---
name: dart-multiline-strings
description: |-
  Guidelines and best practices for refactoring consecutive prints, single-line
  string concatenations, and complex output blocks into triple-quoted multi-line
  string literals (''' or """) in Dart.
license: Apache-2.0
key_features:
  - Triple-quoted multiline strings
  - Print & concatenation refactoring
  - Formatting large text blocks
---

# Dart Multi-line Strings

## 1. When to use this skill

Use this skill when:

- Refactoring consecutive `print()` or `stdout.writeln()` statements into a
  single, cohesive output block.
- Simplifying string literals that span multiple lines, contain embedded
  newlines (`\n`), or use nested indentations.
- Formatting large user-facing text output (like CLI help menus, reports, or
  templated messages) to be readable, maintainable, and performant.

### When NOT to use (Abstention Guardrails)

Do NOT refactor to multiline strings when:

- **Short, Single-Line Outputs**: Strings that comfortably fit on a single line
  (<80 chars) without embedded newlines.
- **Localized String Catalogs**: Strings that are looked up from localization
  bundles (e.g. `intl`, ARB files), where line breaks or formatting must conform
  to external translation tooling.
- **Raw Query Strings with Strict Whitespace Semantics**: Protocol strings,
  strict CSV rows, or queries where indentation whitespace inside triple quotes
  would alter payload semantics or introduce unintended leading spaces.
- **Streaming or Incremental I/O**: Progress bars, spinners, or interactive
  console streams where individual writes occur with delays or flushing between
  lines.

## Discovery

To find candidate code blocks for multi-line string refactoring:

- Look for multiple back-to-back `print()` or `stdout.writeln()` calls inside a
  function, especially inside loops, console views, or CLI controllers.
- Look for single-line strings heavily loaded with `\n` escape sequences.
- Look for multiple string concatenations using the `+` operator or adjacent
  string literal splits that are meant to represent multi-line outputs.

## 2. Guidelines

### Combine Consecutive Outputs

Instead of calling `print()` repeatedly for a multi-line output, group the
contents into a single triple-quoted string literal: `print('''...''')`.

### Explicit and Clean Alignment

In a triple-quoted literal, the exact spacing and formatting inside the quotes
are preserved. Use this to specify indentation levels visually instead of using
manually padded space prefixes (e.g., `'    '`).

### Remove Empty Print Calls

If there are empty `print()` or `print('')` statements serving as vertical
separators between output segments, replace them by letting the trailing newline
of a multi-line string block handle the separation naturally.

### Handling the First Newline

If the opening triple-quote is immediately followed by a newline, the compiler
discards it.

- If you do **not** want a leading blank line, start the string content on a
  fresh line in the source code for clean layout.
- If you **do** want a leading blank line in the output, leave an extra empty
  line inside the triple-quoted block, or use `\n` explicitly at the beginning:
  ```dart
  print('''

  This starts with one blank line above it.''');
  ```

### Handling the Trailing Newline & `print()`

`print()` automatically appends a trailing newline to the printed output.

- If you place the closing `'''` on a new line (`\n''');`), an extra trailing
  blank line will be printed.
- To avoid unintended trailing blank lines, place the closing triple-quotes
  immediately after the final character:
  ```dart
  // ✅ Emits standard output with no extra trailing empty line:
  print('''
  Header
  Content''');
  ```

### Avoiding Ghost Blank Lines in Conditional Interpolations

When injecting optional content via interpolation (`${condition ? '...' : ''}`),
placing the `${...}` on its own line leaves behind its enclosing newline when
the condition evaluates to `''`, producing an empty blank line in the output.

- Include the leading newline _inside_ the conditional string literal so the
  newline only renders when the content is present:
  ```dart
  // ✅ Clean conditional rendering without ghost blank lines:
  print('''
  Branch Details:
      Name: $branch${hasWarning ? '\n    WARNING: $warningMessage' : ''}''');
  ```

### 80-Character Line Limit Exemption

The `lines_longer_than_80_chars` lint rule **automatically ignores** lines
inside multiline string literals. You can write long lines inside triple-quotes
without triggering linter warnings or being forced to break them up.

### Dynamic Switch Expressions inside Interpolation

Leverage Dart 3 switch expressions directly inside string interpolations to
dynamically select and inject optional lines, conditional labels, or helper
instructions. This avoids cluttering the surrounding code with imperatively
constructed strings or multiple `if` statements:

```dart
print('''
Status: ${status.isSuccess ? 'PASS' : 'FAIL'}${switch (status) {
  Status.failed => '\nError details: $errorMessage',
  _ => '',
}}''');
```

## 3. Examples

### Refactoring Consecutive Prints with Indentation

**Avoid:**

```dart
void printGerritView(String branch, String desc, bool hasConflicts) {
  print('Branch Details:');
  print('    Name:        ' + branch);
  print('    Description: ' + desc);
  print('');
  if (hasConflicts) {
    print('    WARNING: This branch has conflicts.');
    print('    Run `git merge origin/main` to resolve.');
  }
}
```

**Prefer:**

```dart
void printGerritView(String branch, String desc, bool hasConflicts) {
  print('''
Branch Details:
    Name:        $branch
    Description: $desc${hasConflicts ? '''

    WARNING: This branch has conflicts.
    Run `git merge origin/main` to resolve.''' : ''}''');
}
```