of Success’ concept.
• Again, if it is possible to mix up the arguments, then they will be mixed.
  • Good luck writing your postmortem and refunding your users!
• That is just one example of leveraging types so that the compiler makes it impossible to do the wrong thing.
• There are many more cases, examples, and techniques.
  • To learn many of them: learn Haskell!

One public method#

• If a business class has more than one public method, then it probably has more than one responsibility.
  • And we want to avoid that!

Static > Dynamic#

• It is easier to understand static relationships.
• It is harder to understand relationships that depend on time or the state of the system.
• Relationships are:
  • Caller-callee relationships
  • Dependencies
  • Control flow
  • ...

Avoid inheritance#

• I have seen it misused more frequently than not.
  • And if it does work, it will probably not be compatible with the rest of these principles, because I simply avoid thinking in terms of inheritance.
• Many authors advise against it.
• It is one of the (if not the) strongest coupling relationships in the toolbox.
  • And that is very risky.

Be wary of dynamic business-level higher-order inputs & outputs#

• Dependencies are static higher-order inputs
  • They are easier to reason about, mock

(Proto idea) Structural equality only#