Update 1.3's C++ concept implementation of Monoid (#259)

Co-authored-by: Alfonso Ros <alfonso.ros@esrlabs.com>

Author: rosds

src/content/1.3/categories-great-and-small.tex

@@ -177,41 +177,42 @@ \section{Monoid as Set}
 so this might be a little confusing to the beginner.)
 
 The closest one can get to declaring a monoid in C++ would be to use the
-(proposed) syntax for concepts.
+C++20 standard's concept feature.
 
 \begin{snip}{cpp}
 template<class T>
-  T mempty = delete;
-  
+struct mempty;
+
 template<class T>
-  T mappend(T, T) = delete;
-  
-template<class M> 
-  concept bool Monoid = requires (M m) { 
-    { mempty<M> } -> M; 
-    { mappend(m, m); } -> M;
-  };
+T mappend(T, T) = delete;
+
+template<class M>
+concept Monoid = requires (M m) {
+    { mempty<M>::value() } -> std::same_as<M>;
+    { mappend(m, m) } -> std::same_as<M>;
+};
 \end{snip}
-The first definition uses a value template (also proposed). A
-polymorphic value is a family of values --- a different value for every
-type.
+The first definition is a structure meant to hold the neutral element for each
+specialization.
 
 The keyword \code{delete} means that there is no default value
 defined: It will have to be specified on a case-by-case basis.
 Similarly, there is no default for \code{mappend}.
 
-The concept \code{Monoid} is a predicate (hence the \code{bool}
-type) that tests whether there exist appropriate definitions of
+The concept \code{Monoid} tests whether there exist appropriate definitions of
 \code{mempty} and \code{mappend} for a given type \code{M}.
 
 An instantiation of the Monoid concept can be accomplished by providing
 appropriate specializations and overloads:
 
 \begin{snip}{cpp}
 template<>
-std::string mempty<std::string> = {""};
+struct mempty<std::string> {
+    static std::string value() { return ""; }
+};
 
-std::string mappend(std::string s1, std::string s2) { 
+template<>
+std::string mappend(std::string s1, std::string s2) {
     return s1 + s2;
 }
 \end{snip}