tcon

Typed configuration that stays in sync with its generated files

By Arpan Bhandari

Source: https://arpan.sh/projects/tcon

A Rust compiler that catches configuration errors and keeps generated files in sync with their typed definitions.

I built tcon to make configuration changes easier to validate and review. A file can be valid JSON or YAML while still containing a misspelled key, an invalid value, or an old setting left behind by a manual edit.

The compiler takes a typed definition and produces the configuration files an application needs. A separate check compares those outputs with the files on disk, so drift can fail CI before deployment.


Choosing predictable output

I made generation deterministic, including key ordering, so a diff reflects a configuration change rather than a change in formatting. The same definitions can produce JSON, YAML, environment files, TOML, and properties files.

Version 1 replaces the generated file completely. I left merging out because it would make ownership less clear: when a generated value conflicts with a manual edit, someone still has to decide which one wins. Full replacement keeps the definition authoritative.

The compiler has no dependencies. That keeps it self-contained, but makes me responsible for the parser, validation, output formats, and diagnostics.


Deciding what the compiler should reject

Validation includes where a file can be written and how secrets enter it. Output paths must remain within the owning workspace, even through symlinks. Secret fields must use environment values and cannot contain defaults.

I also treated error codes as part of the compatibility contract. People can read an error message, but CI scripts need a stable way to interpret it. Versioned fixtures check both successful output and expected failures.

tcon is open source. The language guide [https://github.com/arpan404/tcon/blob/master/docs/dsl-reference.md] and compatibility fixtures [https://github.com/arpan404/tcon/tree/master/compat/v1] describe the current version.