dpans/concept-pprint.tex

% -*- Mode: TeX -*-

\issue{PRETTY-PRINT-INTERFACE}

\beginsubSection{Pretty Printer Concepts}

The facilities provided by the \newterm{pretty printer} permit
\term{programs} to redefine the way in which \term{code} is displayed, 
and allow the full power of \term{pretty printing} to be applied 
to complex combinations of data structures.

Whether any given style of output is in fact ``pretty'' is inherently a
somewhat subjective issue.  However, since the effect of the 
\term{pretty printer} can be customized by \term{conforming programs},
the necessary flexibility is provided for individual \term{programs}
to achieve an arbitrary degree of aesthetic control.

By providing direct access to the mechanisms within the pretty printer
that make dynamic decisions about layout, the macros and functions
\macref{pprint-logical-block}, \funref{pprint-newline}, and
\funref{pprint-indent} make it possible to specify pretty printing
layout rules as a part of any function that produces output.  They also
make it very easy for the detection of circularity and sharing, and
abbreviation based on length and nesting depth to be supported by the
function.

The \term{pretty printer} is driven entirely by dispatch based on
\thevalueof{*print-pprint-dispatch*}.
\Thefunction{set-pprint-dispatch} makes it possible
for \term{conforming programs} to associate new pretty printing 
functions with a \term{type}.

\beginsubsubsection{Dynamic Control of the Arrangement of Output}
\DefineSection{DynamicControlofOutput}
 
The actions of the \term{pretty printer} when a piece of output is too
large to fit in the space available can be precisely controlled.
Three concepts underlie 
the way these operations work---\newterm{logical blocks},
			        \newterm{conditional newlines},
			    and \newterm{sections}.
Before proceeding further, it is important to define these terms.
 
The first line of \thenextfigure\ shows a schematic piece of output.  Each of
the characters in the output is represented by ``\f{-}''.  The positions of
conditional newlines are indicated by digits.  The beginnings and ends of
logical blocks are indicated by ``\f{<}'' and ``\f{>}'' respectively.
 
The output as a whole is a logical block and the outermost section.  This
section is indicated by the \f{0}'s on the second line of Figure 1.  Logical
blocks nested within the output are specified by the macro
\macref{pprint-logical-block}.  Conditional newline positions are specified 
by calls to \funref{pprint-newline}.  Each conditional newline defines 
two sections (one before it and one after it) and is associated with a 
third (the section immediately containing it).
 
The section after a conditional newline consists of: all the output up to,
but not including, (a) the next conditional newline immediately contained
in the same logical block; or if (a) is not applicable, (b) the next
newline that is at a lesser level of nesting in logical blocks; or if (b)
is not applicable, (c) the end of the output.
 
The section before a conditional newline consists of: all the output back
to, but not including, (a) the previous conditional newline that is
immediately contained in the same logical block; or if (a) is not
applicable, (b) the beginning of the immediately containing logical block.
The last four lines in Figure 1 indicate the sections before and after the
four conditional newlines.
 
The section immediately containing a conditional newline is the shortest
section that contains the conditional newline in question.  In \thenextfigure,
the first conditional newline is immediately contained in the section
marked with \f{0}'s, the second and third conditional newlines are immediately
contained in the section before the fourth conditional newline, and the
fourth conditional newline is immediately contained in the section after
the first conditional newline.
 
\code
 <-1---<--<--2---3->--4-->->
 000000000000000000000000000
 11 111111111111111111111111
           22 222
              333 3333
        44444444444444 44444
\endcode
\simplecaption{Example of Logical Blocks, Conditional Newlines, and Sections}
 
Whenever possible, the pretty printer displays the entire contents of a
section on a single line.  However, if the section is too long to fit in
the space available, line breaks are inserted at conditional newline
positions within the section.

\endsubsubsection%{Dynamic Control of the Arrangement of Output}

\beginsubsubsection{Format Directive Interface}

The primary interface to operations for dynamically determining the
arrangement of output is provided through the functions and macros of the
pretty printer.  \Thenextfigure\ shows the defined names related to \term{pretty printing}.

\displaythree{Defined names related to pretty printing.}{
*print-lines*&pprint-dispatch&pprint-pop\cr
*print-miser-width*&pprint-exit-if-list-exhausted&pprint-tab\cr
*print-pprint-dispatch*&pprint-fill&pprint-tabular\cr
*print-right-margin*&pprint-indent&set-pprint-dispatch\cr
copy-pprint-dispatch&pprint-linear&write\cr
format&pprint-logical-block&\cr
formatter&pprint-newline&\cr
}

\Thenextfigure\ identifies a set of \term{format directives} which serve
as an alternate interface to the same pretty printing operations in a 
more textually compact form.

