Lesson 14 of 59

5 mins read

Named Parameters & the required Keyword

In Plain English

Named parameters (`{Type param = default}`) enclosed in curly braces are the cornerstone of Flutter widget APIs and idiomatic Dart code. Unlike positional arguments where order matters, named parameters are labeled at the callsite (`createUser(name: "Alice", age: 30)`), can appear in any order, and use the `required` keyword for mandatory arguments.

The Mental Model

"Named parameters are like filling out labeled form fields on a tax form: it doesn't matter which box you fill first as long as each box is clearly labeled with its name."

Deep Dive: How It Works

Self-Documenting Calls: Callsites like `Button(label: "Submit", isPrimary: true)` make intent unmistakable.

`required` Keyword: Enforces at compile-time that the caller must supply the argument.

Default Values: Optional named parameters can provide defaults (`{int timeoutMs = 5000}`).

Syntax Blueprint

void configure({
  required String host,
  int port = 8080,
  bool enableTls = true,
}) {
  print("$host:$port (TLS: $enableTls)");
}

Named parameter signature with required and default parameter options.

Core Rules to Remember

Order Independence: Callers can pass named arguments in any order.

The required Modifier: Prevents omission of critical arguments without requiring artificial positional parameters.

Default Values: Compile-time constant defaults are automatically applied when omitted.

Common Beginner Traps & How to Fix Them

Mistake:Declaring a non-nullable named parameter without `required` or a default value.

Why it happens: Non-nullable types cannot be null if the caller omits them.

How to fix: Either add `required`, provide a default (`int count = 1`), or make the type nullable (`int? count`).

Pro Tip: Flutter widgets use named constructor parameters for virtually all widget styling and configuration.

Live Interactive Example

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Creating Self-Documenting APIs with Named Parameters

Python 3.12

Output Console

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How it works: Named parameters allowed callsites to supply arguments in arbitrary order with sensible defaults.

Your Turn: Micro Challenge

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

Micro Exercise

Define Function with Named Parameters

Define `void registerUser({required String username, String role = "MEMBER", bool isActive = true})`.
Have it print `User: $username [$role] (Active: $isActive)`.
Call it from `main()` with `username: "dev_alex"`.

Sandbox Output

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