Thanks for clarifying, Robby.
I'm modeling reduction on nested states. In addition to a collection of
straightforward rules, there is a rule that specifies when to reduce a
state-inside-a-state. I can specify this using evaluation contexts, but in
order for the relation to remain deterministic I need to restrict which
nested-states are reduced to a particular subclass.
I'm hoping this little model demonstrates what I'm trying to do:
==================================================================
#lang racket
(require redex)
(define-language L
(State (number
(Event ...) ;; "incoming" events
(Event ...) ;; "outgoing" events
(State ...)))
(Event string)
;; states with no outgoing events
(State-Q (number
(Event ...)
()
(State ...)))
;; states with no incoming or outgoing events
(State-I (number
()
()
(State ...)))
;; Evaluation Contexts
(E hole
(number
()
(Event ...)
(State-I ... E State-Q ...))))
(define red
(reduction-relation
L
;; incoming events just increase a counter
(--> (number
(Event_0 Event_n ...)
(Event ...)
(State ...))
(,(add1 (term number))
(Event_n ...)
(Event ...)
(State ...))
handle)
;; once a State has processed all of its incoming Events, it can receive
an
;; outgoing Event from nested States.
(--> (number
()
(Event_out ...)
(State-Q ...
(number_s
(Event_si ...)
(Event Event_so ...)
(State_s ...))
State ...))
(number
(Event Event_so ...)
(Event_out ...)
(State-Q ...
(number_s
(Event_si ...)
()
(State_s ...))
State ...))
receive)))
#|
Want this to result in:
'((5 () () ((4 () () ()) (4 () () ()))))
|#
(apply-reduction-relation*
red
(term (0 ()
()
((1 ("hi" "hello" "ciao")
("zip" "zap" "zooey")
())
(2 ("where" "fore")
("art" "thou")
())))))
==================================================================
That model allows the top-most State to process incoming Events and receive
Events from its direct children. But, I also want child States to reduce
using
the same rules. To keep my relation deterministic, I only want to step child
States that are also State-Q's. This is how I ended up trying to add a
shortcut like:
[(--> (in-hole E State-Q) (in-hole E State))
(==> State-Q State)]
I was able to get the behavior I want by calling apply-reduction-relation
from
inside a rule:
(--> (in-hole E State-Q)
(in-hole E State)
(side-condition (not (redex-match? L hole (term E))))
(where (State) ,(apply-reduction-relation red (term State-Q))))
But I can't say I felt great about doing so. (The side-condition is needed
to
prevent infinite looping). Is there a cleaner way to achieve the same
result?
Thanks,
Sam Caldwell
On Sat, Dec 12, 2015 at 11:11 AM, Robby Findler <ro...@eecs.northwestern.edu
> wrote:
> I can see why you might have expected that to work that way.
> Unfortunately, it doesn't. The identifiers in those places in
> shortcuts (Add2, x, and n in your examples below) are not pattern
> positions. They are simply identifiers.
>
> In the code you wrote, one could change the rule's left-hand side to
> (+ V_1 V_2) to achieve the desired effect, but maybe that doesn't work
> in your larger model? Perhaps if you explained a little more why
> something like that is problematic, we could be of more use.
>
> Meanwhile, I've pushed a fix to the bug in the error-checking that you
> found, added some more checking, and tried to emphasize this point
> more clearly in the documentation. The commit cbb2d88b would probably
> have been the most helpful to you, but it's backwards incompatible, so
> it may need to be reverted.
>
> Robby
>
> On Fri, Dec 11, 2015 at 3:09 PM, Sam Caldwell <s...@ccs.neu.edu> wrote:
> > Hi,
> >
> > I'm working on a redex model where I want to constrain the shape of terms
> > used
> > to fill the hole in an evaluation context. I thought it would be fairly
> > straightforward to do so using a `with` clause in my reduction-relation,
> but
> > I've run into some difficulty, and consulting the docs [1] left me
> unsure of
> > where I went wrong.
> >
> > In the docs for `reduction-relation`, the old-arrow-name clause of
> shortcuts
> > are defined in terms of patterns and terms. However, the docs later say
> "The
> > left- and right-hand sides of a shortcut definition are identifiers, not
> > patterns and terms." I don't understand what is and is not allowed when
> > using
> > shortcuts.
> >
> > I have a small model that I think illustrates what I'm after. Say I have
> a
> > tiny addition language but for some reason I want to restrict reduction
> to
> > two-argument terms:
> >
> > ==================================================================
> >
> > #lang racket
> >
> > (require redex)
> >
> > (define-language add
> > (M (+ M ...) V)
> > (V natural)
> > (Add2 (+ V V))
> > (E hole (+ V ... hole M ...)))
> >
> > (define red
> > (reduction-relation
> > add
> > (==> (+ V ...)
> > ,(apply + (term (V ...)))
> > +)
> > with
> > [(--> (in-hole E Add2) (in-hole E n))
> > (==> Add2 n)]))
> >
> > ==================================================================
> >
> > This does not behave as I would have expected, for example reducing
> > `(term (+ 1 2 (+ 3 4) 4 (+ 0 1)))` down to `15`.
> >
> > It seems like the issue is that `Add2` in the shortcut is not being used
> as
> > a
> > pattern. For example, if I replace `Add2` with its definition:
> >
> > [(--> (in-hole E (name x (+ V V))) (in-hole E n))
> > (==> x n)]
> >
> > I get the following error message:
> >
> > free-identifier=?: contract violation
> > expected: identifier?
> > given: #<syntax (name x any_2)>
> > argument position: 2nd
> > other arguments.:
> >
> > So my question boils down to:
> > 1) What is/is not allowed in each position when defining a shortcut?
> > 2) How can I constrain what is used to fill a hole, as attempted in my
> > example?
> >
> > Thanks,
> > Sam Caldwell
> >
> > [1]
> >
> http://docs.racket-lang.org/redex/The_Redex_Reference.html#%28form._%28%28lib._redex%2Freduction-semantics..rkt%29._reduction-relation%29%29
> >
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