Amiga Computing - Commodore Is Awesome

Paul Overaa explains
how AlRexxis
de-bugging facilities
an be used to
find errors in scripts
ebugging and testing con be one of the
most time-consuming stages of any software
development, and ARexx scripts
ore, of course, no exception to this. The
use of proper structured design techniques, modular
programming, topdown development and so on
titips, but it will not eliminate all the problems. No
Tatter how conscientious a programmer you ore,
some mistakes are bound to be made.
Sometimes problems will be due to simple syntax
!tors caused by, say, a basic misunderstanding of
ow a statement should be written, or a dyslexic
slip at the keyboard - inadvertently writing Uppre(l
when you meant Upper( for example, or writing a
s
ebecause
you didn't hit the shift key at the right time.
mSuch
mistakes ore usually considered harmless
because they are easily found and therefore easily
i orrected.
- ARexx is very good in this respect and not
conly
will it identify most errors but you con be fairly
oSure
that any ARexx program which gets to a
l noble stage is going to be nigh on 100 per cent
ontacticaly
correct.
If, whilst developing a script, you get past the
n
ol syntax error stage and your program seems
wrun
in some fashion, then often you will be home
hnd
dry. Occasionally, however, things will go
e ong - o program may run to a point arid then for
n a obvious reason moy lust fail to behave as
expected. It's at this stage that we enter the realm
y
debugging proper and one of the most important
o
s available to the modern programmer is easily
u ble trace facilities.
mARexx
supports two basic modes of tracing:
e sly, there is the normal Or 'non-interactive ' troc•
amg
mode which simply alows troce information to
viewed Secondly, there is on 'interactive' mode
n
hich allows the programmer to step through the
t °gram and interact with it as the trace proceeds
ttea
sidin or sterr if available'. It is possible to look
o at the contents of variables, re-execute clauses, and
weven
type in instructions which are subsequently
r xecuted as though they were o part of the
ogram itself.
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An ARa e rs tra cing
console window in
action.
Bug 1
huntin
ALL T r a c e s all clauses as they are executed.
BACKGROUND This mode is the same as OFF except for the fact that the TS
support program con not override it.
COMMANDS A l l command clauses ore traced before being sent to the external
host. Any non- zero return codes are also displayed.
ERRORS C o m m a n d s which result in a non-zero error code are traced.
INTERMEDIATES Clauses and all intermediate results of expression evaluation are
traced (including values of variables, the results of function calls
and expansion of compound names.
LABELS
NORMAL
OFF
RESULTS
SCAN
Traces labels reached either in sequence or via flow control
statements.
Default tracing mode. Similar to the ERRORS mode except that on
error ceiling value (set using the OPTIONS FAILAT instruction
Form) is used to determine whether or not an error trace should
be generated.
Tracing is turned off by this option although this situation can be
overridden by ARexx's TS program.
Traces the results of every expression that is evaluated.
Similar to ALL but it is a parse-only scan so the program does not
ac•ually get exerL tod.
-11110111W I M P
Tablet Sr Options a va ila ble . with AR e sx's TR AC E instruction
THE TRACE INSTRUCTION
The syntax of this instruction takes the
general form—
TRACE [ symbol I stringS I I [VALUE] expression]]
which means that three basic forms of
the instruction exist, namely:
TRACE symbol
TRACE string$
TRACE [VALUE] expression
where the symbol, string or expression
must either represent (or reduce to) a
positive or negative integer, or be one of
the options (or a single letter abbreviation)
shown in table 1. Negative numeric
values, incidentally, indicate a 'suppression
count', which turns ARexx's tracing
off for a given number of clauses. A positive
value given during interactive tracing
indicates that a number of interactive
'pauses for input' are to be skipped
over.
The trace output itself can be sent to
one of three places: Directly back to stdout
(which for programs launched from
111111=Mit ' 1 1 1
AMIGA COMPUTING
FEBRUARY 1996
•.J CU/Shell console is the CLI/Shell window
itself); to a file associated with a
standard error (stderr) file handle (this
can of course be another console window);
or to ARexx's Global Tracing
Console (a special console which can be
opened using the TCO utility program).
Before deciding where trace information
should be sent, AResix checks for
the existence of an stderr file handle, if
you have opened a window or file tied
to this file handle, or have opened the
Global Tracing Console (which is automatically
attached to stderr), then ARexx
will send its information to this destination,
otherwise it will send its output
through the standard output channel
(stdoutl. The benefit of having trace
information sent through a separate
channel is simple - it avoids having
trace output and normal program output
intermixed at the same window.
Of course, what we need now is a few
examples of these facilities in action to
drive home the ideas. Needless to say
this is exactly what's coming in the next
issuel