1 files changed, 37 insertions, 0 deletions
diff --git a/clap/examples/15_custom_validator.rs b/clap/examples/15_custom_validator.rs
new file mode 100644
index 0000000..a5c0d42
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+++ b/clap/examples/15_custom_validator.rs
@@ -0,0 +1,37 @@
+extern crate clap;
+
+use clap::{App, Arg};
+
+fn main() {
+    // You can define a function (or a closure) to use as a validator to argument values. The
+    // function must accept a String and return Result<(), String> where Err(String) is the message
+    // displayed to the user.
+
+    let matches = App::new("myapp")
+                        // Application logic goes here...
+                        .arg(Arg::with_name("input")
+                                     .help("the input file to use")
+                                     .index(1)
+                                     .required(true)
+                                     // You can pass in a closure, or a function
+                                     .validator(is_png))
+                        .get_matches();
+
+    // Here we can call .unwrap() because the argument is required.
+    println!("The .PNG file is: {}", matches.value_of("input").unwrap());
+}
+
+fn is_png(val: String) -> Result<(), String> {
+    // val is the argument value passed in by the user
+    // val has type of String.
+    if val.ends_with(".png") {
+        Ok(())
+    } else {
+        // clap automatically adds "error: " to the beginning
+        // of the message.
+        Err(String::from("the file format must be png."))
+    }
+    // Of course, you can do more complicated validation as
+    // well, but for the simplicity, this example only checks
+    // if the value passed in ends with ".png" or not.
+}