CHAPTER 14 The LISP Stepper 14.1. Simple Use Of Stepping (step s_arg1...) NOTE: The LISP "stepping" package is intended to give the LISP programmer a facility analogous to the Instruction Step mode of running a machine language program. The user interface is through the function (fexpr) step, which sets switches to put the LISP interpreter in and out of "stepping" mode. The most common _s_t_e_p invocations follow. These invocations are usually typed at the top- level, and will take effect immediately (i.e. the next S-expression typed in will be evaluated in stepping mode). ____________________________________________________ (_s_t_e_p _t) ; Turn on stepping mode. (_s_t_e_p _n_i_l) ; Turn off stepping mode. ____________________________________________________ SIDE EFFECT: In stepping mode, the LISP evaluator will print out each S-exp to be evaluated before evaluation, and the returned value after evaluation, calling itself recur- sively to display the stepped evaluation of each argument, if the S-exp is a func- tion call. In stepping mode, the evalua- tor will wait after displaying each S-exp before evaluation for a command character from the console. 9 9The LISP Stepper 14-1 The LISP Stepper 14-2 ____________________________________________________ _S_T_E_P _C_O_M_M_A_N_D _S_U_M_M_A_R_Y Continue stepping recursively. c Show returned value from this level only, and continue stepping upward. e Only step interpreted code. g Turn off stepping mode. (but continue evaluation without stepping). n Step through evaluations without stopping p Redisplay current form in full (i.e. rebind prinlevel and prinlength to nil) b Get breakpoint q Quit d Call debug ____________________________________________________ 14.2. Advanced Features 14.2.1. Selectively Turning On Stepping. If (_s_t_e_p _f_o_o_1 _f_o_o_2 ...) is typed at top level, stepping will not commence immediately, but rather when the evaluator first encounters an S-expression whose car is one of _f_o_o_1, _f_o_o_2, etc. This form will then display at the console, and the evaluator will be in stepping mode waiting for a command character. Normally the stepper intercepts calls to _f_u_n_- _c_a_l_l and _e_v_a_l. When _f_u_n_c_a_l_l is intercepted, the arguments to the function have already been evaluated but when _e_v_a_l is intercepted, the Printed: July 21, 1983 The LISP Stepper 14-3 arguments have not been evaluated. To differen- tiate the two cases, when printing the form in evaluation, the stepper preceded intercepted calls to _f_u_n_c_a_l_l with "f:". Calls to _f_u_n_c_a_l_l are nor- mally caused by compiled lisp code calling other functions, whereas calls to _e_v_a_l usually occur when lisp code is interpreted. To step only calls to eval use: (_s_t_e_p _e) 14.2.2. Stepping With Breakpoints. For the moment, step is turned off inside of error breaks, but not by the break function. Upon exiting the error, step is reenabled. However, executing (_s_t_e_p _n_i_l) inside a error loop will turn off stepping globally, i.e. within the error loop, and after return has be made from the loop. 14.3. Overhead of Stepping. If stepping mode has been turned off by (_s_t_e_p _n_i_l), the execution overhead of having the stepping packing in your LISP is identically nil. If one stops stepping by typing "g", every call to eval incurs a small overhead--several machine instructions, corresponding to the compiled code for a simple cond and one function pushdown. Running with (_s_t_e_p _f_o_o_1 _f_o_o_2 ...) can be more expensive, since a member of the car of the current form into the list (_f_o_o_1 _f_o_o_2 ...) is