MzCOM: Scheme as a Windows COM Object

MzCOM.exe is a Windows COM (i.e., Component Object Model) class wrapper for PLT Scheme.

During normal installation of MzCOM, the executable is registered as a COM object automatically. If you move the PLT Scheme installation folder, re-register MzCOM.exe with

  mzcom.exe /RegServer

The MzCOM.exe executable find DLLs and PLT Scheme library collections relative to its own path.

1 Loading MzCOM

To load a COM object, COM hosts require a COM class name or a ProgID. MzCOM has the class name "MzObj Class" and the ProgID "MzCOM.MzObj.⟨version⟩", where ⟨version⟩ is 4.0.2.

In the Visual BASIC 6 environment, from the Project|References (VB6), check MzCOM 1.0 Type Library. In Visual BASIC .NET, choose Project|Add Reference, and from the COM tab, select MzCOM 1.0 Type Library. In your code, declare a variable, then assign to it:

DIM schemeObject AS MzObj

SET schemeObject = NEW MzObj

From Visual C++:

#include "mzcom.h"

CLSID clsid;

IMzObj *pIMzObj;

CoInitialize(NULL);

CLSIDFromProgID(L"MzCOM.MzObj.<version>",&clsid);

CoCreateInstance(clsid,NULL,CLSCTX_SERVER,IID_IMzObj, (void **)&pIMzObj);

where <version> is the version number. You’ll need the definition of IID_IMzObj (see GUIDs). The header file "mzcom.h" is generated as "src\worksp\mzcom\" when building from the PLT Scheme source distribution. The above C/C++ code is for illustration; your actual code should check return values, of course.

Using mysterx to manipulate COM objects within Scheme, you can load MzCOM with either

  (cci/coclass "MzObj Class")

or

  (cci/progid "MzCOM.MzObj.<version>")

Consult your documentation for loading MzCOM into other COM environments. MzCOM is compiled as a “dual-mode” class, meaning its methods may be called directly or by using OLE Automation.

2 GUIDs

When compiled from the PLT Scheme source distibrution, the directory "src\worksp\mzcom\" contains the file "MzCOM_i.c" that contains GUIDs for MzCOM. Those GUIDs are as follows:

const IID IID_IMzObj =

{0xA604CBA8,0x2AB5,0x11D4,{0xB6,0xD3,0x00,0x60,0x08,0x90,0x02,0xFE}};

const IID LIBID_MZCOMLib =

{0xA604CB9C,0x2AB5,0x11D4,{0xB6,0xD3,0x00,0x60,0x08,0x90,0x02,0xFE}};

const IID DIID__IMzObjEvents =

{0xA604CBA9,0x2AB5,0x11D4,{0xB6,0xD3,0x00,0x60,0x08,0x90,0x02,0xFE}};

const CLSID CLSID_MzObj =

{0xA3B0AF9E,0x2AB0,0x11D4,{0xB6,0xD2,0x00,0x60,0x08,0x90,0x02,0xFE}};

which represent the IMzObj interface, the MzCOM type library, the IMzObjEvents interface, and the MzObj class, respectively.

3 Methods

MzCOM support three COM methods:

• void About(void)

  Takes no arguments and displays an informational dialog.

• BSTR Eval(BSTR input)

  Takes and returns BSTRs (BASIC strings). The returned value is the result of evaluating the input expression, formatted as a string. The input string may contain several S-expressions. The embedded PLT Scheme updates its environment with each evaluation. Therefore, it is possible to define procedures in a call to Eval, and use the procedures in subsequent calls.

• Reset :: void Reset(void)

  Resets the Scheme environment to the initial environment. Also, the custodian for the primary Scheme thread is invoked, shutting all its managed values.

4 Events

MzCOM supports a single event.

• SchemeError()

  Passed a BSTR (BASIC string) that explains the error.

5 Errors

When an error occurs in MzCOM, it creates a COM error object. C and C++ clients can use GetErrorInfo to retrieve error information. Clients implemented in other languages typically have some equivalent means to obtain COM error information.

6 Evaluation thread

The PLT Scheme evaluator runs in a Win32 thread created when MzCOM is loaded. If an expression kills the primary MzScheme thread, as in

  (kill-thread (current-thread))

then the evaluator Win32 thread is also killed. When that happens, subsequent calls to Eval() will fail.

7 Acknowledgments

MzCOM was developed in response to a query by Andre Van Meulebrouck. Andre also did extensive testing with Visual BASIC.