Error Handling

The `AppError` Enum

TMT uses a single centralised error type defined in src/error.rs using the thiserror crate. Every failure mode in the application is represented as a variant of AppError, which gives callers a single type to match on and provides human-readable messages automatically via the Display trait.

Error Propagation

All fallible functions return Result<T, AppError> and use the ? operator to bubble errors upward. The chain terminates in main.rs, which logs the error and exits with a non-zero status code.

format handler error
    │ ?
TranslationService error
    │ ?
app::run
    │ ?
main  ──▶  eprintln! + process::exit(1)

Automatic Conversions

Standard library and third-party error types are automatically converted into AppError variants through From implementations:

Source Type	`AppError` Variant
`std::io::Error`	`AppError::Io`
`csv::Error`	`AppError::Csv`
HTTP / network errors	`AppError::Network` (via `TmtClient`)
Rate-limit responses	`AppError::RateLimit`

Key Error Variants

Variant	When it occurs
`MissingApiToken`	No token provided via `--api-token` or `TMT_API_TOKEN`
`InvalidArgument`	A CLI argument fails a business-logic constraint (e.g. concurrency = 0)
`FileTooLarge`	The input file exceeds `MAX_FILE_SIZE_BYTES`
`FormatMismatch`	Input and output extensions do not match
`UnsupportedFormat`	The file extension is not handled by any format module
`RateLimit`	The API responded with HTTP 429 and backoff was exhausted
`Io`	Any filesystem read/write failure
`Csv`	A parsing error in CSV/TSV content
`DocxParse`	A structural error in a DOCX file
`PdfParse`	A structural error in a PDF file (pdf feature only)

Adding New Error Variants

Because AppError uses thiserror, adding a new variant is straightforward:

#![allow(unused)]
fn main() {
#[derive(Debug, thiserror::Error)]
pub enum AppError {
    // ... existing variants ...

    #[error("my new error: {0}")]
    MyNewError(String),
}
}

Implement From<YourLibraryError> if you want automatic ? conversion from a third-party type.

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