redo-safe-variables and redo-safe-parameters

[Stefan Israelsson Tampe]

Dear friends,

1. I will change the name special variables to
redo-safe-variables. And the parameter semantic I'm using to redo safe 
parameters. Itis clumsy but descriptive.

2. To implement these in a good way we need to touch the fundamentals
of scheme. If we want to. But the concept is difficult and it is
therefore wise to try to lift it as an srfi spec so that rthe whole
scheme commmunity can take advantage of the concept and also help
design it.

3. Before sending it to them I will ask you to comment on the idea and
perhaps help a little to get the spec into a form that does not make
me look like a fool and so that I don't step on any of you huy's toes.

4. I will wait to send it until the next version of guile is outr and
people can sit down and read it.

Here is my draft:
---------------------------------------------------------------------

Dear editors of srfi,

Authors background:
I'm an active developer of scheme centered around the guile scheme 
ecosystem and the developer of guile-log, a logic programming system on 
top of guile.

The background of the proposal.

The background has a plurality of origins that converged into one 
concept.

1. Current scheme is a beautiful language but has a drawback. If you
set! a variable that variable will be boxed and any uses of undo/redo
semantic becomes broken. This is not acute in scheme because we try
to avoid set! and that is well. But there are cases where you would
like to use set!. One example being utilizing a simple generator e.g.

  (next! n) -> (begin (set! n (+ n 1)) n)

This can many times be implemented efficiently and with a good semantic 
in a looping macro for example. To achieve that without using set! 
seams to be difficult and the resulting loop! macro will not mix well 
with trying to implement undo and redo features on top of the system.

2. Emacs lisp is coded very much in an imperative style and with guile
we would like to sell guile scheme as a backend for emacs lisp and
emacs as a whole. One of the selling point to do this could be that we 
could use continuations to seamlessly add undo redo like semantics to 
already written programs in emacs lisp. The problem is twofold here. 

i) Typically emacs variable are dynamical variables. Dynamic variables 
in scheme is modeled on parameters and they posses the property that 
for each dynamic state S and parameter p the evaluation of p under S is 
always the last value settedt e.g. it is working like a local variable 
in the S environment. Because each continuation get's it's own dynamic 
state dynamically, the possibility of freezing a state that can be 
restarted multiple times seams to be there. The problem is that a 
dynamic state can be referenced as a scheme object and used as the
current state multiple times as well for the same continuation and 
hence we must overwrite the initial value at a normal return from the 
dynamic state and hence loose the ability to store and restore 
naturally. This property also prevent a possible optimization to store 
the parameter directly on the stack.

ii) Scheme local variables does also show up in the compilation of emacs
 lisp to guile. The problem with this is that typically lisp programs
in emacs is littered with set!-ing of local variables. And the semantic
for this in scheme is to box them and again you loose the ability to 
redo and undo without a hefty reprogramming or write an advanced 
compiler to scheme - which is typically not a beautiful solution. Again
a good concept for enabling seamless and efficient and well designed 
code that can be undone and redone multiple times is needed.

3. Guile log is a combination of both traditional stack based logic
programming concept and the kanren concept and together with
a separate continuation implementation targeted for logical variables.
 To be able to code all of kanrens concept including a stack as well 
for speed and extra semantics like easy tracing or more generally
to have a dynamic-wind like idioms. The ability to redo and undo is 
really helpful and enables interesting logic programs to be written. So
here it was found out that having variables that dynamically can change 
between behaving like normal variables or during other dynamical 
context behave like a state preserving variable. The problem is that 
everything was based on code living in C and a separate stack 
implementation. So in order to use less library code and take advantage
of fundamental scheme concepts with classy implementation semantics. It
is actually possible with current scheme + parameters to implement 
these concepts, but the code will be slow and risk to be hard to reason 
about.

Rational for lifting this to a srfi request:
The concept have been proven useful and also implementing this in the
right way do touch the fundamentals of scheme in such a way that it
, from the perspective of the author, that help from the greater scheme 
community to improve upon the idea into a good specification is a well
tought out strategy for this concept.

Main Caveats:
1. It is possible to implement functionality in r5rs + parameters, but 
the ideoms runs 10x or more slower then an effective implementation. And 
result in bloated code.

2. The functionality without a good language support risk of messing
up the ability to reason about scheme code. Therefore we want support
in scheme to make it possible to introduce this concept in a sane way.

