Variables, user-written procedures, and user-written functions should not have the same names as IDL functions or procedures. Re-using names of IDL routines can lead to syntax errors or to “hiding” variables. In addition, certain words representing IDL language constructs are strictly forbidden—using any of these reserved words as a variable, procedure, or function name will cause an immediate syntax error. The following table lists all of the reserved words in IDL.

AND	BEGIN	BREAK
CASE	COMMON	COMPILE_OPT
CONTINUE	DO	ELSE
END	ENDCASE	ENDELSE
ENDFOR	ENDFOREACH	ENDIF
ENDREP	ENDSWITCH	ENDWHILE
EQ	FOR	FOREACH
FORWARD_FUNCTION	FUNCTION	GE
GOTO	GT	IF
INHERITS	LE	LT
MOD	NE	NOT
OF	ON_IOERROR	OR
PRO	REPEAT	SWITCH
THEN	UNTIL	WHILE
XOR