Today I Learned

TIL, 2018-04-04, Result Objects and Callee.

Musings, JS and React

  • Formspree is awesome!
  • HTML5 form validation is okay as a first declarative validation layer.

Musings, Ruby

  • undef_method: Only in class definitions, you can probably use this in a file.
  • Getting method names:
    • __method__ to get the name of the method, good for passing to another abstraction thing.
    • __callee__ looks up the name dynamically and refers to the name under which the method was called.
  • Updating with bang methods:
    • ActiveRecord::RecordInvalid inherits ActiveRecord::ActiveRecordError, which inherits from StandardError.
    • When updating via update_attributes, we get ActiveModel::Errors, not ActiveRecord::RecordInvalid.
  • Is the adapter design pattern an Anti-corruption layer?
  • Single vs Multi-tenant SaaS:
    • Create a database, provision some servers, add a load balancer, add some caching, and call it a day.
    • Any-scale system: a system that’s built to remove scale from that equation.
    • Multi-tenant: Load balancer, gateway, subnets, kinesis, lambdas, Redshift…
    • Multi-tenant: Teams share resources. Higher cost up front but each new cx doesn’t really cost much.
    • Single-tenant: More expensive except for the human aspect.
    • At Kumu, every project is backed by a CouchDB database.
  • Rubocop tells us to put mixins in separate attachments.
  • Ruby and Amazon Lambda: Currently, it doesn’t support Ruby, but you can use it as a go-between for your S3 to Rails use case.
  • lambda or proc?
  • Flaky Ruby tests:
    • Capybara: Avoid dangerous methods that do not wait: visit(path) and current_path, all(selector), accessors (text, value, title). Use safe methods that do wait: find(selector), find_field, has_selector?.
    • DB pollution: database_cleaner: transaction vs truncation. Truncation is required when running request specs with Capybara. The thread running your Rails server uses a separate database connection from the one running your test.
    • Non-deterministic attributes: Don’t use faker, just hardcode.
    • ID-dependent things: When tests run in a transaction, the auto-increment value still runs. Truncation = auto-increment will be reset.
    • To reproduce: run RSpec using the same seed, run the entire list of files, use the --bisect flag.
  • Sentinel value: “A special value in the context of an algorithm which uses its presence as a condition of termination.”

Result Objects: Errors without Exceptions


  • Resonad gem:
    • Errors are part of the return value, not an exception.
    • Expected errors are available through result.error, and unexpected bugs are exceptions.
    • All expected error cases are automatically “caught”, without having to guess what they are.
result = register_new_user(params)
if result.success?
  • When not to use:
    • When the method should always succeed.
    • When there is only a single failure case, just return nil or whatever.
    • If you can recover Locally, and you can just return a Null Object.
  • Result objects are appropriate:
    • When an operation can fail in multiple different ways.
    • When operations need to be chained together, and each individual operation can fail.
    • When it’s important for callers to know that the method can fail.
    • Works with service/interactor/command objects.
  • Implementation: Resonad, dry-monads, Github::Result, monadic, result-monad.

  • Refactoring to data: “This is an example of what I call “data-driven design” or just “thinking in data”. All the specific details are described in data: numbers, strings, arrays, hashes, structs, etc. All the behaviour (i.e. the implementation) is generic, and controlled by the data.”
  • When thinking of inheritance, ask yourself if it is possible to think of the inheritance chain in terms of just data (array, hash, and do a concretion based on that).

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