> I'm not completely following the design goals here because it seems
> like the desire to reduce only State-Qs could be achieved by writing
> rules that reduced only State-Qs (not arbitrary states). Or are you
> saying that State-Qs are only allowed in the context? If so, then you
> could write E differently?
I don't think I can constrain the rules to only operate on State-Qs
because the *outermost* State needs to reduce, and it does not need to
be a State-Q. I think this would lead to duplicating all of the
reduction rules; once for the outermost State and once for internal
State-Qs.
It's not clear to me how I would change the definition of E to achieve this.
> you could rewrite it using define-judgment-form
This seems fairly promising - looking at the docs I don't see any
reason define-judgment-form couldn't do what I want in a
straightforward manner.
Thanks for the help,
Sam Caldwell
On Sun, Dec 13, 2015 at 5:17 PM, Robby Findler <ro...@eecs.northwestern.edu>
wrote:
> I'm not completely following the design goals here because it seems
> like the desire to reduce only State-Qs could be achieved by writing
> rules that reduced only State-Qs (not arbitrary states). Or are you
> saying that State-Qs are only allowed in the context? If so, then you
> could write E differently? I think that all of this is expressible
> without using a feature like you want in Redex, but I'm not really
> getting precisely what you want so I'm hesitant to make more concrete
> suggestions.
>
> Regardless, if for some reason it is better to express your rewriting
> relation in the way that has actual premises, then you could rewrite
> it using define-judgment-form. Relations defined with
> define-judgment-form that have the mode (I O) or (O I) can be used
> with 'traces' and 'stepper' (as well as show-derivations).
>
> hth,
> Robby
>
>
> On Sat, Dec 12, 2015 at 8:21 PM, Sam Caldwell <s...@ccs.neu.edu> wrote:
> > 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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