Apply it to the GroceryList

xtofl · Feb 10, 2019 · 4b85733 · 4b85733
1 parent ff01021
commit 4b85733
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Showing 2 changed files with 29 additions and 7 deletions.
diff --git a/monoid/groceries_monoid.hpp b/monoid/groceries_monoid.hpp
@@ -3,14 +3,17 @@
 #include "groceries.h"
 #include "monoid.hpp"
 
+const auto intmapmonoid =
+    lean::fmonoid<std::map<std::string, int>>(
+        lean::monoid(0, std::plus<int>{})
+    );
+
 const auto grocery_monoid = lean::monoid(
-    GroceryList{},
-    [](auto a, auto b){
-        for (const auto &ib: b.items) {
-            a.items[ib.first] += ib.second;
-        }
-        return a;
-    });
+    GroceryList{intmapmonoid.mempty},
+    [](auto a, auto b) -> GroceryList{
+        return {intmapmonoid.mappend(a.items, b.items)};
+    }
+);
 
 template<>
 GroceryList overloading::mempty<GroceryList>() { return {}; }

diff --git a/monoid/monoid.md b/monoid/monoid.md
@@ -806,6 +806,25 @@ EXPECT_EQ(
     )
 );
 ```
+--
+
+## After
+
+```C++
+const auto intmapmonoid =
+    lean::fmonoid<std::map<std::string, int>>(
+        lean::monoid(0, std::plus<int>{})
+    );
+
+const auto grocery_monoid = lean::monoid(
+    GroceryList{intmapmonoid.mempty},
+    [](auto a, auto b) -> GroceryList {
+        return {intmapmonoid.mappend(a.items, b.items)};
+    }
+);
+```
+
+(We could dig deeper and also extract the `{ ... .items}`)
 
 --