INPUT Execution – Error Handling

Errors can occur while parsing the input – in one of the input parse codes. When an error occurs, after it is reported, the already parsed values in the temporary input values stack need to be thrown away and execution needs to resume at the beginning of the INPUT statement, except instead of issuing the prompt again, the cursor will be positioned at the beginning of the input, which will contain the previously erroneous input to allow the user to correct the input instead of reentering it (the traditional “redo from start”).

The temporary input values stack can't simply be reset (setting the internal index to -1, the empty stack indicator) because elements may contain allocated string values, which need to be deleted to prevent memory leaks. Like the evaluation stack, the elements in the temporary input values stack won't have any indicator what data type they are. Consider the basic format of the INPUT statement (only up to the parsing codes is shown):

InputBegin InputParseType1 InputParseType2 InputParseType3'End' ...

Say an error occurs on the second parse code (the program execution pointer will be pointing at next code, the InputParseType3, in other words, the pointer is incremented after reading each program code word, then the code read is executed by calling its run-time handler). Execution needs to be backed up until the InputBegin is reached (reading the program codes in reverse).

As each code is passed, one element will be popped from the temporary input values stack. If the code passed is an InputParseStr code, then the element popped is a string that needs to be deleted. The beginning is reached when a non input parse code is reached (it could be InputBegin, InputBeginStr or InputBeginTmp).  The stack will now be empty.

The INPUT begin code will be executed again and the begin code will call the get input routine. The get input routine will normally allocates the temporary input values stack. For error recovery, it will see that the stack is already allocated, so instead of outputting the prompt and saving the cursor position to the beginning of the input, it will restore the saved cursor position and get the input starting with the previously entered erroneous input. Execution will then resume with the corrected input.