I will describe my problem on example.
I'll get (play '(left nothing right left)). Some of the names in the list are real procedures, others i want to skip.
(define (left)
'left
)
I need to interpret procedures with names in the list. What is the solution?
When I try ( (car '(left nothing right left))) I get error : procedure application: expected procedure, given: left (no arguments)
(car '(left nothing right left)) evaluates to the symbol left, which is the name of a procedure, but not actually a procedure, so you can't call it.
You'll want to build an association list mapping symbols to procedures:
(define actions `((left . ,(lambda () 'left))
(right . ,(lambda () 'right))
(nothing . ,(lambda () (display "I'm staying put")))))
then you can call the appropriate function for the first element in your list as
((cdr (assoc (car '(left nothing right left)) actions)))
You can also use quasiquoting to construct a list containing a mixture of symbols you want evaluated and others you don't, e.g.
(play `(,left nothing nothing ,right nothing))
left and right will expand to whatever you've defined them as (such as a procedure) while nothing is not un-quoted so it will be left as a symbol. play would then have to test each member to see if it's a procedure, something like:
(define (play xs)(for-each (lambda (x)(if (procedure? x)(x) x)) xs))
Related
I'm trying to add completion at point for frame-local variables from backtrace-frames during invocations of read--expression by debugger-eval-expression or edebug-eval-expression.
I constructed the following completion table to add frame-local variables to the already available table for local elisp variables,
;; completion table for locals in current frame
(defvar my-backtrace-locals-completion-table
(completion-table-in-turn
(completion-table-dynamic
(lambda (_string)
(when-let* ((idx (backtrace-get-index)) ;backtrace.el
(frame (nth idx backtrace-frames)))
(backtrace-frame-locals frame)))
'do-switch-buffer)
elisp--local-variables-completion-table)) ;elisp-mode.el
which seems to work fine, eg. to reproduce
(1) evaluate
;; debug-on-error = t
(let ((my-local-var '(1 2))) (mapcan #'car this-local-var))
(2) from debugger's second frame, evaluate with eval-expression
(funcall my-backtrace-locals-completion-table "my-" nil t)
returns expected ("my-local-var").
The problem is following the above steps, but calling instead calling debugger-eval-expression doesn't work. The environment where the table is evaluated isn't finding a backtrace-frame (with or without do-switch-buffer).
How can I define the table to be evaluated in the proper buffer?
emacs v27.0.50
The completion table above doesn't quite return the expected candidates for debugger-eval-expression. The environment where the expression is evaluated has locals from frames higher than, but not including, the one at point in the Backtrace buffer.
So, the available locals should be only those from higher frames, eg.
(eval-when-compile (require 'dash))
(defvar my-backtrace-locals-completion-table
(completion-table-dynamic
(lambda (_string)
(when backtrace-frames
(--mapcat
(-some->> (backtrace-frame-locals it) (--map (car it)))
(nthcdr (1+ (backtrace-get-index)) backtrace-frames))))
'do-switch-buffer))
Then, redefining debugger-eval-expression's interactive spec to use the new locals table in place of the normal elisp table provides the correct completions (passing the 'do-switch-buffer arg completion-table-dynamic to find completions in the original buffer).
(defun my-backtrace#eval (orig-fn exp &optional nframe)
(interactive
(let ((elisp--local-variables-completion-table
my-backtrace-locals-completion-table))
(list (read--expression "[my] Eval in stack frame: "))))
(apply orig-fn (list exp nframe)))
(advice-add 'debugger-eval-expression :around #'my-backtrace#eval)
I'm not sure if the question title is appropriate but here is what I wonder:
From the repl, I wanted to get the list of bindings defined in the current module. After some searching this seemed like a good solution:
(define (racket-symbols-set)
(list->set (namespace-mapped-symbols (module->namespace 'racket))))
(define (namespace-symbols-set)
(list->set (namespace-mapped-symbols)))
(define (module-bindings)
(set->list (set-subtract
(namespace-symbols-set)
(racket-symbols-set))))
so, calling (module-bindings) returns a list of symbols. But if I try to call a symbol from that result, such as doing ((first (module-bindings))), I get a "application: not a procedure" error although the first symbol is a procedure.
How do I call the corresponding procedure of that symbol?
You can look up the value of a namespace variable using namespace-variable-value. And since your namespace-symbols-set just uses the current namespace, which is also the default namespace for namespace-variable-value, using it is very simple.
