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Tracein is a facility for very low level tracing/stepping, i.e. to show you
everything that takes place within a particular piece of code.

(TRACEIN fn {T} loc1 loc2 ...)   [NLAMBDA NOSPREAD]
Tracein steps or traces everything that happens in a piece of code.  The
first argument is the name of the function in which that code is to be found.
It can also be a list of the form (fn test) where test will be evaluated to
decide whether to actually trace.  The default condition is T.  (Note: the
condition NIL is also treated as T - if you want never to break try (NOT T).)
If fn is followed by T, the code will always be traced (as if you always typed
T to the first prompt described below).
The other arguments are editor location specifications.  (TRACEIN FN) is
like (BREAKIN FN) - it allows you to find the desired location in the editor.
When you type OK the current expression is the answer.  (TRACEIN fn loc1)
does a (BREAKIN fn (AROUND loc1) <some trickery>).  Since TRACEIN works by
calling BREAKIN, it can be undone by calling UNBREAK.
When the stepper enters an expression it prints the function and prompts with
the string "->".  The acceptable responses are P (rettyprint the form you are
about to eval), B (reak), C (ontinue to step), E (val the form with no more
questions or tracing) and T (race everything in the form without asking).
After the expression has been evaluated, it prints the value and prompts with
the string "<-".  The acceptable responses are P (rettyprint the value),
B (reak), E (valuate the rest of the broken form without printing or asking)
and C (ontinue stepping with the next form).
Note that it only makes sense to tracein expressions that will be 
evaluated.  You will get into trouble if you attempt to trace a clause of
a COND, e.g. ((ATOM X) NIL), because it will be interpreted as call to the
function (ATOM X).  Also, it is the DWIMIFY'd version of your code that
will be traced.  Thus you may see PROG executing where you typed FOR.
Note that a break condition of (NOT T) is useful for avoiding sections of
code.  E.g. if the code is (F (G (H))), you can see F and H but not G by
doing (TRACEIN <fn> F H) and (TRACEIN (<fn> (NOT T)) G).

(WATCH form)    [NLAMBDA SPREAD]
Watch is the stepper/tracer used by TRACEIN.  It is controlled by a free
variable StepAction (initially NIL meaning step).  Other possible values are
T (meaning trace) and EVAL meaning no more printing or questions.
Watch is a handy function to insert by hand in strategic places in your code.
However, unlike TRACEIN, this method of tracing will not be undone by UNBREAK.
Watch rebinds the free variable INDENT# to determine how far to indent the
output.  It also rebinds PRINTLEVEL when it shows the results of a computation
to WATCHPLEVEL.  This gives you a way to control the amount of detail you see.

Notice that TRACEIN traces every subexpression that it EXPECTS to eventually
be evaluated.  It doesn't expect the arguments of NLAMBDAs to be evaluated,
but sometimes they are.  Users can tell TRACEIN what will be evaluated by
adding an EVL-FIX property to functions that TRACEIN does not understand.
(Maybe some masterscope expert can figure out how to use the masterscope
templates to get most of the effect of EVL-FIX properties described below.)
The system functions are supposed to be already understood, but if that
understanding is wrong (or the user wants to change it for any other reason)
the same tactic will work.  The EVL-FIX property is a pattern to be applied
to the tail of a function call.  A pattern element of T means that the
corresponding argument will be evaluated (and thus ought to be traced). 
NIL means that it will not.  Thus the pattern for SETQ is (NIL T).  If
a pattern element is itself a list, then that argument is not to be traced,
but it is expected to be a list which will in turn be matched with the
pattern element to determine whether its elements will be traced.
In addition, the special pattern element TAIL allows the pattern element
after it to be applied to as many arguments as necessary to exhaust the list.
For example, the pattern for COND is (TAIL (TAIL T)).  The pattern for
SELECTQ is (T TAIL (NIL TAIL T) T).
If the CAR of a pattern is TEST, the next element is an expression which
is evaluated.  If the result is not NIL it will be traced.  Also, as a 
special case, if a pattern element is an atom other than T or NIL, it is
expected to be a function which will be applied to the corresponding argument,
and if the result is not NIL then the argument will be traced.  If the CAR
of a pattern is EVAL, the next element is evaluated, and what it returns is
used as the pattern.  In such code, the free variable EXP is the function
call that the resulting pattern is supposed to describe.  In addition, the
variable NOEMBED may be set to T to indicate that the function call itself
is not to be traced.  This feature is not normally needed, but LAMBDA is
an example where it is.

Implementation

TRACEIN itself simply calls BREAKIN.  If a break occurs (as determined by
the break condition), the code that is evaluated (instead of BRKEXP) is
a new version of BRKEXP in which the forms to be evaluated are embedded in 
calls to WATCH (the stepper/tracer).  This form is computed the first time
it is needed, and is stored in CLISPARRAY.  Thus the WATCHified version
will automatically go away (and have to be recomputed) if you ever change
the expression.  (Detail: before WATCHification, the expression is DWIMified.
WATCHification of an expression which already has a CLISP translation stores
the WATCHified version of the CLISP translation as the CLISP translation of
the CLISP translation of the original form.  Thus, when the break condition
is false, the original CLISP translation is used.  When the expression is
changed, the CLISP translation goes away, and so the WATCHification (as well
as the DWIMification) is redone.)

DonC (Don Cohen) @ ISIF