NOTE-2-9-3-PPR-CHANGE.html -- ACL2 Version 6.3

### NOTE-2-9-3-PPR-CHANGE

change in pretty-printing for ACL2 Version_2.9.3

    Major Section:  NOTE-2-9-3

We have improved pretty-printing in ACL2 Version_2.9.3 to handle keywords a little differently. To see a discussion of the basics of this change, see note-2-9-3. In this note we describe it in considerable detail.

Those who wish to understand the ACL2 pretty-printer's implementation can now find considerably more comments on it in the source code. In this note, we do not focus on the implementation. Rather, we motivate the change and show how the improved prettyprinter performs.

Why do we want better keyword handling? Imagine a macro that builds a new state from an old state by changing the values in the affected fields, leaving everything else unchanged. One could write

    (modify th s :key1 val1 :key2 val2 :key3 val3)

where the three keys identify fields in the state.

To make it easier to read new concrete states, we may have a function that prints them ``relative'' to a given base state, expressing the new state as a modification of the given base state. So we may find ourselves prettyprinting modify forms like that above.

The previous prettyprinter will sometimes print the form above as follows.

    (modify th s :key1
            val1
            :key2 val2 :key3 val3)

This can be unpleasant to read, because of the way `:key1` and `val1` are separated. Here is an example of the old prettyprinter and the new one, both printing an expression from the ACL2 source code in a width of 40:

    Old:
    (ADD-TO-TAG-TREE
     'ASSUMPTION
     (MAKE
      ASSUMPTION :TYPE-ALIST TYPE-ALIST
      :TERM TERM :REWRITTENP REWRITTENP
      :IMMEDIATEP IMMEDIATEP :ASSUMNOTES
      (LIST
       (MAKE
            ASSUMNOTE :CL-ID
            NIL :RUNE RUNE :TARGET TARGET)))
     TTREE)

    New:
    (ADD-TO-TAG-TREE
         'ASSUMPTION
         (MAKE ASSUMPTION
               :TYPE-ALIST TYPE-ALIST
               :TERM TERM
               :REWRITTENP REWRITTENP
               :IMMEDIATEP IMMEDIATEP
               :ASSUMNOTES
               (LIST (MAKE ASSUMNOTE
                           :CL-ID NIL
                           :RUNE RUNE
                           :TARGET TARGET)))
         TTREE)

Basically the change we made forces the prettyprinter to print each `:key` on a new line unless they all fit on a single line. So we would now get either

    (modify th s :key1 val1 :key2 :val2 :key3 val3)

or

    (modify th s
            :key1 val1
            :key2 val2
            :key3 val3)

Furthermore, we fixed it so that if `val1` (say) is a big s-expression we may still print it on the same line as its key. The old prettyprinter enforced the rule that if you wanted to print `(foo a b)` and `b` gets broken up into several lines, then it has to start on a new line. Thus, we'd never print

    (foo a (bbb
            (mum x)))

but would print instead

    (foo a
         (bbb
          (mum x)))

Now, if a is a keyword, we can print the first way.

So here are some nice examples of prettyprinted keyword forms. All of these are printed for a page of width 40.

    <--            40 chars               ->
    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

    (MODIFY TH S :KEY1 V1 :KEY2 V2)

    (MODIFY TH S :KEY1 V1 :KEY2 V2 :KEY3 V3)

    (MODIFY TH S1                               ; Because of the extra char
            :KEY1 V1                            ; in S1 the flat size exceeds
            :KEY2 V2                            ; 40 and we break it.
            :KEY3 V3)

The old ppr would have printed this as:

    (MODIFY
         TH S1 :KEY1 V1 :KEY2 V2 :KEY3 V3)

Returning to new examples:

    <--            40 chars               ->
    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

    (MODIFY TH S
            :KEY1 (IF (IF X Y Z) AAAA BBBB)
            :KEY2 VAL2
            :KEY3 VAL3)

Now we extend `AAAA` and `BBBB` by one char each, so it would overflow the right margin if printed as above, and we get:

    <--            40 chars               ->
    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

    (MODIFY
         TH S
         :KEY1 (IF (IF X Y Z) AAAAX BBBBX)
         :KEY2 VAL2
         :KEY3 VAL3)

If we make these names even longer we force the value off the line containing `:key1`:

    (MODIFY
         TH S
         :KEY1
         (IF (IF X Y Z) AAAAXXXXX BBBBXXXXX)
         :KEY2 VAL2
         :KEY3 VAL3)

Here are some examples from the ACL2 source code, printed in 40 characters:

