Undocumented WolframLibraryData::evaluateExpression

Question

It seem there's this promising function in the callback object provided to MathLink C functions:

struct st_WolframLibraryData
{
    /* ... */
    int (*evaluateExpression)(WolframLibraryData, char *, int, mint, void *);
    /* ... */
};

If this does what it seems it does, it would save the boring and lengthy code that comes with managing a MLINK object for some purpose. But I'm lost at interpreting the arguments (and if the second one is supposedly the string to execute, why is it not const?).

Answer 1

I have done a brief reverse engineering, and this is the result.

Here's how to interpret the function:

int // 0 = LIBRARY_NO_ERROR for success, 6 = LIBRARY_FUNCTION_ERROR for bad syntax
evaluateExpression(WolframLibraryData _ignored, char* expression, int ResultType, mint ResultStuff1, void* ResultStuff2);

So the problem is that the function tries to store the result of the expression in some internal format using the last two arguments... unless ResultType == 6.

So, this wrapper comes handy if you want to evaluate some hardcoded expressions in your C library:

int MathematicaEval(WolframLibraryData wd, const char* expression){
    return wd->evaluateExpression(0, (char*)(expression), 6, 0, 0);
}