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Lecture 8: Metaprogramming

Build systems​

make:

  • consults Makefile
  • the things named on the right-hand side are dependencies, and the left-hand side is the target
  • the indented block is a sequence of programs to produce the target from those dependencies
  • the first directive is the default goal
paper.pdf: paper.tex plot-data.png
	pdflatex paper.tex

plot-%.png: %.dat plot.py
	./plot.py -i $*.dat -o $@

Dependency management​

Repository

  • hosts a large number of dependencies in a single place
  • Examples:
    • Ubuntu package repositories for Ubuntu system packages, accessed through apt tool
    • PyPI for Python librarires

Sematic versioning:

  • major.minor.path
  • e.g. code written for Python 3.5 might run fine on Python 3.7, but possibly not on 3.4
  1. If a new release does not change the API, increase the patch version
  2. If you add to your API in a backwards-compatible way, increase the minor version
  3. If you change the API in a non-backwards-compatible way, increase the major version

Lock files

  • a file that lists the exact version currently depending on of each dependency
  • vendoring: copy all the code of your dependencies into your own project

Continuous integration systems​

  • CI: umbrella term for "stuff that runs whenever your code changes"
  • Examples:
    • event triggered:
      • general: Travis CI, Azure Pipelines, GitHub Actions
      • website: GitHub Pages
    • dependency version: Dependabot, alert when there are updates or security vulnerabilities, keeping dependencies up to date
  • How it works:
    • roughly: you add a file (rule/recipe) to your repository that describes what should happen when various things happen to that repository
    • more detailed: when the event triggers, the CI provider spins up virtual machines, runs the commands in your "recipe", and then usually notes down the results somewhere
  • Most common rule/recipe:
    • when someone pushes code (or submit pull requests), run the test suite
    • check your code style everytime you commit
  • You might set it up so that you are notified if the test suite stops passing, or so that a little badge appears on your repository (e.g. top of README.md) as long as the tests pass

A brief aside on testing:

  • Test suite: a collective term for all the tests
  • Unit test: a “micro-test” that tests a specific feature in isolation
  • Integration test: a “macro-test” that runs a larger part of the system to check that different feature or components work together.
  • Regression test: a test that implements a particular pattern that previously caused a bug to ensure that the bug does not resurface.
  • Mocking: to replace a function, module, or type with a fake implementation to avoid testing unrelated functionality. For example, you might “mock the network” or “mock the disk”.

Exercises​

TO-DO