Suggested Spec:
We need
1. dynamic-wind from r5rs with the addition that the winder gets the 
continuation k, as an argument to the lambda.

2. Variables referenced indexed by the dynamic state of a continuation
 k, (D k) and a scheme object o and an integer i. It should be marked 
for garbage collection when (D k) and o is not reachable anymore. 
the getter and setter will then be (here v is a scheme object value),

  (storage-ref  k o i)
  (storage-set! k o i v)

3. A function to return a unique scheme object,
  (make-id)

4. The new variable kind will be called a redo-safe-variable and the 
concept will be marked with a ~, like with ! a macro or function that 
spread this concept should be marked with ~. Else if the semantic 
follow what is expected by the standard use the usual symbol 
nomenclature.

5. We will add three predicate functions that is indexed by the 
continuation k, and will be referenced by the continuation. They 
represent the possibility to dynamically change the meaning of the 
variable between a normal and redo safe state. We will introduce them 
as,

  (perform-wind-guard? k v)
  (perform-unwind-guard? k v)
  (perform-unwind-parameter-guard? k v)

They have the restrictions: they are evaluated to #t if v is #t and #f 
when v is #f.


6. There should be a setter and a getter of the predicate functions
above attached to a continuation k
  (redo-wind-predicate-set! k f)
  (redo-wind-predicate-ref k)
  (redo-unwind-predicate-set! k f)
  (redo-unwind-predicate-ref k)
  (redo-unwind-parameter-predicate-set! k f)
  (redo-unwind-parameter-predicate-ref k)

There is no semantics connected to the values that the function gives on
#f and #t.

7. A ordinary variable could be made a tilde variable by the idiom

  (with-redo-variables ((s v) ...) code ...),

with s ... being variable identifiers, v ... scheme expressions and
code ... is the code where s will lexically be (possibly) redo safe. The
semantics is depending on the usage of s inside code ... an s will
either be in guarded state and included in the list (s' v') ... or be
not changed at all and keep it's usual semantic inside code ... . 
So for the case where ((s' v') ...) is a nonempty list define the 
semantics as. let i ... = be a sequence of integers starting from 0 and
incremented one unit a time, the same length as s' ... .

8)

   (let ((last? #f)     
         (first? #t)
         (id   (make-id)))
     (dynamic-wind 
        (lambda (k)
          (set! last? #f)
          (when (and (not first?) (perform-wind-guard? k v'))
              (set! s' (storage-ref (D k) id i)))
          ...
          (set! first #f))
        (lambda ()
           (call-with-values 
               (lambda () body ...)
            (lambda ret
               (set! last? #t)
               (apply values ret))))
        
        (lambda (k)
           (when (not last?)
              (when (perform-unwind-guard? k v')
                  (storage-set! (D k) id s'))
               ...))))

9. With a parameter scheme object that behaves well with redo and undo 
will be called a redo-safe-parameter.

10. A ordinary parameters could be made a tilde parameters by the idiom

  (with-redo-parameters ((p u v) ...) code ...),

with p ... being evaluated to parameter objects, u ... scheme 
expressions represented the usual parameter values and v ... again 
control objects to direct the usage of tilde semantics or not for the 
variable dynamically. code ... is the code where p will lexically be 
 (possibly) redo safe. The
semantics is. depending on the usage of p inside code ... and p will
either be in guarded state and included in the list (p' u' v') ... or be
not changed at all and keep it's usual semantic inside code ... . So 
for the case where ((p' u' v') ...) is a nonempty list define the 
semantics as. let i ... = be a sequence of integers starting from 0 and
incremented one unit a time, the same length as p' ... .

11)

   (let ((last? #f)
         (first? #t)          
         (id   (make-id)))
     (dynamic-wind 
        (lambda (k)
          (set! last? #f)
          (let ((temp (p')))
             (if first?
                 (p' u')
                 (p' (storage-ref (D k) id i)))
             (storage-set! (D k) id i temp))
          ...
          (set! first? #f))
        (lambda ()
           (call-with-values 
               (lambda () body ...)
            (lambda ret
               (set! last? #t)
               (apply values ret))))
        
        (lambda (k . l)
           (let ((temp (p')))
             (p' (storage-ref (D k) id i))
             (unless (and (perform-unwind-property-guard? k v') last?)
             (storage-set! (D k) id i temp))))))

12) A variable a can be referenced as (set! a v) as usual and as a
simple variable reference. Added to this we introduce (set~ a v) and
(~ a) with the semantic:

13) A parameter value will be normally used with (p) and (p v). Using
the parameter the same way will in tilde context will be marked with
(~ (p)) and (~ (p v)).