%%Only of interest historically. -kmp
%In addition, it permits one would have to abandon the use of \funref{format}
%when interacting with the pretty printer.
 
\displaythree{Format directives related to Pretty Printing}{
\formatOp{I}&\formatOp{W}&\formatOp{<...~:>}\cr
\formatOp{:T}&\formatOp{/.../}&\formatOp{_}\cr
}

\endsubsubsection%{Format Directive Interface}

\beginsubsubsection{Compiling Format Strings}
\DefineSection{CompilingFormatStrings}

\issue{FORMAT-STRING-ARGUMENTS:SPECIFY}
A \term{format string} is essentially a program in a special-purpose language
that performs printing, and that is interpreted by \thefunction{format}.
\Themacro{formatter} provides the efficiency of using a \term{compiled function} 
to do that same printing but without losing the textual compactness of \term{format strings}.

A \newterm{format control} is either a \term{format string} or a \term{function}
that was returned by the \themacro{formatter}.
\endissue{FORMAT-STRING-ARGUMENTS:SPECIFY}

\endsubsubsection%{Compiling Format Strings}

\beginsubsubsection{Pretty Print Dispatch Tables}
\DefineSection{PPrintDispatchTables}
 
\issue{GENERALIZE-PRETTY-PRINTER:UNIFY}

%KAB: Are all type specifiers really valid?
%Waters: Yes.
%KMP: Actually, CONS was not originally valid, but has been added due to a cleanup.
A \newterm{pprint dispatch table} is a mapping from keys to pairs of values.  
Each key is a \term{type specifier}.  
The values associated with a key are
     a ``function'' (specifically, a \term{function designator} or \nil)
%% Per X3J13. -kmp 05-Oct-93
% and a ``numerical priorities'' (specifically, a \term{real}).
 and a ``numerical priority'' (specifically, a \term{real}).
Basic insertion and retrieval is done based on the keys with the equality
of keys being tested by \funref{equal}.

When \varref{*print-pretty*} is \term{true}, 
the \newterm{current pprint dispatch table} (in \varref{*print-pprint-dispatch*})
controls how \term{objects} are printed.
The information in this table takes precedence over
all other mechanisms for specifying how to print \term{objects}.
In particular, it 
%overrides
has priority over
user-defined \funref{print-object} \term{methods} 
\issue{DEFSTRUCT-PRINT-FUNCTION-AGAIN:X3J13-MAR-93}
%and print functions for \term{structures} 
\endissue{DEFSTRUCT-PRINT-FUNCTION-AGAIN:X3J13-MAR-93}
because the \term{current pprint dispatch table} is consulted first.

%%Just trying to simplify wording to fit context. -kmp 27-Aug-93
%The function to use when \term{pretty printing} an \term{object} is chosen 
The function is chosen from the \term{current pprint dispatch table}
by finding the highest priority function 
%% Again, this should follow from context.
%from the \term{current pprint dispatch table} 
that is associated with a \term{type specifier} that matches the \term{object};
%KAB: What if there are several matches with equal priority?
%Waters: It's not well-defined.
%KMP: I've added this text to clarify that point:
if there is more than one such function, 
it is \term{implementation-dependent} which is used.

However, if there is no 
%% better parallel construction. -kmp 27-Aug-93
%specification
information in the table
about how to \term{pretty print} a particular kind of \term{object}, 
% it is then printed using the standard mechanisms as if
% \varref{*print-pretty*} were \term{false}.
a \term{function} is invoked which uses \funref{print-object} to print the \term{object}.
The value of \varref{*print-pretty*} is still \term{true} 
when this function is \term{called},
and individual methods for \funref{print-object} might still elect to
produce output in a special format conditional on \thevalueof{*print-pretty*}.
\endissue{GENERALIZE-PRETTY-PRINTER:UNIFY}

\endsubsubsection%{Pretty Print Dispatch Tables}

\beginsubsubsection{Pretty Printer Margins}

A primary goal of pretty printing is to keep the output between a pair of
margins. 
%This used to say:
%  The left margin is set at the column where the output begins.
%KMP asked:
%  Does this mean that
%   a. the cursor is moved 
%   b. the position at the cursor is assumed to be zero
%   c. that the position at the cursor becomes the new left margin?
%Waters replied (c), so the following new sentence was made:
The column where the output begins is taken as the left margin.
%This used to say:
%  If this cannot be determined, the left margin is set to zero.
%KMP asked:
%  Does this mean ``assumed to be''
%Waters replied yes, so the following new sentence was made:
If the current column cannot be determined at the time output begins,
the left margin is assumed to be zero.
The right margin is controlled by \varref{*print-right-margin*}.