For example, to invoke the procedure associated with the first item in the list returned by your module-bindings procedure:
((namespace-variable-value (car (module-bindings))))
Alternatively, specify your preferred namespace as the fourth argument of the namespace-variable-value call.
You need to evaluate that symbol in order for it to return the corresponding procedure.
> (define (foo) 'bar)
> (eval 'foo)
#<procedure:foo>
> ((eval 'foo))
'bar
Hence in your case
((eval (car (module-bindings))))
will call the first procedure of the list returned by module-bindings.
This time my problem is mostly with Racklog. I guess. Could also be the Racket syntax this time.
The idea is rather simple. I have a logic-base made up of places and objects and I just wanted to try out printing all the objects using the %bag-of primitive.
My logic-base looks like this:
(define %contains
(%rel ()
[('bridge 'phaser)]
[('engine_room 'toolkit)]
[('toolkit 'screwdriver)]
[('toolkit 'tricorder)]
[('inventory '(communicator, no_tea))]
)
)
Now I have my predicate which is the following one. It should be simply called with the query "(%which () (%list_objects 'toolkit))" and then give out all the items inside the toolkit for example.
(define %list_objects
(%rel (place)
[(place)
(%which (objects)
(%let (x)
(%bag-of x (%contains place x)
objects)))]
)
)
The weird thing is when I just thake the part from the "%which (objects)...)" onwards and throw it directly into the listener, it works perfectly fine. But if I'm using it inside the predicate, it throws this exception:
"application: not a procedure;
expected a procedure that can be applied to arguments
given: '((objects screwdriver tricorder))
arguments...: [none]"
I tried rearranging the code several times, but right now I'm quite stumped about what I did wrong. I would appreciate a little hint what I as a total newbee to Scheme and Racket missed out here. My thanks in advance!
The problem is that the goal (%which ...) returns an answer (not a new relation). Therefore %list_objects can't be used in the way you want.
Maybe this works for you?
#lang racket
(require racklog)
(define %contains
(%rel ()
[('bridge 'phaser)]
[('engine_room 'toolkit)]
[('toolkit 'screwdriver)]
[('toolkit 'tricorder)]
[('inventory '(communicator, no_tea))]))
(define %list_objects
(%rel (place)
[(place)
(%let (x) (%contains place x))]))
(%which (x) (%list_objects x))
(%more)
(%which (bag) (%let (x) (%bag-of x (%list_objects x) bag)))
(define tools-in-toolkit (map cdar (%find-all (tool) (%contains 'toolkit tool))))
(define %in-toolkit
(%rel (tool)
[(tool) (%member tool tools-in-toolkit)]))
(%find-all (tool) (%in-toolkit tool))
Output:
'((x . bridge))
'((x . engine_room))
'((bag bridge engine_room toolkit toolkit inventory))
'(((tool . screwdriver)) ((tool . tricorder)))
In Exercise 35.4.2 from HtDP, I've implemented the GUI and have a button called "Remove" which invokes a callback function. Here it is:
(define (cb-remove x)
((lambda (name result)
(cond
[(number? result) (remove-name name address-book)]
[else (draw-message msg "Not found")]))
(string->symbol (text-contents label-name))
(lookup (string->symbol (text-contents label-name)) address-book)))
When I run this, I get the following message: button-callback: result of type <Boolean> expected, your function produced #<set!-result>. The problem is that I have to call set! in order to change the address book. However, the result of set! is (void), which cannot cannot be combined with a Boolean type. How can I avert this problem? Thanks for any insight.
Simple:
(begin (set! foo bar) #t)
(defspel game-action (command subj obj place &rest rest)
`(defspel ,command (subject object)
`(cond ((and (eq *location* ',',place)
(eq ',subject ',',subj)
(eq ',object ',',obj)
(have ',',subj))
,#',rest)
(t '(i cant ,',command like that.)))))
Thats the code from http://www.lisperati.com/actions.html for the 'macro defining macro'. I can't seem to convert it appropriately to scheme. Can someone explain to me the process of creating this same sort of thing in Scheme?
This kind of macro is actually much simpler in Scheme, since you can do it all with define-syntax-rule (in standard Scheme code you will need define-syntax + syntax-rules). You basically do the same thing, minus the whole quote/unquote mess.
(defspel (game-action command subj obj place rest ...)
(defspel (command subject object)
(cond [(and (eq? *location* 'place)
(eq? 'subject 'subj)
(eq? 'object 'obj)
(have 'subj))
rest ...]
[else '(i cant command like that.)])))
And since this is actually most of the code, I ported the whole thing to PLT -- see the post on the mailing list.