Improve documentation of set command (#13269)

Source/cmSetCommand.h

@@ -52,7 +52,7 @@ public:
    */
   virtual const char* GetTerseDocumentation() const
     {
-    return "Set a CMAKE variable to a given value.";
+    return "Set a CMake, cache or environment variable to a given value.";
     }
   
   /**
@@ -63,26 +63,37 @@ public:
     return
       "  set(<variable> <value>\n"
       "      [[CACHE <type> <docstring> [FORCE]] | PARENT_SCOPE])\n"
-      "Within CMake sets <variable> to the value <value>.  <value> is expanded"
-      "  before <variable> is set to it.  If CACHE is present, then the "
-      "<variable> is put in the cache. <type> and <docstring> are then "
-      "required. <type> is used by the CMake GUI to choose a widget with "
-      "which the user sets a value.  The value for <type> may be one of\n"
+      "Within CMake sets <variable> to the value <value>.  "
+      "<value> is expanded before <variable> is set to it.  "
+      "Normally, set will set a regular CMake variable. "
+      "If CACHE is present, then the <variable> is put in the cache "
+      "instead, unless it is already in the cache. "
+      "See section 'Variable types in CMake' below for details of "
+      "regular and cache variables and their interactions. "
+      "If CACHE is used, <type> and <docstring> are required. <type> is used "
+      "by the CMake GUI to choose a widget with which the user sets a value. "
+      "The value for <type> may be one of\n"
       "  FILEPATH = File chooser dialog.\n"
       "  PATH     = Directory chooser dialog.\n"
       "  STRING   = Arbitrary string.\n"
       "  BOOL     = Boolean ON/OFF checkbox.\n"
       "  INTERNAL = No GUI entry (used for persistent variables).\n"
-      "If <type> is INTERNAL, then the <value> is always written into the "
-      "cache, replacing any values existing in the cache.  If it is not a "
-      "cache variable, then this always writes into the current makefile. The "
-      "FORCE option will overwrite the cache value removing any changes by "
-      "the user.\n"
+      "If <type> is INTERNAL, the cache variable is marked as internal, "
+      "and will not be shown to the user in tools like cmake-gui. "
+      "This is intended for values that should be persisted in the cache, "
+      "but which users should not normally change. INTERNAL implies FORCE."
+      "\n"
+      "Normally, set(...CACHE...) creates cache variables, but does not "
+      "modify them. "
+      "If FORCE is specified, the value of the cache variable is set, even "
+      "if the variable is already in the cache. This should normally be "
+      "avoided, as it will remove any changes to the cache variable's value "
+      "by the user.\n"
       "If PARENT_SCOPE is present, the variable will be set in the scope "
       "above the current scope. Each new directory or function creates a new "
       "scope. This command will set the value of a variable into the parent "
       "directory or calling function (whichever is applicable to the case at "
-      "hand).\n"
+      "hand). PARENT_SCOPE cannot be combined with CACHE.\n"
       "If <value> is not specified then the variable is removed "
       "instead of set.  See also: the unset() command.\n"
       "  set(<variable> <value1> ... <valueN>)\n"
@@ -90,7 +101,58 @@ public:
       "values.\n"
       "<variable> can be an environment variable such as:\n"
       "  set( ENV{PATH} /home/martink )\n"
-      "in which case the environment variable will be set.";
+      "in which case the environment variable will be set.\n"
+      "*** Variable types in CMake ***\n"
+      "In CMake there are two types of variables: normal variables and cache "
+      "variables. Normal variables are meant for the internal use of the "
+      "script (just like variables in most programming languages); they are "
+      "not persisted across CMake runs. "
+      "Cache variables (unless set with INTERNAL) are mostly intended for "
+      "configuration settings where the first CMake run determines a "
+      "suitable default value, which the user can then override, by editing "
+      "the cache with tools such as ccmake or cmake-gui. "
+      "Cache variables are stored in the CMake cache file, and "
+      "are persisted across CMake runs. \n"
+      "Both types can exist at the same time with the same name "
+      "but different values. "
+      "When ${FOO} is evaluated, CMake first looks for "
+      "a normal variable 'FOO' in scope and uses it if set. "
+      "If and only if no normal variable exists then it falls back to the "
+      "cache variable 'FOO'.\n"
+      "Some examples:\n"
+      "The code 'set(FOO \"x\")' sets the normal variable 'FOO'. It does not "
+      "touch the cache, but it will hide any existing cache value 'FOO'.\n"
+      "The code 'set(FOO \"x\" CACHE ...)' checks for 'FOO' in the cache, "
+      "ignoring any normal variable of the same name. If 'FOO' is in the "
+      "cache then nothing happens to either the normal variable or the cache "
+      "variable. If 'FOO' is not in the cache, then it is added to the "
+      "cache.\n"
+      "Finally, whenever a cache variable is added or modified by a command, "
+      "CMake also *removes* the normal variable of the same name from the "
+      "current scope so that an immediately following evaluation of "
+      "it will expose the newly cached value.\n"
+      "Normally projects should avoid using normal and cache variables of "
+      "the same name, as this interaction can be hard to follow. "
+      "However, in some situations it can be useful. "
+      "One example (used by some projects):"
+      "\n"
+      "A project has a subproject in its source tree. The child project has "
+      "its own CMakeLists.txt, which is included from the parent "
+      "CMakeLists.txt using add_subdirectory(). "
+      "Now, if the parent and the child project provide the same option "
+      "(for example a compiler option), the parent gets the first chance "
+      "to add a user-editable option to the cache. "
+      "Normally, the child would then use the same value "
+      "that the parent uses. "
+      "However, it may be necessary to hard-code the value for the child "
+      "project's option while still allowing the user to edit the value used "
+      "by the parent project. The parent project can achieve this simply by "
+      "setting a normal variable with the same name as the option in a scope "
+      "sufficient to hide the option's cache variable from the child "
+      "completely. The parent has already set the cache variable,  so the "
+      "child's set(...CACHE...) will do nothing, and evaluating the option "
+      "variable will use the value from the normal variable, which hides the "
+      "cache variable.";
     }
   
   cmTypeMacro(cmSetCommand, cmCommand);