Blocking Commands

Where it's going to get complicated for my interactive compiler is with the blocking commands. Examples of blocking commands include IF/THEN/ELSE/ENDIF, FOR/NEXT, DO/LOOP and SELECT/CASE. These commands are entered on multiple lines and can be entered in any order. A regular compiler matches the blocks during compilation and any unmatched blocks are reported as “context” errors. Generally an interpreter does not catch these context errors until run time since the program lines can be entered in any order. Ideally, an interactive compiler should catch as many errors as possible as soon as possible and before run time, including these context errors.  For run time speed, the less checking done at run time, the faster it will run the program.

Brown suggests ignoring these context errors when the lines are entered and having a “pre-run” module that goes through the program checking for context errors before the program is run. Brown briefly discusses an alternative method where tables are kept as these context errors are created in a table and removing entries as the context errors are resolved, but decides this method is too complicated and opts for the simpler approach of having a pre-run module.

I disagree with the need to have a pre-run module, which would delay the starting of the program when the RUN command is entered.  This takes away one of the advantages of an interactive compiler and would make it appear to be simply compiling the program before running. I plan on taking Brown's table idea further. It's difficult to explain, but essentially as lines are entered, a table with information on the blocks in the code is built as the lines are entered and the number of current context errors is kept track of. These context errors are different from syntax errors that can be caught immediately when a line is entered. With a block table and context error outstanding count, when a RUN command is issued, this count simply needs to be checked and if not zero, the errors are reported and the program is not run.