Dart Long Lines

1. When to use this skill

Use this skill when:

• Writing Dart code that might exceed the 80-column limit.
• Refactoring code to comply with the lines_longer_than_80_chars lint. Reference: https://dart.dev/tools/linter-rules/lines_longer_than_80_chars

When NOT to use (Abstention Guardrails)

Do NOT break lines or alter code when:

• Unbreakable URLs & URIs: Lines contain long HTTP/HTTPS URLs, import URIs, or file paths where breaking them across lines would invalidate the link.
• Complex Regular Expression Literals: Long RegExp patterns (RegExp(r'...')) where inserting whitespace or line breaks changes the matching semantics.
• ASCII Art, Tables, or Preformatted Diagrams: Preformatted text blocks, table rows, or ASCII diagrams inside comments or strings where wrapping lines destroys visual alignment.
• Prior to Automated Formatting: Do NOT manually break code lines before running dart format. Let dart format handle code wrapping automatically; only manually wrap comments, docs, and multi-line strings that the formatter does not re-wrap.

Discovery

To find lines that exceed the limit:

Automated Analysis

The most reliable way to find long lines is to use the Dart analyzer:

• Command: dart analyze
• Lint: lines_longer_than_80_chars

Manual Search

To search for long lines using regex:

• Regex: ^.{81,}$ (Matches any line with 81 or more characters).

2. Guidelines

Format First

Always run dart format before manually breaking long lines. The formatter often automatically fixes long lines, especially in generated code, and applies standard Dart styling rules.

Code Comments

Break long code comments (//) cleanly at word boundaries to ensure lines do not exceed 80 characters. Maintain tight formatting and avoid unnecessary vertical space.

Documentation Comments (///)

• Apply the same line-breaking rules as for code comments.
• Avoid breaking markdown link blocks like [name] or [text](http://example.com) across lines. Place them on their own line if they exceed the limit.
• Start doc comments with a single summary sentence, followed by a blank line before the rest of the comment. It is okay to break this first sentence across multiple lines to fit the 80-column limit.
• Avoid unresolved references or dangling sentences.

Long Strings

• Use adjacent string literals (e.g., 'part 1 ' 'part 2') to break long strings. Break at word boundaries.
• If a single-line string contains newline characters (\n) or if there are consecutive print statements, consider migrating to a multi-line string literal (''').

Format and Analyze After Changes

• Run dart format and dart analyze after making changes.
• Be aware that splitting strings may trigger new lints (e.g., prefer_single_quotes if a double-quoted string is split into parts that no longer contain single quotes).

3. Examples

Documentation Comment Link

Avoid:

/// This is a long doc comment that contains a link to [a very long
/// URL](http://example.com/very/long/url/that/exceeds/eighty/chars).

Prefer:

/// This is a long doc comment that contains a link to [a very long URL][ref].
///
/// [ref]: http://example.com/very/long/url/that/exceeds/eighty/chars

Adjacent String Literals

Prefer:

final longString = 'This is a very long string that needs to be broken '
    'across multiple lines to stay under the limit.';

Multi-line String Migration

Avoid:

print('This is line 1\nThis is line 2 that is also quite long\nThis is line 3 which makes the whole thing exceed eighty characters');

Prefer:

print('''This is line 1
This is line 2 that is also quite long
This is line 3 which makes the whole thing exceed eighty characters''');