Traversable / Foldable

I am the Host. This is Haskell · medium, lesson 1: Traversable / Foldable. Read the teaching and the stills before the checks. The quizzes are open book and test the subject — Haskell itself — not product rules or layout trivia.

You already know Introductory and Easy. Medium assumes those habits and asks for sharper tools.

Foldable collapses structures.

Traversable maps with effects (traverse).

Lists/Maybe/Either are instances.

Prefer traverse over manual recursion when clear.

Put Traversable / Foldable in a sentence you could teach a friend: what changes, what stays the same, and what you expect from the still.

Change one dial at a time when you experiment, then re-run. Batch changes invent ghost bugs.

Predict the still on paper before you type. Prediction turns running into confirmation.

Name each token in the still aloud. Hesitation marks the exact gap.

When something fails, read the last error line first. Calm retries beat rewriting everything.

Open-book means the page is a tool. Pick the claim that matches what we traced.

Keep mistaken traces beside corrected ones. The diff is the real lesson.

Label the runtime or toolchain you are using. Boring mismatches still burn evenings.

main = print (sum [1,2,3])  -- Foldable sum

Sit with that still. Trace it top to bottom. Name each piece. The exercise asks you to match its essence.

If something fails when you try it yourself, change one thing. Read the error. Return to the still. I will not rush you.

That is the lesson. When every quiz and exercise on this page is green, mark it passed. Next lesson when you are ready.