Foreign values are resolved dynamically/lazily (and "cached" thereafter), so you don't need to define all of them right away. If needed and missing during runtime, an exception will be thrown. Using a source-level debugger which supports C++ will reveal the missing implementation easily enough.

To implement foreign values to be exported:

1. Create a C++ source file (typically in your_workspace/ffi)
2. #include purescript.h and any other C/C++ dependencies
3. Use macros FOREIGN_BEGIN( Module_Name ) and FOREIGN_END to delineate your block of exported values.
   For example:
   

   #include "purescript.h"
   
   // Tested with package v4.0.0
   
   FOREIGN_BEGIN( Data_Semigroup )
   
   exports["concatString"] = [](const boxed& s1) -> boxed {
       return [=](const boxed& s2) -> boxed {
           return unbox<string>(s1) + unbox<string>(s2);
       };
   };
   
   FOREIGN_END

See the standard library implementations for more examples