Lesson 37 of 59

5 mins read

Records: Anonymous Composite Tuples in Dart 3

In Plain English

Introduced in Dart 3, **Records** (`(TypeA, {TypeB name})`) are anonymous, aggregate, immutable tuple values. Records allow functions to return multiple values type-safely without having to create throwaway DTO classes. Records support positional fields (`$1`, `$2`) and named fields (`rec.name`).

The Mental Model

"A Record is like a partitioned bento box: you can hand someone a single box containing rice in compartment $1, salad in compartment $2, and salmon in the labeled "mainDish" compartment."

Deep Dive: How It Works

Anonymous & Strongly Typed: `(String, int)` is a concrete type with value equality out of the box.

Field Access: Positional fields are accessed via `$1`, `$2`, etc.; named fields are accessed by name (`record.fieldName`).

Value Equality: Two records with identical field values evaluate as equal (`==`) automatically.

Syntax Blueprint

// Return multiple values
(double lat, double lng) getCoordinates() {
  return (37.7749, -122.4194);
}

// Usage with destructuring
final (lat, lng) = getCoordinates();

Record return type and pattern destructuring.

Core Rules to Remember

Multiple Return Values: Eliminates boilerplate throwaway wrapper classes.

Built-In Value Equality: Records automatically implement `==` and `hashCode` based on contents.

Positional and Named: Mix positional elements and labeled named fields seamlessly.

Common Beginner Traps & How to Fix Them

Mistake:Single-element positional record syntax `(42)` instead of `(42,)`.

Why it happens: `(42)` is parsed as parenthesized integer expression 42.

How to fix: Include a trailing comma `(42,)` for single-element positional records.

Pro Tip: Combine records with pattern destructuring `final (name, :age) = getUser();` for clean JSON parsing.

Live Interactive Example

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Returning and Destructuring Records

Python 3.12

Output Console

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How it works: Returned a record with positional and named fields, then destructured fields cleanly.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Return Min and Max as a Record

Define `(int min, int max) getBounds(List<int> list) => (list.reduce((a, b) => a < b ? a : b), list.reduce((a, b) => a > b ? a : b));`.
In `main()`, call `getBounds([10, 5, 20, 15])` and print `Min: ${b.$1}, Max: ${b.$2}`.

Sandbox Output

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