BBC BASIC Programmers' Reference

by Jon Ripley, May 2011

The detokeniser function documented on this wiki allows us to write an ON ERROR statement that lists the source code of the errant line when an error occurs. The FNErrorLine function returns the source code of the line the last error occurred in.

The following ON ERROR handler prints the error followed by the line of code it occurred in:

        ON ERROR ON ERROR OFF:PRINT REPORT$": "FNErrorLine:END

The full code for FNErrorLine including the detokeniser is as follows:

        DEF FNErrorLine
        LOCAL I%,P%,line$ : I% = !408 : P% = PAGE
        WHILE ?P% AND P% + ?P% < I% P% += ?P% : ENDWHILE
        FOR I%=0 TO ?P%-1:line$ += CHR$P%?I% : NEXT I%
        =FNdetokenise(line$)
 
        DEF FNdetokenise(S$)
        IF S$ = "" OR S$ = CHR$0+CHR$255+CHR$255 THEN =""
        PRIVATE Keywd$()
        IF !^Keywd$()=0 THEN
          DIM Keywd$(160)
          Keywd$() = "AND","DIV","EOR","MOD","OR","ERROR","LINE","OFF",\
          \"STEP","SPC","TAB(","ELSE","THEN","","OPENIN","PTR",\
          \"PAGE","TIME","LOMEM","HIMEM","ABS","ACS","ADVAL","ASC",\
          \"ASN","ATN","BGET","COS","COUNT","DEG","ERL","ERR",\
          \"EVAL","EXP","EXT","FALSE","FN","GET","INKEY","INSTR(",\
          \"INT","LEN","LN","LOG","NOT","OPENUP","OPENOUT","PI",\
          \"POINT(","POS","RAD","RND","SGN","SIN","SQR","TAN",\
          \"TO","TRUE","USR","VAL","VPOS","CHR$","GET$","INKEY$",\
          \"LEFT$(","MID$(","RIGHT$(","STR$","STRING$(","EOF","SUM","WHILE",\
          \"CASE","WHEN","OF","ENDCASE","OTHERWISE","ENDIF","ENDWHILE","PTR",\
          \"PAGE","TIME","LOMEM","HIMEM","SOUND","BPUT","CALL","CHAIN",\
          \"CLEAR","CLOSE","CLG","CLS","DATA","DEF","DIM","DRAW",\
          \"END","ENDPROC","ENVELOPE","FOR","GOSUB","GOTO","GCOL","IF",\
          \"INPUT","LET","LOCAL","MODE","MOVE","NEXT","ON","VDU",\
          \"PLOT","PRINT","PROC","READ","REM","REPEAT","REPORT","RESTORE",\
          \"RETURN","RUN","STOP","COLOUR","TRACE","UNTIL","WIDTH","OSCLI",\
          \"","CIRCLE","ELLIPSE","FILL","MOUSE","ORIGIN","QUIT","RECTANGLE",\
          \"SWAP","SYS","TINT","WAIT","INSTALL","","PRIVATE","BY","EXIT"
        ENDIF
        LOCAL flag%, O$, i%, ch%
        IF ASCS$ = LENS$ AND ASC RIGHT$(S$,1) = 13 THEN
          ch% = ASC MID$(S$,2)+256*ASC MID$(S$,3)
          IF ch% O$=STR$ch%+" "
          S$ = MID$(S$, 4, LENS$-4)
          IF S$="" THEN =LEFT$(O$, LENO$-1)
        ENDIF
        FOR i%=1 TO LENS$
          IF ASC MID$(S$,i%)=34 flag%EOR=1
          IF flag% AND 1 THEN
            O$ += MID$(S$, i%, 1)
          ELSE
            ch% = ASCMID$(S$, i%)
            CASE TRUE OF
              WHEN ch% >= 17 AND ch% < 128:O$ += CHR$ch%
              WHEN ch% = &8D
                ch% = ASC MID$(S$,i%+1)
                lo% = ((ch% << 2) AND &C0) EOR ASC MID$(S$, i%+2)
                hi% = ((ch% << 4) AND &C0) EOR ASC MID$(S$, i%+3)
                O$+ = STR$(lo% + 256*hi%)
                i% += 3
              OTHERWISE O$ += Keywd$(ch% EOR 128)
            ENDCASE
          ENDIF
        NEXT i%
        =O$