IMPORT FGL module

The IMPORT FGL instruction imports module symbols.

Syntax

IMPORT FGL modulename

modulename is the identifier (without the file extension) of the module to be imported.

Usage

With IMPORT FGL modulename, the symbols of the named .42m module can be referenced in the current module.

Important: At runtime, the imported modules are only loaded on demand, when the program flow reaches an instruction that uses an element of the imported module. For example, when calling a function or when assigning a (public) module variable of the imported module.

The name specified after the IMPORT FGL instruction is case-sensitive.

The imported module symbols that can be referenced are:

Public functions
Public constants
Public types
Public module variables

The PRIVATE/PUBLIC modifiers can be used to hide / publish symbols to other modules. Functions are by default public, for backward compatibility. The next example declares a module variable that can be used by other modules, and a private function to be used only locally:

PUBLIC DEFINE custlist DYNAMIC ARRAY OF RECORD
  id INT,
  name VARCHAR(50),
  address VARCHAR(200)
END RECORD
...
PRIVATE FUNCTION myfunction()
...

When importing modules with the IMPORT FGL instruction, you instruct the fglcomp compiler and fglrun runtime system to load/check the specified modules, and there is no longer a need to link programs or use libraries.

With IMPORT FGL, the compiler can check the number of parameters and returning values in functions calls, and the completion in source code editors is improved as it can suggest all imported symbols.

Imported modules should be compiled before compiling the importing module. The FGLLDPATH environment variable specifies the directories to search for the .42m modules used by IMPORT FGL.

However, if the 42m file of the imported module is not existing, or is older as the corresponding source file, fglcomp will automatically compile the imported module. To avoid implicit compilation of imported modules, use the --implicit=none option of fglcomp. If the .42m file exists but the .4gl source file cannot be found, fglcomp imports the .42m file as is.

Important: Auto-compilation of imported modules is only supported if the imported module is in the current directory. Modules located in other directories and found width FGLLDPATH must alredy be compiled.

No circular references are allowed. For example when module A imports module B, which in turn imports module A, you cannot compile one of the modules because the 42m file of the imported module is needed. Thus fglcomp will give error -8403, indicating that the imported module cannot be found:

Module "mod_a.4gl":

IMPORT FGL module_b
FUNCTION func_a()
  CALL func_b()
END FUNCTION

Module "mod_b.4gl":

IMPORT FGL module_a
FUNCTION func_b()
  CALL func_a()
END FUNCTION

Traditional linking is still supported for backward compatibility. To ease migration from traditional linking to imported modules, you can mix IMPORT FGL usage with fgllink. By default, even when IMPORT FGL is used, fglcomp does not raise an error if a referenced function is not found in the imported modules. This is mandatory to compile the 42m file to be linked later with the module defining the missing function. Use the -W implicit or the --resolve-calls option to check for imported functions.

When the -W implicit option is used and at least one IMPORT FGL is defined in the module, fglcomp will print warning -8406 for any referenced function that cannot be found in the imported modules. This option is silently ignored if no IMPORT FGL is used in the module.

To enable full symbol resolution by the compiler, use the --resolve-calls option. This option will force the compiler to check all function symbols referenced in a module, and raise error -8406, if a symbol could not be found in the imported modules. This option is typically used in programs that are only using IMPORT FGL and do not longer use the link phase.

When migrating existing projects using traditional linking, after compiling all the .4gl sources, consider using the --print-imports option of fglrun to print the IMPORT FGL suggestions for all the modules specified in the command line. This option will try to resolve all symbols as during linking, but instead of producing a .42r program, it will list the import instructions to be added in each module, and thus avoid linking:

$ cat main.4gl
MAIN
    CALL func1()
END MAIN
$ cat mod1.4gl
FUNCTION func1()
   CALL func2()
END FUNCTION
$ cat mod2.4gl
FUNCTION func2()
   CALL func1()
END FUNCTION

$ fglrun --print-imports main.42m mod1.42m mod2.42m
-- in main.4gl
IMPORT FGL mod1

-- in mod1.4gl
IMPORT FGL mod2

-- in mod2.4gl
# Cyclic import: IMPORT FGL mod1
#   caused by CALL func1

If a symbol is defined twice with the same name in two different modules, the symbol must be qualified by the name of the module. This feature overcomes the traditional 4gl limitation requiring unique function names within a program. In the next example, both imported modules define the same "init()" function, but this can be resolved by adding the module name followed by a dot before the function names:

IMPORT FGL orders 
IMPORT FGL customers 
MAIN
  CALL orders.init()
  CALL customers.init()
  ...
END MAIN

If a symbol is defined twice with the same name in the current and the imported module, an unqualified symbol will reference the current module symbol. The next example calls the "init()" function with and without a module qualifier, the second call will reference the local function:

IMPORT FGL orders 
MAIN
  CALL orders.init()  -- orders module function 
  CALL init()  -- local function 
  ...
END MAIN
FUNCTION init()
  ...
END FUNCTION

Example

Module "account.4gl":

PRIVATE DEFINE current_account VARCHAR(20)

PUBLIC FUNCTION set_account(id)
  DEFINE id VARCHAR(20)
  LET current_account = id 
END FUNCTION
...  -- File: myutils.4gl 
PRIVATE DEFINE initialized BOOLEAN

PUBLIC TYPE t_prog_info RECORD
        name STRING,
        version STRING,
        author STRING
     END RECORD

PUBLIC FUNCTION init()
  LET initialized = TRUE
  ...
END FUNCTION

PUBLIC FUNCTION fini()
  LET initialized = FALSE
  ...
END FUNCTION

Module "myutils.4gl":

PRIVATE DEFINE initialized BOOLEAN

PUBLIC TYPE t_prog_info RECORD
        name STRING,
        version STRING,
        author STRING
     END RECORD

PUBLIC FUNCTION init()
  LET initialized = TRUE
  ...
END FUNCTION

PUBLIC FUNCTION fini()
  LET initialized = FALSE
  ...
END FUNCTION

Module "program.4gl":

IMPORT FGL myutils 
IMPORT FGL account 
DEFINE filename STRING
DEFINE proginfo t_prog_info  -- Type is defined in myutils 
MAIN
  LET proginfo.name = "program"
  LET proginfo.version = "0.99"
  LET proginfo.author = "scott"
  CALL myutils.init()  -- with module prefix
  CALL set_account("CFX4559")  -- without module prefix 
END MAIN