Elm Compiler and File‑System Cache: Interoperability of Cache Key with Compiler Version

Compiler Cache and Versioning

The Elm compiler does not currently include its own version number or command-line flags (such as --optimize or --debug) in the hash used to generate .elmi cache files. The cache key is derived primarily from the source file contents and the version constraints defined in elm.json.

Likely Cause of Build Failures

Because the compiler version is absent from the hash, upgrading the Elm compiler does not automatically trigger a cache bust. If a newer version of the compiler changes how interface files are generated or how types are resolved, the compiler may attempt to use stale .elmi files created by a previous version. This mismatch often manifests as unexpected type errors or incorrect binary output that does not align with the current source code.

Recommended Resolution Steps

To ensure build integrity after upgrading the Elm compiler or changing critical build flags, you must manually invalidate the file-system cache. Use one of the following methods:

• Force a clean build: Run the make command with the clean flag to force the regeneration of all interface files:

  elm make src/Main.elm --output=main.js --clean

• Manual cache removal: Delete the global Elm cache directory to clear all stored metadata and interface files:

  rm -rf ~/.elm/cache

Current Status and Proposals

There are known discussions and open issues within the Elm community regarding the modification of the cache key to incorporate the compiler version and build flags. However, as of the current stable releases, these changes have not been merged into the core compiler. Until an automated version-aware hashing mechanism is implemented, manual cache clearing remains the standard operational procedure for upgrades.

Verification

To verify if a stale cache is affecting your project, perform the following sequence:

1. Run elm --version to confirm the current installed version.
2. Attempt a standard elm make. If it fails with opaque type errors, proceed to the next step.
3. Execute rm -rf ~/.elm/cache and run elm make again. If the build succeeds, the previous failure was caused by cache interoperability issues.