Val err details

README.md

@@ -47,7 +47,7 @@ let mut u = Username::new(" valid name \n\n").unwrap();
 assert_eq!(u.get(), "valid name"); // now we're talking!
 
 // This also works for mutations:
-assert!(matches!(u.try_mutate(|u| *u = "   ".to_owned()), Err(prae::ValidationError)));
+assert!(matches!(u.try_mutate(|u| *u = "   ".to_owned()), Err(prae::ValidationError { .. })))
 ```
 
 Now our `Username` trims provided value automatically.

prae/Cargo.toml

@@ -16,5 +16,6 @@ prae_macro = { version = "0.2", path = "../prae_macro" }
 serde = { version = "1.0", optional = true }
 
 [dev-dependencies]
+assert_matches = "1.5"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"

prae/src/core.rs

@@ -1,23 +1,30 @@
 use core::hash::Hash;
-use std::fmt;
 use std::ops::{Deref, Index};
+use std::{error::Error, fmt};
 
-/// Default validation error. It is used for [`define!`](prae_macro::define) macro with `ensure`
-/// keyword.
-#[derive(PartialEq)]
-pub struct ValidationError;
-
-impl std::error::Error for ValidationError {}
+/// Used for [`define!`](prae_macro::define) macro with `ensure` keyword.
+#[derive(Clone, Debug)]
+pub struct ValidationError {
+    /// The name of the type where this error originated.
+    type_name: &'static str,
+}
 
-impl fmt::Debug for ValidationError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        writeln!(f, "provided value is not valid")
+impl ValidationError {
+    /// Create a new error with the input value that failed.
+    pub fn new(type_name: &'static str ) -> Self {
+        ValidationError { type_name }
     }
 }
 
+impl Error for ValidationError {}
+
 impl fmt::Display for ValidationError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        writeln!(f, "provided value is not valid")
+        write!(
+            f,
+            "failed to create {}: provided value is invalid",
+            self.type_name
+        )
     }
 }