\endsubsubsection%{Pretty Printer Margins}

\endsubSection%{Pretty Printer Concepts}

\beginsubSection{Examples of using the Pretty Printer}
\DefineSection{PrettyPrinterExamples}

As an example of the interaction of logical blocks, conditional newlines,
and indentation, consider the function \f{simple-pprint-defun} below.  This
function prints out lists whose \term{cars} are \macref{defun} in the 
standard way assuming that the list has exactly length \f{4}.
 
\code
(defun simple-pprint-defun (*standard-output* list)
  (pprint-logical-block (*standard-output* list :prefix "(" :suffix ")")
    (write (first list))
    (write-char #\\Space)
    (pprint-newline :miser)
    (pprint-indent :current 0)
    (write (second list))
    (write-char #\\Space)
    (pprint-newline :fill)
    (write (third list))
    (pprint-indent :block 1)
    (write-char #\\Space)
    (pprint-newline :linear)
    (write (fourth list))))
\endcode

Suppose that one evaluates the following:

\code
(simple-pprint-defun *standard-output* '(defun prod (x y) (* x y)))
\endcode
 
If the line width available is greater than or equal to \f{26}, then all of the
output appears on one line.  If the line width available is reduced to \f{25},
a line break is inserted at the 
linear-style conditional newline\idxtext{linear-style conditional newline}
before the
\term{expression} \f{(* x y)}, producing the output shown.  The
\f{(pprint-indent :block 1)} causes \f{(* x y)} to be printed at a relative
indentation of \f{1} in the logical block.
 
\code
 (DEFUN PROD (X Y) 
   (* X Y))
\endcode 

If the line width available is \f{15}, a line break is also inserted at the
fill style conditional newline before the argument list.  The call on
\f{(pprint-indent :current 0)} causes the argument list to line up under the
function name.
 
\code
(DEFUN PROD
       (X Y)
  (* X Y))
\endcode
 
If \varref{*print-miser-width*} were greater than or equal to 14, the example 
output above would have been as follows, because all indentation changes 
are ignored in miser mode and line breaks are inserted at 
miser-style conditional newlines.\idxtext{miser-style conditional newline}
 
\code
 (DEFUN
  PROD
  (X Y)
  (* X Y))
\endcode 

As an example of a per-line prefix, consider that evaluating the following
produces the output shown with a line width of \f{20} and
\varref{*print-miser-width*} of \nil.
 
\code
 (pprint-logical-block (*standard-output* nil :per-line-prefix ";;; ")
   (simple-pprint-defun *standard-output* '(defun prod (x y) (* x y))))
 
 ;;; (DEFUN PROD
 ;;;        (X Y)
 ;;;   (* X Y))
\endcode
 
As a more complex (and realistic) example, consider the function \f{pprint-let}
below.  This specifies how to print a \specref{let} \term{form} in the traditional
style.  It is more complex than the example above, because it has to deal with
nested structure.  Also, unlike the example above it contains complete code to 
readably print any possible list that begins with the \term{symbol} \specref{let}.
The outermost \macref{pprint-logical-block} \term{form} handles the printing of
the input list as a whole and specifies that parentheses should be printed in the
output.  The second \macref{pprint-logical-block} \term{form} handles the list 
of binding pairs.  Each pair in the list is itself printed by the innermost
\macref{pprint-logical-block}.  (A \macref{loop} \term{form} is used instead of
merely decomposing the pair into two \term{objects} so that readable output will
be produced no matter whether the list corresponding to the pair has one element,
two elements, or (being malformed) has more than two elements.)   
A space and a 
fill-style conditional newline\idxtext{fill-style conditional newline}
are placed after
each pair except the last.  The loop at the end of the topmost
\macref{pprint-logical-block} \term{form} prints out the forms in the body
of the \specref{let} \term{form} separated by spaces and 
linear-style conditional newlines.
 
