The `-darity-checks' option inserts code to check for arity violations. Unfortunately, it's not that simple: you have to link with a prelude that was also built with arity checks. If you have one, then great; otherwise...
The `-darity-checks-C-only' option inserts the self-same arity checking code into `.hc' files, but doesn't compile it into the `.o' files. We use this flag with the `-keep-hc-file-too', where we are keeping `.hc' files around for debugging purposes.