difference between 'traonsform' and 'and_then' in monadic interface

yielding · yielding · commit fea00f82088e · 2023-09-04T10:49:14.000+09:00
diff --git a/cpp.fp/expected/main.cpp b/cpp.fp/expected/main.cpp
@@ -35,7 +35,10 @@ auto parse_number(string_view str) -> expected<double, parse_error>
 auto print_error(parse_error e) -> expected<double, string>
 {
   if (e == parse_error::invalid_input)
+  {
+    logger.()
     return unexpected("error : invalid input");
+  }
 
   if (e == parse_error::overflow)
     return unexpected("error : overflow");
diff --git a/cpp.fp/optional/optional.cpp b/cpp.fp/optional/optional.cpp
@@ -23,8 +23,11 @@ int main(int argc, char* argv[])
 {
   vector_opt<string> v = { "1234", "15 foo", "bar", "42", "5000", " 5" };
 
+  // NOTICE
+  // an operation which changes stored variable -> transform
+  // an operation which could itself fail -> and_then
   auto filter = [](auto&& o) {
-    return o.and_then(to_int) // flatmap from str to int
+    return o.and_then(to_int)   // flatmap from str to int
             .transform([](int n) { return n + 1; })
             .transform([](int n) { return to_string(n); })
             .or_else  ([]        { return optional("null"s); });