\code
 (defun pprint-let (*standard-output* list)
   (pprint-logical-block (nil list :prefix "(" :suffix ")")
     (write (pprint-pop))
     (pprint-exit-if-list-exhausted)
     (write-char #\\Space)
     (pprint-logical-block (nil (pprint-pop) :prefix "(" :suffix ")")
       (pprint-exit-if-list-exhausted)
       (loop (pprint-logical-block (nil (pprint-pop) :prefix "(" :suffix ")")
               (pprint-exit-if-list-exhausted)
               (loop (write (pprint-pop))
                     (pprint-exit-if-list-exhausted)
                     (write-char #\\Space)
                     (pprint-newline :linear)))
             (pprint-exit-if-list-exhausted)
             (write-char #\\Space)
             (pprint-newline :fill)))
     (pprint-indent :block 1)
     (loop (pprint-exit-if-list-exhausted)
           (write-char #\\Space)
           (pprint-newline :linear)
           (write (pprint-pop)))))
\endcode
 
Suppose that one evaluates the following with \varref{*print-level*} being 4, 
and \varref{*print-circle*} being \term{true}.

\code
 (pprint-let *standard-output*
             '#1=(let (x (*print-length* (f (g 3))) 
                       (z . 2) (k (car y)))
                   (setq x (sqrt z)) #1#))
\endcode
 
If the line length is greater than or equal to \f{77}, the output produced
appears on one line.  However, if the line length is \f{76}, line breaks are
inserted at the linear-style conditional newlines separating the forms in
the body and the output below is produced.  Note that, the degenerate
binding pair \f{x} is printed readably even though it fails to be a list; a
depth abbreviation marker is printed in place of \f{(g 3)}; the binding pair
\f{(z . 2)} is printed readably even though it is not a proper list; and
appropriate circularity markers are printed.

\code
 #1=(LET (X (*PRINT-LENGTH* (F #)) (Z . 2) (K (CAR Y))) 
      (SETQ X (SQRT Z))
      #1#)
\endcode

If the line length is reduced to \f{35}, a line break is inserted at one of the
fill-style conditional newlines separating the binding pairs.
 
\code
 #1=(LET (X (*PRINT-PRETTY* (F #))
          (Z . 2) (K (CAR Y)))
      (SETQ X (SQRT Z))
      #1#)
\endcode
 
Suppose that the line length is further reduced to \f{22} and \varref{*print-length*} is
set to \f{3}. In this situation, line breaks are inserted after both the first
and second binding pairs.  In addition, the second binding pair is itself
broken across two lines.  Clause (b) of the description of fill-style
conditional newlines (\seefun{pprint-newline}) 
prevents the binding pair \f{(z . 2)} from being printed
at the end of the third line.  Note that the length abbreviation hides the
circularity from view and therefore the printing of circularity markers
disappears.
 
\code
 (LET (X
       (*PRINT-LENGTH*
        (F #))
       (Z . 2) ...)
   (SETQ X (SQRT Z))
   ...)
\endcode
 
The next function prints a vector using ``\f{\#(...)}'' notation.
      
\code
(defun pprint-vector (*standard-output* v)
  (pprint-logical-block (nil nil :prefix "#(" :suffix ")")
    (let ((end (length v)) (i 0))
      (when (plusp end)
        (loop (pprint-pop)
              (write (aref v i))
              (if (= (incf i) end) (return nil))
              (write-char #\\Space)
              (pprint-newline :fill))))))
\endcode

Evaluating the following with a line length of 15 produces the output shown.
 
\code
 (pprint-vector *standard-output* '#(12 34 567 8 9012 34 567 89 0 1 23))
 
 #(12 34 567 8 
   9012 34 567 
   89 0 1 23)
\endcode

As examples of the convenience of specifying pretty printing with 
\term{format strings}, consider that the functions \f{simple-pprint-defun}
and \f{pprint-let} used as examples above can be compactly defined as follows.
(The function \f{pprint-vector} cannot be defined using \funref{format}
because the data structure it traverses is not a list.)
 
\code
(defun simple-pprint-defun (*standard-output* list)
  (format T "~:<~W ~@_~:I~W ~:_~W~1I ~_~W~:>" list))

(defun pprint-let (*standard-output* list)
  (format T "~:<~W~{\hat}~:<~@\{~:<~@\{~W~{\hat}~_~\}~:>~{\hat}~:_~\}~:>~1I~@\{~{\hat}~_~W~\}~:>" list)) 
\endcode

In the following example, the first \term{form} restores
\varref{*print-pprint-dispatch*} to the equivalent of its initial value.
The next two forms then set up a special way to pretty print ratios.
Note that the more specific \term{type specifier} has to be associated
with a higher priority.
 
