book: add enum documentation

Author: Kijewski

book/src/creating_templates.md

@@ -118,6 +118,85 @@ recognized:
   struct HelloTemplate<'a> { ... }
   ```
 
+## Templating `enum`s
+
+You can add derive `Template`s for `struct`s and `enum`s.
+If you add `#[template()]` only to the item itself, both item kinds work exactly the same.
+But with `enum`s you also have the option to add a specialized implementation to one, some,
+or all variants:
+
+```rust
+#[derive(Debug, Template)]
+#[template(path = "area.txt")]
+enum Area {
+    Square(f32),
+    Rectangle { a: f32, b: f32 },
+    Circle { radius: f32 },
+}
+```
+
+```jinja2
+{%- match self -%}
+    {%- when Self::Square(side) -%}
+        {{side}}^2
+    {%- when Self::Rectangle { a, b} -%}
+        {{a}} * {{b}}
+    {%- when Self::Circle { radius } -%}
+        pi * {{radius}}^2
+{%- endmatch -%}
+```
+
+will give you the same results as:
+
+```rust
+#[derive(Template, Debug)]
+#[template(ext = "txt")]
+enum AreaPerVariant {
+    #[template(source = "{{self.0}}^2")]
+    Square(f32),
+    #[template(source = "{{a}} * {{b}}")]
+    Rectangle { a: f32, b: f32 },
+    #[template(source = "pi * {{radius}}^2")]
+    Circle { radius: f32 },
+}
+```
+
+As you can see with the `ext` attribute, `enum` variants inherit most settings of the `enum`:
+`config`, `escape`, `ext`, `syntax`, and `whitespace`.
+Not inherited are: `block`, and `print`.
+
+If there is no `#[template]` annotation for an `enum` variant,
+then the `enum` needs a default implementation, which will be used if `self` is this variant.
+A good compromise between annotating only the template, or all its variants,
+might be using the `block` argument on the members:
+
+```rust
+#[derive(Template, Debug)]
+#[template(path = "area.txt")]
+enum AreaWithBlocks {
+    #[template(block = "square")]
+    Square(f32),
+    #[template(block = "rectangle")]
+    Rectangle { a: f32, b: f32 },
+    #[template(block = "circle")]
+    Circle { radius: f32 },
+}
+```
+
+```jinja2
+{%- block square -%}
+    {{self.0}}^2
+{%- endblock -%}
+
+{%- block rectangle -%}
+    {{a}} * {{b}}
+{%- endblock -%}
+
+{%- block circle -%}
+    pi * {{radius}}^2
+{%- endblock -%}
+```
+
 ## Documentation as template code
 [#documentation-as-template-code]: #documentation-as-template-code
 

testing/tests/enum.rs

@@ -3,6 +3,99 @@ use std::fmt::{Debug, Display};
 
 use rinja::Template;
 
+#[test]
+fn test_book_example() {
+    #[derive(Template, Debug)]
+    #[template(
+        source = "
+            {%- match self -%}
+                {%- when Self::Square(side) -%}      {{side}}^2
+                {%- when Self::Rectangle { a, b} -%} {{a}} * {{b}}
+                {%- when Self::Circle { radius } -%} pi * {{radius}}^2
+            {%- endmatch -%}
+        ",
+        ext = "txt"
+    )]
+    enum AreaWithMatch {
+        #[template(source = "{{self.0}}^2", ext = "txt")]
+        Square(f32),
+        #[template(source = "{{a}} * {{b}}", ext = "txt")]
+        Rectangle { a: f32, b: f32 },
+        #[template(source = "pi * {{radius}}^2", ext = "txt")]
+        Circle { radius: f32 },
+    }
+
+    assert_eq!(AreaWithMatch::Square(2.0).render().unwrap(), "2^2");
+    assert_eq!(
+        AreaWithMatch::Rectangle { a: 1.0, b: 2.0 }
+            .render()
+            .unwrap(),
+        "1 * 2",
+    );
+    assert_eq!(
+        AreaWithMatch::Circle { radius: 3.0 }.render().unwrap(),
+        "pi * 3^2"
+    );
+
+    #[derive(Template, Debug)]
+    enum AreaPerVariant {
+        #[template(source = "{{self.0}}^2", ext = "txt")]
+        Square(f32),
+        #[template(source = "{{a}} * {{b}}", ext = "txt")]
+        Rectangle { a: f32, b: f32 },
+        #[template(source = "pi * {{radius}}^2", ext = "txt")]
+        Circle { radius: f32 },
+    }
+
+    assert_eq!(AreaPerVariant::Square(2.0).render().unwrap(), "2^2");
+    assert_eq!(
+        AreaPerVariant::Rectangle { a: 1.0, b: 2.0 }
+            .render()
+            .unwrap(),
+        "1 * 2",
+    );
+    assert_eq!(
+        AreaPerVariant::Circle { radius: 3.0 }.render().unwrap(),
+        "pi * 3^2"
+    );
+
+    #[derive(Template, Debug)]
+    #[template(
+        source = "
+            {%- block square -%}
+                {{self.0}}^2
+            {%- endblock -%}
+            {%- block rectangle -%}
+                {{a}} * {{b}}
+            {%- endblock -%}
+            {%- block circle -%}
+                pi * {{radius}}^2
+            {%- endblock -%}
+        ",
+        ext = "txt"
+    )]
+    enum AreaWithBlocks {
+        #[template(block = "square")]
+        Square(f32),
+        #[template(block = "rectangle")]
+        Rectangle { a: f32, b: f32 },
+        #[template(block = "circle")]
+        Circle { radius: f32 },
+    }
+
+    assert_eq!(AreaWithBlocks::Square(2.0).render().unwrap(), "2^2");
+    assert_eq!(
+        AreaWithBlocks::Rectangle { a: 1.0, b: 2.0 }
+            .render()
+            .unwrap(),
+        "1 * 2",
+    );
+    assert_eq!(
+        AreaWithBlocks::Circle { radius: 3.0 }.render().unwrap(),
+        "pi * 3^2"
+    );
+}
+
 #[test]
 fn test_simple_enum() {
     #[derive(Template, Debug)]