14) If a local variable a is ever touched through (set! a v) then it 
will not be in a guarded state else if it is lexically never touched by 
set~ inside code ... in the with-redo-parameters it will not be in a 
guarded state.

15) If a parameter p inside code ... in with-redo-parameters is touched 
by (p v) it will not be in a guarded state else if it is never touched 
by
(~ (p v)) it will not be in a guarded state.

16) for top level variables the rule is different. Here there is a 
recommendation that top level-variables should be marked by ~ if they 
are 
ever touched with set~. The only language support is that they will 
not be guarded if set~ never appears inside code ... in 
with-redo-variables.

We will add to this a specification to support tail call's.

17. Define property S as. A closure object has property S if it 
includes an object of property S or ~. The return value of a function 
has property S if any of the arguments , the function included has 
property S. redo-safe-parameters have property S. In variable bindings 
the S property carries over to the binding identifier. The value of
(relax-s-property c) does not have the S property.

18. If the with-redo-... construct is in tail call 
position and a function call is located at a tail call position in 
code ... then if the value of the function does not have the S property 
it can be moved out of the construct and a proper tail call should be 
taken.

19. If the dynamic extent is never referenced outside of the internals
during the body code ... for any of the constructs, then a call in 
tail-call position should be a proper tail-call.

Finally we want support for defining lexical regions of code that is have
the property of securing the old behavior of scheme

20. (redo-transfer-to-user (a ...) code ...)
a ... are identifiers of with-redo-safe-variables and 
with-redo-safe-parameters origin. If they are referenced both as ~ 
and as ordinary variables an error should be thrown.

21. If a is a redo-safe variable then:
i) a is set!-ed, Then nothing is done
with a. 

ii) it is referenced but not set! then it be guarded by
  (let ((a a)) code ...)

iii) else nothing is done and the identity of a passes through.

22. If p is a redo-safe parameter then:
i) If it is found that it is used lexically in both ~ context and 
standard context an error will be trown.
ii) If it is lexically referenced in ~ context nothing will be done to 
the identity a.
iii) Else we will guard it with
(with-parameters ((a (a))) code ...)

Rational and discussion:
5) the rational for #t passthrough is to help the important case where
 we know that we want to store variables for a redo. after optimising 
redo properties. Putting it to #t and noting that ~ 
variables are never placed lexically in a lambda, and the variable 
beeing local we can just use unboxed values and set the values directly 
on the stack. #f is really just to be consistant and an opertunity for 
macros to turn off the construct.

6) I have only used the semantic f :- (lambda (x) #f) and (lambda (x) 
#t)
as predicates. It is possible to imagin cases where undo-redo is used 
as in algorithms in a hierachial manner. Then eg constructs like
(lambda (x) (< 4 x)) could be a good pattern to create that pretty 
advanced reason engine. Executing conminuations is also usually somewhat 
heavy and the extra burden to set the predicate will probably not cause 
much harm in the executiom speed.

* The redo safe variables will always be saved independently of the 
dynamic state and that is true for redo-safe parameters as well.

Example 1

(define-syntax-rule (next! i) (begin (set! i (+ i 1)) i))
(define-syntax-rule (next~ i) (begin (set~ i (+ i 1)) i))
(define-syntax-rule (for (x from k) code ...)
  (let ((x (- k 1)))
    (with-redo-variables ((x #t))
      (let loop ()
        (set~ x (+ x 1))
        (redo-transfer-to-user (x)
           code ...)))))

(for (x from 0) (f x))
;; redo safe, behaves as if x is just a local variable that is never 
;; set!-ed. The code is well optimized and as fast as if non boxed 
;; values are used

(for (x from 0) (f (lambda () (+ x (next! x)))))
;; not redo safe, the code works as if set! is used under the hood

(for (x from 0) (f (lambda () (+ x (next~ x)))))
;; Error, user is confused about mixing ~ context with non ~ context

(for (x from 0) (f (lambda () (+ (~ x) (next~ x)))))
;; redo safe, it is obvious that x behaves as a redo safe variable