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bfa02fd418
This ports our example testing over to [trycmd](https://docs.rs/) so we can: - More thoroughly test our examples - Provide always-up-to-date example usage The old way of testing automatically picked up examples. This new way requires we have a `.md` file that uses the example in some way. Notes: - Moved overall example description to the `.md` file - I added cross-linking between related examples - `14_groups` had a redundant paragraph (twice talked about "one and only one"
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You can define a function (or a closure) to use as a validator to argument values. The
function must accept a &str and return Result<(), String> where Err(String) is the
message displayed to the user.
$ 15_custom_validator input.png
The .PNG file is: input.png
$ 15_custom_validator input.txt
? failed
error: Invalid value for '<input>': the file format must be png.
For more information try --help
This is especially useful when using custom types.