added default 'unnamed exercise module' for exercise modules that are…

… unnamed. without this, unnamed module exported as .txt files rather than .ml files

src/haz3lweb/app/editors/mode/ExercisesMode.re

@@ -175,8 +175,10 @@ module Update = {
 
   let export_exercise_module = (exercises: Model.t): unit => {
     let exercise = Model.get_current(exercises);
-    let module_name = exercise.editors.module_name;
-    let filename = exercise.editors.module_name ++ ".ml";
+    let module_name =
+      StringUtil.isEmptyOrWhitespace(exercise.editors.module_name)
+        ? "Unnamed Exercise Module" : exercise.editors.module_name;
+    let filename = module_name ++ ".ml";
     let content_type = "text/plain";
     let contents =
       Exercise.export_module(module_name, {eds: exercise.editors});

src/util/StringUtil.re

@@ -63,3 +63,8 @@ let unescape_linebreaks: string => string =
   Re.Str.global_replace(Re.Str.regexp("\\\\n"), "\n");
 
 let trim_leading = Re.Str.global_replace(Re.Str.regexp("\n[ ]*"), "\n");
+
+let isEmptyOrWhitespace = str => {
+  let trimmed = String.trim(str);
+  String.length(trimmed) == 0;
+};