test: add example for getting error generated from helper [#512]

examples/error.rs

@@ -3,11 +3,44 @@ extern crate handlebars;
 #[macro_use]
 extern crate serde_json;
 
-use std::error::Error;
+use std::error::Error as StdError;
 
-use handlebars::Handlebars;
+use handlebars::{Context, Handlebars, Helper, Output, RenderContext, RenderError};
+use thiserror::Error;
 
-fn main() -> Result<(), Box<dyn Error>> {
+#[derive(Debug, Error)]
+pub enum HelperError {
+    #[error("db error")]
+    DbError,
+    #[error("api error")]
+    ApiError,
+}
+
+/// A helper that raise error according to parameters
+pub fn error_helper(
+    h: &Helper,
+    _: &Handlebars,
+    _: &Context,
+    _: &mut RenderContext,
+    _: &mut dyn Output,
+) -> Result<(), RenderError> {
+    let param = h
+        .param(0)
+        .ok_or(RenderError::new("Param 0 is required for error helper."))?;
+    match param.value().as_str() {
+        Some("db") => Err(RenderError::from_error(
+            "helper error",
+            HelperError::DbError,
+        )),
+        Some("api") => Err(RenderError::from_error(
+            "helper error",
+            HelperError::ApiError,
+        )),
+        _ => Ok(()),
+    }
+}
+
+fn main() -> Result<(), Box<dyn StdError>> {
     env_logger::init();
     let mut handlebars = Handlebars::new();
 
@@ -36,5 +69,17 @@ fn main() -> Result<(), Box<dyn Error>> {
     println!("{}", be1.source().unwrap());
     println!("{:?}", be1.source().unwrap().source());
 
+    // process helper error
+    handlebars.register_helper("err", Box::new(error_helper));
+    let e2 = handlebars
+        .render_template("{{err \"db\"}}", &json!({}))
+        .unwrap_err();
+    match e2.source().unwrap().downcast_ref::<HelperError>() {
+        Some(HelperError::DbError) => {
+            println!("Detected error from helper: db error",)
+        }
+        _ => {}
+    }
+
     Ok(())
 }