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<h1 id="errors"><a class="header" href="#errors">Errors</a></h1>
<p>towl uses typed errors throughout, built with <code>thiserror</code>. Each module defines its own error enum, and the top-level <code>TowlError</code> aggregates them.</p>
<h2 id="error-hierarchy"><a class="header" href="#error-hierarchy">Error Hierarchy</a></h2>
<pre><code class="language-text">TowlError
├── TowlConfigError
├── TowlScannerError
│ └── TowlParserError
│ └── TowlCommentError
├── TowlOutputError
│ ├── FormatterError
│ └── WriterError
├── TowlGitHubError
├── TowlProcessorError
├── TowlTuiError
└── TowlLlmError
</code></pre>
<h2 id="towlerror"><a class="header" href="#towlerror"><code>TowlError</code></a></h2>
<p>Top-level error type used by the CLI binary. All sub-error types convert automatically via <code>#[from]</code>.</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub enum TowlError {
Config(TowlConfigError),
Scanner(TowlScannerError),
Output(TowlOutputError),
GitHub(TowlGitHubError),
Processor(TowlProcessorError),
Tui(TowlTuiError),
Llm(TowlLlmError),
}
<span class="boring">}</span></code></pre></pre>
<h2 id="towlconfigerror"><a class="header" href="#towlconfigerror"><code>TowlConfigError</code></a></h2>
<p>Errors during configuration loading, initialisation, and validation.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>PathTraversalAttempt(PathBuf)</code></td><td>Config path contains <code>..</code></td></tr>
<tr><td><code>ConfigAlreadyExists(PathBuf)</code></td><td><code>towl init</code> without <code>--force</code> on existing file</td></tr>
<tr><td><code>WriteToFileError(PathBuf, io::Error)</code></td><td>Failed to write config file</td></tr>
<tr><td><code>UnableToParseToml(toml::ser::Error)</code></td><td>TOML serialisation failure</td></tr>
<tr><td><code>CouldNotCreateConfig(ConfigError)</code></td><td>Config crate loading error</td></tr>
<tr><td><code>GitRepoNotFound { message }</code></td><td>Not inside a git repository</td></tr>
<tr><td><code>GitRemoteNotFound { message }</code></td><td>No <code>origin</code> remote</td></tr>
<tr><td><code>GitInvalidUrl { url, message }</code></td><td>Cannot parse owner/repo from remote URL</td></tr>
<tr><td><code>TooManyConfigPatterns { field, count, max_allowed }</code></td><td>Pattern array exceeds 100 entries</td></tr>
<tr><td><code>ConfigValueTooLong { field, length, max_length }</code></td><td>Config string exceeds 512 characters</td></tr>
<tr><td><code>ContextLinesOutOfRange { value, min, max }</code></td><td>Context lines outside 1..=50</td></tr>
<tr><td><code>RateLimitDelayTooHigh { value, max }</code></td><td>Rate limit delay exceeds maximum</td></tr>
</tbody></table>
</div>
<h2 id="towlscannererror"><a class="header" href="#towlscannererror"><code>TowlScannerError</code></a></h2>
<p>Errors during directory walking and file reading.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>UnableToWalkFile(ignore::Error)</code></td><td>Directory traversal error</td></tr>
<tr><td><code>ParsingError(TowlParserError)</code></td><td>Parser failure (propagated)</td></tr>
<tr><td><code>UnableToReadFileAtPath(PathBuf, io::Error)</code></td><td>File I/O error</td></tr>
<tr><td><code>InvalidPath { path }</code></td><td>Path could not be canonicalised</td></tr>
<tr><td><code>FileTooLarge { path, size, max_allowed }</code></td><td>File exceeds 10 MB</td></tr>
<tr><td><code>TooManyTodos { path, count, max_allowed }</code></td><td>File exceeds 10,000 TODOs</td></tr>
</tbody></table>
</div>
<h2 id="towlparsererror"><a class="header" href="#towlparsererror"><code>TowlParserError</code></a></h2>
<p>Errors during regex compilation and TODO extraction.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>InvalidRegexPattern(String, regex::Error)</code></td><td>Regex failed to compile</td></tr>
<tr><td><code>UnknownConfigPattern(TowlCommentError)</code></td><td>Pattern matched but type unknown</td></tr>
<tr><td><code>RegexGroupMissing</code></td><td>Pattern lacks a capture group <code>(.*)</code></td></tr>
<tr><td><code>PatternTooLong(usize, usize)</code></td><td>Pattern exceeds 256 characters</td></tr>
<tr><td><code>TooManyTotalPatterns { count, max_allowed }</code></td><td>Total patterns across all categories exceeds 50</td></tr>
</tbody></table>
</div>
<h2 id="towlcommenterror"><a class="header" href="#towlcommenterror"><code>TowlCommentError</code></a></h2>
<p>Errors in comment type resolution.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>UnknownTodoType { comment }</code></td><td>String does not map to a known <code>TodoType</code></td></tr>
</tbody></table>
</div>
<h2 id="towloutputerror"><a class="header" href="#towloutputerror"><code>TowlOutputError</code></a></h2>
<p>Errors during formatting and writing.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>InvalidOutputPath(String)</code></td><td>Missing/wrong extension, terminal format with file path</td></tr>
