add let helper as an example

examples/block_helper_macro_let.rs

@@ -0,0 +1,65 @@
+use handlebars::{
+    BlockParamHolder, Context, Handlebars, Helper, Output, RenderContext, RenderError,
+    RenderErrorReason,
+};
+use serde_json::{json, Value};
+
+// a custom block helper to bind a variable name to a value
+pub fn helper_let<'reg, 'rc>(
+    h: &Helper<'rc>,
+    _r: &'reg Handlebars<'reg>,
+    _ctx: &'rc Context,
+    rc: &mut RenderContext<'reg, 'rc>,
+    _out: &mut dyn Output,
+) -> Result<(), RenderError> {
+    let name_param = h
+        .param(0)
+        .ok_or_else(|| RenderErrorReason::ParamNotFoundForIndex("let", 0))?;
+
+    let handlebars::ScopedJson::Constant(Value::String(name_constant)) = name_param.value else {
+        return Err(RenderErrorReason::ParamTypeMismatchForName(
+            "let",
+            "0".to_string(),
+            "constant string".to_string(),
+        )
+        .into());
+    };
+
+    let value = h
+        .param(1)
+        .as_ref()
+        .map(|v| v.value().to_owned())
+        .ok_or_else(|| RenderErrorReason::ParamNotFoundForIndex("let", 2))?;
+
+    let block = rc.block_mut().unwrap();
+
+    block.set_block_param(name_constant, BlockParamHolder::Value(value));
+
+    Ok(())
+}
+
+fn main() -> Result<(), RenderError> {
+    // create the handlebars registry
+    let mut handlebars = Handlebars::new();
+
+    handlebars.register_helper("let", Box::new(helper_let));
+
+    let input = r#"
+{{#if foo}}
+{{let "mixin_classes" "foo-bar baz"}}
+{{else}}
+{{let "mixin_classes" "quux"}}
+{{/if}}
+
+<div class="content-main {{mixin_classes}}">
+    <p> content </p>
+</div>
+"#;
+
+    println!(
+        "{}",
+        handlebars.render_template(input, &json!({"foo": true}))?
+    );
+
+    Ok(())
+}