    (DEFTHM
     ALPHORDER-ANTI-SYMMETRIC
     (IMPLIES (AND (NOT (CONSP X))
                   (NOT (CONSP Y))
                   (ALPHORDER X Y)
                   (ALPHORDER Y X))
              (EQUAL X Y))
     :HINTS
     (("Goal"
       :IN-THEORY
       (UNION-THEORIES
        '(STRING< SYMBOL-<)
        (DISABLE
           CODE-CHAR-CHAR-CODE-IS-IDENTITY))
       :USE
       ((:INSTANCE SYMBOL-EQUALITY (S1 X)
                   (S2 Y))
        (:INSTANCE BAD-ATOM<=-ANTISYMMETRIC)
        (:INSTANCE
             CODE-CHAR-CHAR-CODE-IS-IDENTITY
             (C Y))
        (:INSTANCE
             CODE-CHAR-CHAR-CODE-IS-IDENTITY
             (C X)))))
     :RULE-CLASSES
     ((:FORWARD-CHAINING
       :COROLLARY
       (IMPLIES
          (AND (ALPHORDER X Y)
               (NOT (CONSP X))
               (NOT (CONSP Y)))
          (IFF (ALPHORDER Y X) (EQUAL X Y)))
       :HINTS
       (("Goal"
         :IN-THEORY (DISABLE ALPHORDER))))))

Here is that same one, printed in a width of 60.

    (DEFTHM
     ALPHORDER-ANTI-SYMMETRIC
     (IMPLIES (AND (NOT (CONSP X))
                   (NOT (CONSP Y))
                   (ALPHORDER X Y)
                   (ALPHORDER Y X))
              (EQUAL X Y))
     :HINTS
     (("Goal"
         :IN-THEORY
         (UNION-THEORIES
              '(STRING< SYMBOL-<)
              (DISABLE CODE-CHAR-CHAR-CODE-IS-IDENTITY))
         :USE ((:INSTANCE SYMBOL-EQUALITY (S1 X)
                          (S2 Y))
               (:INSTANCE BAD-ATOM<=-ANTISYMMETRIC)
               (:INSTANCE CODE-CHAR-CHAR-CODE-IS-IDENTITY (C Y))
               (:INSTANCE CODE-CHAR-CHAR-CODE-IS-IDENTITY
                          (C X)))))
     :RULE-CLASSES
     ((:FORWARD-CHAINING
          :COROLLARY (IMPLIES (AND (ALPHORDER X Y)
                                   (NOT (CONSP X))
                                   (NOT (CONSP Y)))
                              (IFF (ALPHORDER Y X) (EQUAL X Y)))
          :HINTS (("Goal" :IN-THEORY (DISABLE ALPHORDER))))))

Just for comparison, here is the above printed in 60 columns by the old prettyprinter.

    (DEFTHM
     ALPHORDER-ANTI-SYMMETRIC
     (IMPLIES (AND (NOT (CONSP X))
                   (NOT (CONSP Y))
                   (ALPHORDER X Y)
                   (ALPHORDER Y X))
              (EQUAL X Y))
     :HINTS
     (("Goal" :IN-THEORY
              (UNION-THEORIES
                   '(STRING< SYMBOL-<)
                   (DISABLE CODE-CHAR-CHAR-CODE-IS-IDENTITY))
              :USE
              ((:INSTANCE SYMBOL-EQUALITY (S1 X)
                          (S2 Y))
               (:INSTANCE BAD-ATOM<=-ANTISYMMETRIC)
               (:INSTANCE CODE-CHAR-CHAR-CODE-IS-IDENTITY (C Y))
               (:INSTANCE CODE-CHAR-CHAR-CODE-IS-IDENTITY
                          (C X)))))
     :RULE-CLASSES
     ((:FORWARD-CHAINING
           :COROLLARY
           (IMPLIES (AND (ALPHORDER X Y)
                         (NOT (CONSP X))
                         (NOT (CONSP Y)))
                    (IFF (ALPHORDER Y X) (EQUAL X Y)))
           :HINTS
           (("Goal" :IN-THEORY (DISABLE ALPHORDER))))))

Of course, given that you cannot tell for sure whether the keywords you're seeing are part of a keyword/value parameter list or part of some constant containing random keywords, the prettyprinter can't solve the problem perfectly. We just tried to make it work nicely on well-formed keyword/value parameter lists.

For example, here is a form printed by the each prettyprinter.

    Old:
    (MEMBER
     X
     '(:MONDAY
          :MON :TUESDAY :TUES :WEDNESDAY
          :WED :THURSDAY :THURS :FRIDAY
          :FRI :SATURDAY :SAT :SUNDAY :SUN))

    New:
    (MEMBER X
            '(:MONDAY :MON
                      :TUESDAY :TUES
                      :WEDNESDAY :WED
                      :THURSDAY :THURS
                      :FRIDAY :FRI
                      :SATURDAY :SAT
                      :SUNDAY :SUN))

The new way is not how one would print it by hand! But then, neither is the old way.