\code
 (setq *print-pprint-dispatch* (copy-pprint-dispatch nil))

 (set-pprint-dispatch 'ratio
   #'(lambda (s obj)
       (format s "#.(/ ~W ~W)" 
                 (numerator obj) (denominator obj))))

 (set-pprint-dispatch '(and ratio (satisfies minusp))
   #'(lambda (s obj)
       (format s "#.(- (/ ~W ~W))" 
               (- (numerator obj)) (denominator obj)))
   5)

 (pprint '(1/3 -2/3))
 (#.(/ 1 3) \#.(- (/ 2 3)))
\endcode

The following two \term{forms} illustrate the definition of 
pretty printing functions for types of \term{code}.  The first
\term{form} illustrates how to specify the traditional method 
for printing quoted objects using \term{single-quote}.  Note
the care taken to ensure that data lists that happen to begin
with \misc{quote} will be printed readably.  The second form 
specifies that lists beginning with the symbol \f{my-let}
should print the same way that lists beginning with \specref{let}
print when the initial \term{pprint dispatch table} is in effect.
 
\code
 (set-pprint-dispatch '(cons (member quote)) () 
   #'(lambda (s list)
       (if (and (consp (cdr list)) (null (cddr list)))
          (funcall (formatter "'~W") s (cadr list))
          (pprint-fill s list))))
 
 (set-pprint-dispatch '(cons (member my-let)) 
                      (pprint-dispatch '(let) nil))
\endcode
 
The next example specifies a default method for printing lists that do not
correspond to function calls.  Note that the functions \funref{pprint-linear},
\funref{pprint-fill}, and \funref{pprint-tabular} are all defined with
optional \param{colon-p} and \param{at-sign-p} arguments so that they can 
be used as \funref{pprint dispatch functions} as well as \formatOp{/.../} 
functions.

\code
 (set-pprint-dispatch '(cons (not (and symbol (satisfies fboundp))))
                      #'pprint-fill -5)
 
 ;; Assume a line length of 9
 (pprint '(0 b c d e f g h i j k))
 (0 b c d
  e f g h
  i j k)
\endcode 

This final example shows how to define a pretty printing function for a
user defined data structure.
 
\code
 (defstruct family mom kids)
 
 (set-pprint-dispatch 'family
   #'(lambda (s f)
       (funcall (formatter "~@<#<~;~W and ~2I~_~/pprint-fill/~;>~:>")
               s (family-mom f) (family-kids f))))
\endcode
 
The pretty printing function for the structure \f{family} specifies how to
adjust the layout of the output so that it can fit aesthetically into
a variety of line widths.  In addition, it obeys 
the printer control variables \varref{*print-level*},
\varref{*print-length*}, \varref{*print-lines*},
\varref{*print-circle*}
%% There's no such var.  (Flanagan pointed this out in private mail.) -kmp 26-Jul-93
%, \varref{*print-shared*}
and \varref{*print-escape*},
and can tolerate several different kinds of malformity in the data structure.
The output below shows what is printed out with a right margin of \f{25},
\varref{*print-pretty*} being \term{true}, \varref{*print-escape*} being \term{false},
and a malformed \f{kids} list.
 
\code
 (write (list 'principal-family
              (make-family :mom "Lucy"
                           :kids '("Mark" "Bob" . "Dan")))
        :right-margin 25 :pretty T :escape nil :miser-width nil)
 (PRINCIPAL-FAMILY
  #<Lucy and
      Mark Bob . Dan>)
\endcode
  
\issue{DEFSTRUCT-PRINT-FUNCTION-AGAIN:X3J13-MAR-93}
Note that a pretty printing function for a structure is different from
%the structure's print function.
the structure's \funref{print-object} \term{method}.
While
%print functions
\funref{print-object} \term{methods}
are permanently associated with a structure,
pretty printing functions are stored in 
\term{pprint dispatch tables} and can be rapidly changed to reflect 
different printing needs.  If there is no pretty printing function for 
a structure in the current \term{pprint dispatch table},
% the print function (if any) 
its \funref{print-object} \term{method}
is used instead.
\endissue{DEFSTRUCT-PRINT-FUNCTION-AGAIN:X3J13-MAR-93}

\endsubSection%{Examples of using the Pretty Printer}

\beginsubsection{Notes about the Pretty Printer's Background}

For a background reference to the abstract concepts detailed in this
section, see \XPPaper.  The details of that paper are not binding on
this document, but may be helpful in establishing a conceptual basis for
understanding this material.

\endsubsection%{Notes about the Pretty Printer's Background}

\endissue{PRETTY-PRINT-INTERFACE}