How to use the "%bag-of" primitive inside a predicate in Racklog - scheme

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)))

Related

Temporary Variable Assistance (Scheme)

I'm making a random sentence generator using Scheme (Pretty Big), and I'm having trouble defining temporary variables. I want to make something like this:
<NOUN1> <VERB1> <NOUN2> <but> <NOUN2> <VERB1> <NOUN2> <also>
Example: Sharks eat fish, but fish eat fish also.
I have word lists, and functions to choose a word from said list. Then, I use append to create a function. I am able to do:
(define (sentence)
(append (getNoun) '(and) (getNoun) (getVerb)))
However, I am unable to figure out a way to temporarily define a variable.
I have this so far:
(define (sentence1)
(append (getNoun)
(lambda (verb getVerb)
(noun getNoun))
(verb) (noun) '(but) (noun) (verb) (noun)))
Hints/Help please?
You are looking for let.
http://docs.racket-lang.org/reference/let.html
Here is an example usage:
(define (my-proc age)
(let ([age-plus-10 (+ age 10)])
(printf "age is ~a" age)
(printf "age-plus-10 is ~a" age-plus-10)))
Notice how we can temporarily define age-plus-10 and then use it later.

Simple interpreter in Scheme

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))

How to inspect/export/serialize a (guile) Scheme environment

I'd like to export or replicate a scheme environment in another guile process. The algorithm I'm imagining would do something like this to serialize:
(map (lambda (var val) (display (quasiquote (define ,var ,val))
(newline))
(get-current-environment))
And then I'd read/eval that on the other end.
However, while there are functions that return the current environment, they are in some internal format that I can't just map across. How can I "walk" the environment as the above? Alternatively, how else can I replicate an environment into another process?
you may decompose the so-called "current-environment" like this:
(define (get-current-binding-list)
(let* ((e (current-module)) ;; assume checking current-module
(h (struct-ref e 0)) ;; index 0 is current vars hashtable
)
(hash-map->list cons h) ;; return a vars binding list
))
and you can call (get-current-binding-list) to get variables binding list in current-module.
Please note that each element in this list is a pair of symbol and variable type, say, (symbol-name . variable-type). So you may print it like this:
for a instance ,you got a var binding:
(define abc 5)
then:
(let ((vl (get-current-binding-list)))
(assoc-ref vl 'abc)
)
==> #<variable 9bb5108 value: 5>
This result is a "variable type" of variable "abc". You can get it's value with variable-ref procedure.
So you can trace all the bindings and do something ,in your code ,it's simply print var-name and var-value.
I know my answer is too brief, but I think there's enough information to help you to find more details in the manual.
Hope this will help you.
You can't really serialize Scheme environment. I don't known even it's possible to (portably) serialize continuations. Oh, and don't forget about FFIs. Ports and threads are unserializable too.

Why can't this macro execute?

I have the following dumb test:
(define-syntax a
(lambda (stx)
(syntax-case stx ()
[(k e s) #'(let ((show display)) (e s))])))
(a show "something")
Why can't this work? (The error shows in DrRacket is expand: unbound identifier in module in: show.
However, The following can work:
(define-syntax a
(lambda (stx)
(syntax-case stx ()
[(k e s)
(with-syntax ((show (datum->syntax #'k 'show)))
#'(let ((show display)) (e s)))])))
(a show "something")
Then WHY?
Um, I assume that you're trying this out after reading the blog post I mentioned in an earlier answer -- but that blog post is explaining exactly this issue. Specifically, your first example has two different show identifiers, one that is bound by the macro, and a different one that is coming from the toplevel use (and is unbound). OTOH, in the second case you're creating a show with the lexical context of the user code.

PLT Scheme: Converting one of the macros in 'Casting SPELs in LISP'

(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.

Resources