Years ago (2006) I wrote a compiler for TI BASIC, the dialect of BASIC that I learned on my TI 99/4A (many many years earlier). This is an “ancient” computer language and one of the first that I learned. (I had a few years of experience with Apple Basic on Apple IIe before getting a TI.)
I was reminded of this project tonight when I added my github repositories to my LinkedIn profile. There is now a slightly larger chance that someone might actually see this code. Which leads me to my point: there is a lot of bad code in that repository. I hadn’t really seen the “refactoring light” at that point in my career and there are large chunks of that code base that are badly in need of it.
The one saving grace is that there are actual tests included with the project. I took code examples from the reference manual and typed them in as separate programs. I also coded up the expected output from each program in python scripts. I then have a Makefile that compiles each sample program, runs it, and pipes the output to the python script that checks that each line of output matches what is expected.
So this means that I can be fairly confident in refactoring some of this code 6 years later. I hardly know where to start however. I think the best thing to do is to just dive into the Main method and starting performing lots of ExtractMethod.
Switching Topics: About BASIC Badness
Grade school and high school gave me about 9 years of programming in BASIC. I arrived at college with years of programming experience and yet had never heard of a compiler. I recall snickering to myself about how archaic any language must be that requires a compiler (a whole separate tool) before it could be run. I couldn’t fathom the purpose of such a tool. (At this time we didn’t have the plethora of scripting languages that also have this great feature of my original BASIC.)
I started learning about “structured” programming as it was called at the time. I didn’t learn any object oriented languages (they were fairly new and not taught). I learned Pascal and the C. We learned not OO but how to use structured constructs. Again, I snickered to myself. What could this terminology mean.
Well, any way, I learned how to program in C and Pascal and Fortran, and then later in life on jobs I learned C++, Visual Basic, and eventually C#, Java and others. I became so comfortable with my new languages and I don’t recall there every being a moment when I realized the code I was writing was completely different from BASIC.
Later in life I heard many of the famous quotes by Edsger W. Dijkstra like
It is practically impossible to teach good programming to students that have had a prior exposure to BASIC: as potential programmers they are mentally mutilated beyond hope of regeneration.
I didn’t really understand what was meant. It had been years since I had studied/used BASIC but couldn’t recall it being bad. Well, when I started the compiler project that is the topic of this post I got to become reacquainted with BASIC and see it in a fresh light. It is truly frightening and I wonder how I ever got any program to work. Here is just one small example program (no indentation, no “structure”, no white space, no symbolic names for objects or methods to help explain what is going on):
[VB] 100 REM 101 REM 108 DIM STACK$(100) 109 REM 110 GOSUB 1000 130 INPUT “String: “:STRING$ 140 FOR I = 1 TO LEN(STRING$) 150 CHAR$ = SEG$(STRING$, I, 1) 151 STACKVAL$ = CHAR$ 155 LEFTBRACKET = (CHAR$=”(“) + (CHAR$=”[”) + (CHAR$=”{“) 156 RIGHTBRACKET = (CHAR$=”)”) + (CHAR$=”]”) + (CHAR$=”}”) 160 IF LEFTBRACKET THEN 180 170 IF RIGHTBRACKET THEN 190 175 GOTO 250 180 GOSUB 2000 185 GOTO 250 190 GOSUB 3000 195 MATCH = ((STACKVAL$=”(“)*(CHAR$=”)”) + (STACKVAL$=”[”)*(CHAR$=”]”) + (STACKVAL$=”{“)*(CHAR$=”}”)) 200 IF MATCH=0 THEN 300 250 NEXT I 260 GOSUB 4000 270 IF STACKCOUNT<>0 THEN 300 280 PRINT “Match” 290 GOTO 301 300 PRINT “No match detected at pos “&STR$(I) 301 INPUT “Another String (Y or N): “:AGAIN$ 302 IF (AGAIN$=”Y”) THEN 110 310 END 999 REM 1000 STACKIDX = -1 1010 STACKVAL$ = “” 1020 RETURN 1999 REM 2000 STACKIDX = STACKIDX + 1 2010 STACK$(STACKIDX) = STACKVAL$ 2020 RETURN 3000 REM 3010 REM 3020 REM 3030 IF (STACKIDX > -1) THEN 3060 3040 STACKVAL$ = “” 3050 GOTO 3080 3060 STACKVAL$ = STACK$(STACKIDX) 3070 STACKIDX = STACKIDX - 1 3080 RETURN 4000 REM 4010 REM 4020 REM 4030 STACKCOUNT = STACKIDX + 1 4040 RETURN [/VB]
Actually, this isn’t nearly as bad as it was “back in the day”. I turned on a little syntax highlighting. Also, before I copied this code I removed the REMARKS and just left placeholders for where they should go. Also, I actually did try to structure this program. Try to see if you can figure out what it is doing and then look at this version with remarks and a little white space.
[VB]
100 REM Checks a string for matching brackets 101 REM ‘(‘, ‘)’, ‘[’, ‘]’, ‘{‘, ‘}’
108 DIM STACK$(100)
109 REM Initialize Stack 110 GOSUB 1000 130 INPUT “String: “:STRING$
140 FOR I = 1 TO LEN(STRING$) 150 CHAR$ = SEG$(STRING$, I, 1) 151 STACKVAL$ = CHAR$ 155 LEFTBRACKET = (CHAR$=”(“) + (CHAR$=”[”) + (CHAR$=”{“) 156 RIGHTBRACKET = (CHAR$=”)”) + (CHAR$=”]”) + (CHAR$=”}”) 160 IF LEFTBRACKET THEN 180 170 IF RIGHTBRACKET THEN 190 175 GOTO 250 180 GOSUB 2000 185 GOTO 250 190 GOSUB 3000 195 MATCH = ((STACKVAL$=”(“)*(CHAR$=”)”) + (STACKVAL$=”[”)*(CHAR$=”]”) + (STACKVAL$=”{“)*(CHAR$=”}”)) 200 IF MATCH=0 THEN 300 250 NEXT I 260 GOSUB 4000 270 IF STACKCOUNT<>0 THEN 300 280 PRINT “Match” 290 GOTO 301 300 PRINT “No match detected at pos “&STR$(I) 301 INPUT “Another String (Y or N): “:AGAIN$ 302 IF (AGAIN$=”Y”) THEN 110 310 END
999 REM SUBROUTINE 1000 Initializes the stack to be empty 1000 STACKIDX = -1 1010 STACKVAL$ = “” 1020 RETURN
1999 REM This subroutine pushes STACKVAL$ onto stack 2000 STACKIDX = STACKIDX + 1 2010 STACK$(STACKIDX) = STACKVAL$ 2020 RETURN
3000 REM This subroutine pops a value off of the stack 3010 REM and puts it into variable STACKVAL$. If the 3020 REM stack is empty then STACKVAL$ will get the empty string 3030 IF (STACKIDX > -1) THEN 3060 3040 STACKVAL$ = “” 3050 GOTO 3080 3060 STACKVAL$ = STACK$(STACKIDX) 3070 STACKIDX = STACKIDX - 1 3080 RETURN
4000 REM This routine updates STACKCOUNT with the number 4010 REM of items in the STACK. STACKCOUNT is only reliable 4020 REM if this subroutine is called before inspecting it. 4030 STACKCOUNT = STACKIDX + 1 4040 RETURN
[/VB]
It was a lot of fun learning to program in TI BASIC, but boy I sure don’t miss it.