Last updated on Sunday, November 02, 2025

Week 7 Debrief: optional find-lazy callback and laziness

Congrats on completing your 7th week of CS-214! Here is a round-up of interesting questions; we kept things short to allow you to focus on the midterm.

Administrivia

• We don’t have a lecture room on Monday, so we’ll have to use a recording again. Sorry for trouble; we’re hoping this is the last time. You can find the videos, as well as a complete transcript of the code, in the syllabus and on Moodle.

• We won’t have a lecture on Wednesday at all, since we have the midterm.

• We have made the find-lazy callback optional to give you time to prepare for the midterm. Good luck!

• Registration is open for unguided-callback checkoffs! The dates are November 3 (Monday), November 7 (Friday), and November 10 (Monday). See the original announcement for more details about registration.

Interesting Ed questions

• What is an API?

• Does the type of image matter in Scalashop tests?

• Callback:

  • What kind of tests should I write for my callback? Must it be unit tests?

• Midterm prep:

  • Why can D("Hello") have type D[Any] if D is contravariant?
  • If we’re proving something about l1 and l2, don’t we have to induct twice?
  • Can we write equality proofs by making the two sides meet in the middle?
  • What does .par.foldLeft do?

• Scala:

  • What does l.map { case _ => ... } mean?
  • What’s the difference between assert and require?
  • Why does matching on a Vector yield a “not exhaustive” warning?

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