This OpenMaple function is part of the MCallBackVector structure passed as an argument to StartMaple.

The callBackCallBack function executes when the Maple callback function is evaluated.  The callback function is application dependent, and can do whatever the driving application directs it to do.

The prototype for the function you can assign to the entry in the MCallBackVector must look like the following.

The output parameter contains the text version of the parameters passed to the Maple callback function.

On return, the callBackCallBack function returns either a NULL pointer or a pointer to a string containing a valid Maple expression. If the callBackCallBack function returns NULL, the Maple callback function returns nothing.  Otherwise the return value is parsed assuming it contains a Maple command string.  Maple parses the expression but does not evaluate it, and returns it as the result of the callback function.

This function can be used to explicitly pass intermediate values of a computation to your code and to return a value to Maple.

If no callBackCallBack function is provided, any attempt to execute the Maple callback function produces an error (reported using the errorCallBack function or textCallBack function).

The data parameter contains the same data as passed to StartMaple in the user_data parameter.

Source code for a callBackCallBack example is provided in the samples/OpenMaple/HelpExamples subdirectory of your Maple installation.

$\mathrm{callback}\left(x^2+y\right)$