add field_lens! macro

src/lenses/fields.rs

@@ -0,0 +1,12 @@
+#[macro_export]
+macro_rules! field_lens {
+    ($type:ident, $field:ident) => {
+        lens(
+            |v: $type| v.$field,
+            |mut u: $type, v| {
+                u.$field = v;
+                u
+            },
+        )
+    };
+}

src/lenses/mod.rs

@@ -1,3 +1,5 @@
+mod fields;
+
 mod identity;
 pub use identity::id;
 
@@ -164,6 +166,18 @@ mod tests {
         assert_eq!(l(hello), 8);
     }
 
+    #[test]
+    fn can_make_lens_for_field() {
+        // making a lens
+        let l = crate::field_lens!(Hello, hey);
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello), 8);
+
+        let hello = Hello { hey: 8 };
+        assert_eq!(l(hello, |v| v + 1), Hello { hey: 9 });
+    }
+
     #[test]
     fn can_make_lens_out_of_funcs() {
         // making a lens