required at each call to eval. 14.4. Evalhook and Funcallhook There are hooks in the FRANZ LISP interpreter to permit a user written function to gain control of the evaluation process. These hooks are used by the Step package just described. There are two hooks and they have been strategically placed in the two key func- tions in the interpreter: _e_v_a_l (which all interpreted code goes through) and _f_u_n_c_a_l_l (which all compiled code goes through if (_s_s_t_a_t_u_s _t_r_a_n_s_l_i_n_k _n_i_l) has been done). The hook in _e_v_a_l is compatible with Maclisp, but there is no Maclisp equivalent of the hook in _f_u_n_- _c_a_l_l. 9 9 Printed: July 21, 1983 The LISP Stepper 14-4 To arm the hooks two forms must be evaluated: (*_r_s_e_t _t) and (_s_s_t_a_t_u_s _e_v_a_l_h_o_o_k _t). Once that is done, _e_v_a_l and _f_u_n_c_a_l_l do a special check when they enter. If _e_v_a_l is given a form to evaluate, say (_f_o_o _b_a_r), and the symbol `evalhook' is non nil, say its value is `ehook', then _e_v_a_l will lambda bind the symbols `evalhook' and `funcallhook' to nil and will call ehook passing (_f_o_o _b_a_r) as the argument. It is ehook's responsibility to evaluate (_f_o_o _b_a_r) and return its value. Typically ehook will call the func- tion `evalhook' to evaluate (_f_o_o _b_a_r). Note that `evalhook' is a symbol whose function binding is a system function described in Chapter 4, and whose value binding, if non nil, is the name of a user writ- ten function (or a lambda expression, or a binary object) which will gain control whenever eval is called. `evalhook' is also the name of the _s_t_a_t_u_s tag which must be set for all of this to work. If _f_u_n_c_a_l_l is given a function, say foo, and a set of already evaluated arguments, say barv and bazv, and if the symbol `funcallhook' has a non nil value, say `fhook', then _f_u_n_c_a_l_l will lambda bind `evalhook' and `funcallhook' to nil and will call fhook with arguments barv, bazv and foo. Thus fhook must be a lexpr since it may be given any number of arguments. The function to call, foo in this case, will be the _l_a_s_t of the arguments given to fhook. It is fhooks responsibility to do the function call and return the value. Typically fhook will call the function _f_u_n_- _c_a_l_l_h_o_o_k to do the funcall. This is an example of a funcallhook function which just prints the arguments on each entry to funcall and the return value. 9 9 Printed: July 21, 1983 The LISP Stepper 14-5 ____________________________________________________ -> (_d_e_f_u_n _f_h_o_o_k _n (_l_e_t ((_f_o_r_m (_c_o_n_s (_a_r_g _n) (_l_i_s_t_i_f_y (_1- _n)))) (_r_e_t_v_a_l)) (_p_a_t_o_m "_c_a_l_l_i_n_g ")(_p_r_i_n_t _f_o_r_m)(_t_e_r_p_r) (_s_e_t_q _r_e_t_v_a_l (_f_u_n_c_a_l_l_h_o_o_k _f_o_r_m '_f_h_o_o_k)) (_p_a_t_o_m "_r_e_t_u_r_n_s ")(_p_r_i_n_t _r_e_t_v_a_l)(_t_e_r_p_r) _r_e_t_v_a_l)) fhook -> (*_r_s_e_t _t) (_s_s_t_a_t_u_s _e_v_a_l_h_o_o_k _t) (_s_s_t_a_t_u_s _t_r_a_n_s_l_i_n_k _n_i_l) -> (_s_e_t_q _f_u_n_c_a_l_l_h_o_o_k '_f_h_o_o_k) calling (print fhook) ;; now all compiled code is traced fhookreturns nil calling (terpr) returns nil calling (patom "-> ") -> returns "-> " calling (read nil Q00000) (_a_r_r_a_y _f_o_o _t _1_0) ;; to test it, we see what happens when returns (array foo t 10) ;; we make an array calling (eval (array foo t 10)) calling (append (10) nil) returns (10) calling (lessp 1 1) returns nil calling (apply times (10)) returns 10 calling (small-segment value 10) calling (boole 4 137 127) returns 128 ... there is plenty more ... ____________________________________________________ 9 9 Printed: July 21, 1983