<tr><td><code>UnableToFormatTodos(FormatterError)</code></td><td>Formatter failure</td></tr>
<tr><td><code>UnableToWriteTodos(WriterError)</code></td><td>Writer failure</td></tr>
</tbody></table>
</div>
<h2 id="formattererror"><a class="header" href="#formattererror"><code>FormatterError</code></a></h2>
<p>Errors in output formatting.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>SerializationError(String)</code></td><td>JSON/TOML/CSV serialisation failure</td></tr>
<tr><td><code>IntegerOverflow(usize)</code></td><td>Count exceeds safe integer bounds</td></tr>
</tbody></table>
</div>
<h2 id="writererror"><a class="header" href="#writererror"><code>WriterError</code></a></h2>
<p>Errors in output writing.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>IoError(io::Error)</code></td><td>File system I/O error</td></tr>
<tr><td><code>PathTraversal(PathBuf)</code></td><td>Output path contains <code>..</code></td></tr>
</tbody></table>
</div>
<h2 id="towlgithuberror"><a class="header" href="#towlgithuberror"><code>TowlGitHubError</code></a></h2>
<p>Errors from GitHub API interactions.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>ApiError { message, source }</code></td><td>General GitHub API failure</td></tr>
<tr><td><code>AuthError</code></td><td>401 response -- invalid or expired token</td></tr>
<tr><td><code>RateLimitExceeded { retry_after_secs }</code></td><td>403 with rate limit message</td></tr>
<tr><td><code>IssueAlreadyExists { title }</code></td><td>Duplicate detected before creation</td></tr>
<tr><td><code>RepositoryNotFound { owner, repo }</code></td><td>404 response -- owner/repo not found</td></tr>
<tr><td><code>MissingToken</code></td><td><code>TOWL_GITHUB_TOKEN</code> not set or empty</td></tr>
</tbody></table>
</div>
<h2 id="towlprocessorerror"><a class="header" href="#towlprocessorerror"><code>TowlProcessorError</code></a></h2>
<p>Errors from replacing TODO comments with issue links in source files.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>FileReadError(PathBuf, io::Error)</code></td><td>Failed to read source file</td></tr>
<tr><td><code>FileWriteError(PathBuf, io::Error)</code></td><td>Failed to write modified file</td></tr>
<tr><td><code>LineOutOfBounds { path, line, total_lines }</code></td><td>TODO line number exceeds file length</td></tr>
<tr><td><code>CommentPrefixNotFound { path, line }</code></td><td>Column offset points past end of line</td></tr>
<tr><td><code>PathOutsideRoot { path, root }</code></td><td>File is outside the repository root</td></tr>
<tr><td><code>InvalidIssueUrl { url }</code></td><td>URL does not start with <code>https://github.com/</code></td></tr>
</tbody></table>
</div>
<h2 id="towlllmerror"><a class="header" href="#towlllmerror"><code>TowlLlmError</code></a></h2>
<p>Errors from LLM API interactions and analysis.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>ApiError { message, status }</code></td><td>LLM API returned a non-200 status</td></tr>
<tr><td><code>AuthError</code></td><td>401 -- invalid or missing API key</td></tr>
<tr><td><code>RateLimited { retry_after_secs }</code></td><td>429 -- too many requests</td></tr>
<tr><td><code>ParseError { message }</code></td><td>LLM response could not be parsed as valid JSON</td></tr>
<tr><td><code>NotConfigured</code></td><td><code>TOWL_LLM_API_KEY</code> environment variable not set</td></tr>
<tr><td><code>UnsupportedProvider { provider }</code></td><td>Provider is not "claude", "openai", "claude-code", or "codex"</td></tr>
<tr><td><code>IoError { message }</code></td><td>File I/O error during context gathering</td></tr>
</tbody></table>
</div>
<p><code>is_retryable()</code> returns <code>true</code> for <code>RateLimited</code>, <code>ApiError</code> with status >= 500, and <code>ApiError</code> with no status (network failures).</p>
<h2 id="towltuierror"><a class="header" href="#towltuierror"><code>TowlTuiError</code></a></h2>
<p>Errors from the interactive terminal UI.</p>
<div class="table-wrapper"><table><thead><tr><th>Variant</th><th>Cause</th></tr></thead><tbody>
<tr><td><code>Io(io::Error)</code></td><td>Terminal I/O error from crossterm or ratatui</td></tr>
</tbody></table>
</div>
<h2 id="error-propagation"><a class="header" href="#error-propagation">Error Propagation</a></h2>
<p>Errors propagate upward using <code>?</code> and <code>#[from]</code>:</p>
<pre><code class="language-text">TowlCommentError --> TowlParserError --> TowlScannerError --> TowlError
FormatterError --> TowlOutputError --> TowlError
WriterError --> TowlOutputError --> TowlError
TowlGitHubError ----------------------------> TowlError
TowlProcessorError ---------------------------> TowlError
TowlTuiError ----------------------------> TowlError
TowlLlmError ----------------------------> TowlError
</code></pre>
<p>All errors implement <code>std::fmt::Display</code> with human-readable messages and <code>std::error::Error</code> for standard error handling.</p>
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