Session 3: Data modeling

Objectives

By the end of this session, students can:

• Declare a record type with named fields and construct a value with the tagged constructor syntax.
• Copy and update a record and explain why the original is left unchanged.
• Match on a record, binding a subset of fields with the { x } shorthand.
• Work over lists with List.map, List.filter, and List.fold, using lambdas and operator sections.
• Group values by position with tuples and destructure them in a match arm.

Prerequisites

Sessions 1 and 2 (immutability, functions, match, ADTs). Python dictionaries and dataclasses give useful points of comparison.

Demo script

The thread is choosing the right shape for data: a record for one thing with named parts, a collection for many things, a tuple for a short positional pairing. Keep the Python panel visible so students see records become frozen dataclasses.

1. Open the Lesson 6 exercise. Run check and read the hole note: the hole has type int. Fill it with 100 and Run. Draw attention to the last line: opened.balance still prints 0.
2. Explain the copy-and-update line { opened with balance = 100 }. It builds a fresh Account and leaves opened alone, which is why the original balance is unchanged. Connect this back to the immutability from Session 1.
3. Show the Python panel: Account compiled to a @dataclass(frozen=True), and the update compiled to a new Account(...) call. The immutability is literal in the output.
4. Break it live: change Account { name = "Ada", balance = 0 } to drop the balance field. The compiler reports the missing field. Restore it. This shows records require all their fields.
5. Move to collections. Open the Lesson 7 exercise. Run check and show the two hole notes. The first hole wants int -> int -> int. Fill it with the operator section (+), fill the second with 0, and Run.
6. Make the totality point: there is no ns[10] bracket indexing in Pyfun. List.get 10 ns returns an Option, and Option.withDefault supplies the fallback, so an out-of-range lookup cannot crash. This is the same lesson from Session 2 applied to collections.
7. Move to tuples. Open the Lesson 8 exercise. Show that List.zip names scores produced a list of pairs and that the single case (name, score) arm destructures each pair. Fill the hole with f"{name}: {score}" and Run.
8. Close by contrasting the three shapes with one line each: a record when parts have names, a list when there are many of the same thing, a tuple for a short fixed pairing like a name and a score.

Assigned exercises

• In class: the Lesson 6 exercise and the Lesson 7 exercise, both driven from the demo.
• Homework: the Lesson 8 exercise (build one line per pair, then join). Ask students to note how the tuple pattern binds both parts at once.

Common misconceptions

• “A record is a dictionary, so I can add keys later.” Correction: a record’s fields are fixed by its type. It compiles to a frozen dataclass, and every field must be present when you build it.
• “{ opened with balance = 100 } mutates opened.” Correction: copy-and-update returns a new record. The original is untouched, which is why opened.balance still reads 0.
• “List.get 10 ns will raise IndexError like Python indexing.” Correction: there is no bracket indexing. List.get returns an Option, so an out-of-range index is None, a value you handle.
• “(+) and (*) 2 are odd syntax I have to memorize.” Correction: an operator in parentheses is just that operator as a function, and supplying one argument ((*) 2) is the same partial application from Session 1.
• “A one-element tuple (x) groups one value into a tuple.” Correction: (x) is just x in parentheses. There is no one-element tuple. A real tuple has two or more elements, and () is unit.

Timing

• 10 min: recap Session 2, then motivate choosing a data shape.
• 20 min: demo steps 1 to 4 (records and copy-and-update).
• 20 min: demo steps 5 to 6 (collections and totality).
• 15 min: demo steps 7 to 8 (tuples and the summary).
• 20 min: students work the Lesson 6 and Lesson 7 exercises in the playground.
• 5 min: recap and assign the Lesson 8 exercise as homework.

Answer keys: answer-keys.md