clap/examples/15_custom_validator.md

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.

```bash
$ 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](12_types_values.md).
test: More thoroughly test examples 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" 2021-11-10 22:15:30 +00:00			`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.`

			```bash
			`$ 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](12_types_values.md).`