build: enhance FYPP error messaging and add defensive checks in stdlib.cmake (#1137)

* build: enhance fypp error messaging for better onboarding

* address reviewer feedback: simplify error message and fix logic typo

* Final changes

* Suggested Changes

* updated

Author: srinjoy933

CMakeLists.txt

@@ -4,8 +4,8 @@ cmake_minimum_required(VERSION 3.14.0)
 set(CMAKE_USER_MAKE_RULES_OVERRIDE ${CMAKE_CURRENT_SOURCE_DIR}/config/DefaultFlags.cmake)
 
 project(fortran_stdlib
-	LANGUAGES Fortran C
-        DESCRIPTION "Community driven and agreed upon de facto standard library for Fortran"
+  LANGUAGES Fortran C
+  DESCRIPTION "Community driven and agreed upon de facto standard library for Fortran"
 )
 
 # Read version from file
@@ -116,8 +116,19 @@ endif()
 
 # --- find preprocessor
 find_program(FYPP fypp)
-if(NOT FYPP)
-  message(FATAL_ERROR "Preprocessor fypp not found! Please install fypp following the instructions in https://fypp.readthedocs.io/en/stable/fypp.html#installing")
+if(NOT FYPP OR "${FYPP}" MATCHES "NOTFOUND")
+  message(FATAL_ERROR 
+    "\n [stdlib Build Error]: Preprocessor 'fypp' not found!\n"
+    " ------------------------------------------------------------------------\n"
+    " The fypp preprocessor is required to generate Fortran source from templates.\n\n"
+    " Please install fypp following the instructions at https://github.com/aradi/fypp\n\n"
+    " To diagnose your installation, please run the following in your terminal:\n"
+    "   Windows:      'where fypp' and 'where python'\n"
+    "   Linux/macOS:  'which fypp' and 'which python'\n\n"
+    " Ensure the executable is in your PATH or set it manually during configuration:\n"
+    "   cmake -B build -DFYPP_EXECUTABLE=/path/to/fypp\n"
+    " ------------------------------------------------------------------------\n"
+  )
 endif()
 
 # Custom preprocessor flags
@@ -156,4 +167,4 @@ install(EXPORT ${PROJECT_NAME}-targets
 )
 
 # Export a pkg-config file
-include(${PROJECT_SOURCE_DIR}/config/export_pc.cmake)
+include(${PROJECT_SOURCE_DIR}/config/export_pc.cmake)

cmake/stdlib.cmake

@@ -10,6 +10,12 @@
 #
 function(preprocess preproc preprocopts srcext trgext srcfiles trgfiles)
 
+  # Defensive check: Ensure the preprocessor path is valid before defining custom commands.
+  # This prevents "empty command" errors during the build phase.
+  if(NOT preproc OR "${preproc}" MATCHES "NOTFOUND")
+    message(FATAL_ERROR "preprocess function called with invalid preprocessor path: '${preproc}'")
+  endif()
+
   set(_trgfiles)
   foreach(srcfile IN LISTS srcfiles)
     get_filename_component(filename ${srcfile} NAME)
@@ -87,8 +93,8 @@ function(configure_stdlib_target target_name regular_sources_var fypp_files_var
     endif()
 
     set(LIB_MOD_DIR ${CMAKE_CURRENT_BINARY_DIR}/mod_files/${target_name}/)
-    #set(INSTALL_MOD_DIR "${CMAKE_INSTALL_MODULEDIR}/${target_name}")
     set(INSTALL_MOD_DIR "${CMAKE_INSTALL_MODULEDIR}")
+    
     # We need the module directory before we finish the configure stage since the
     # build interface might resolve before the module directory is generated by CMake
     if(NOT EXISTS "${LIB_MOD_DIR}")
@@ -115,7 +121,7 @@ endfunction()
 # Determine if a module will be compiled
 #
 # Defines a CMake function that creates an ON/OFF option for a given stdlib module,
-#sets a compile definition accordingly, and prints its enabled/disabled status.
+# sets a compile definition accordingly, and prints its enabled/disabled status.
 #
 # Args:
 #     module [in]: Name of the module to be compiled
@@ -133,4 +139,4 @@ function(check_modular module)
         add_compile_definitions(STDLIB_${umodule}=0)
     endif()
 
-endfunction()
+endfunction()