G'day Fletch,

Overall, there are a lot of input parameters; currently, there are about 50 possible messages that can arise when the sanity checks are run. Most of these are not related to any file checks: "A can't be zero-length"; "B must be an integer"; "C can't be greater than D"; and so on. There are nine file-related checks as outlined in the OP.

I wanted to get all sanity checks performed prior to starting the processing proper; which begins with opening the input file. I considered it to be more useful to advise the user upfront that there was a specific problem with the directory or file parameter supplied, rather than offering a generic $! message from open, such as "File not found", "Permission denied", or similar.

Your suggestion of doing the open first with supplied values (i.e. dir_param/file_param) and then dealing with failure might be a better way to go. Something like:

open ... or do { ... };
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With the do determining more specific messages: "Directory was not supplied"; "File specified does not exist"; and so on.

If users get a specific message, it may indicate something they can fix themselves, especially if it's a simple typo; e.g. file parameter supplied as file.tx but should be file.txt. When presented with a generic message, users may not be able to investigate themselves and may, unfortunately, provide the oft seen error report: "It didn't work".