CLHS: Issue PRETTY-PRINT-INTERFACE Writeup

Issue PRETTY-PRINT-INTERFACE Writeup

Version 3 (by Guy Steele Jr) supersedes version 2 and is changed from

version 1 as follows: adds a functional interface to supplement the

interface through FORMAT, and reflects comments by Barrett and

Pierson.

Version 4 (by Dick Waters) is changed from version 3 as follows: The

short summary is updated to reflect the functional interface. The

functional interface is changed following suggestions made by Dave Moon.

The proposal is amended in a few minor ways to increase the

compatibility with variable width fonts. Additional discussion has been

added with regard to the advantages of XP with respect to handling

detection and abbreviation, the interaction with CLOS, and

the extended type specifier CONS used by XP.

Version 5 (by Waters, Haflich, Laubsch, Loosemore, Pierson, and van Roggen).

On June 27 it was voted 12 to 4 in a plenary session of X3J13 that an add

hoc committee on pretty printing be created containing the above members

with the charter of ironing out the final details of the pretty print

interface and that if this committee could come to a unanimous decision on

the interface, the interface would become part of the draft Common Lisp

standard without the need of a further vote by the full committee. The

unanimity of the add hoc committee has been signaled by having each member

of the committee send a message to X3J13 to this effect.

In addition to the main vote above, five straw polls were taken. Three

of these votes were overwhelming; however, two appeared too close to be

definitive. In order of the clarity of the outcome the straw votes were:

16-0 in favor of the basic functional interface,

10-4 against the inclusion of #"...",

12-7 in favor of having some way to convert a FORMAT string into a function,

11-8 in favor of allowing FORMAT to take a function as well as a string,

10-8 against the pretty printer extensions of FORMAT.

The subcommittee followed these recommendations except for the last.

(See the end of the discussion section.)

Version 5 is changed from version 4 as follows: The names of the

functions and variables have been changed to better reflect that (for the

most part) they are a group that applies only to pretty printing. The

variables *DEFAULT-RIGHT-MARGIN* and *LAST-ABBREVIATED-PRINTING* have been

eliminated. The readmacro construct #"..." has been eliminated and a new

macro FORMATTER introduced in its place. The way *PRINT-LINES*

abbreviation is indicated has been improved to ensure that the delimiters

will be balanced in the output and to ensure that the reader will complain

if you later try to read the output. The macros LOGICAL-BLOCK-POP and

LOGICAL-BLOCK-COUNT have been eliminated and the functionality they

supported provided in a slightly different form by two new macros

PPRINT-POP and PPRINT-EXIT-IF-LIST-EXHAUSTED. The macro

DEFINE-PRINT-DISPATCH has been replaced by a function SET-PPRINT-DISPATCH.

The proposal has been reorganized to show that the functional interface is

more fundamental than the FORMAT interface, to clarify exactly what happens

in error situations, and to indicate that the examples are merely examples

and not part of the proposal.

================================================================================

Issue: PRETTY-PRINT-INTERFACE

References: Description of XP by Dick Waters (attached)

*PRINT-PRETTY* (CLtL p. 371)

WRITE (CLtL p. 382)

PPRINT (CLtL p. 383)

FORMAT (CLtL pp. 385-407)

FORMAT ~T directive (CLtL pp. 398-399)

FORMAT ~< directive (CLtL pp. 404-406)

Related issues:

Category: CLARIFICATION CHANGE ADDITION

Edit history: Version 1, 24-Feb-89 by Steele

Version 2, 15-Mar-89 by Steele and Waters

Version 3, 15-Mar-89 by Steele

Version 4, 22-Mar-89 by Waters

Version 5, 20-Jul-89 by Waters et al

Problem description:

At present, Common Lisp provides no specification whatsoever of how

pretty-printing is to be accomplished, and no way for the user to control

it. In particular, there is no protocol by which a user can write a

print-function for a structure, or a method for PRINT-OBJECT, that will

interact smoothly with the built-in pretty-printer in a portable manner.

Proposal (PRETTY-PRINT-INTERFACE:XP):

Adopt the interfaces and protocols of the XP pretty-printer by Dick Waters,

described in full in the attached document. Here is a very brief summary

of the proposal.

New variables: *PRINT-PPRINT-DISPATCH*

*PRINT-MISER-WIDTH*

*PRINT-RIGHT-MARGIN*

*PRINT-LINES*

New functions: COPY-PPRINT-DISPATCH

SET-PPRINT-DISPATCH

New macros: PPRINT-LOGICAL-BLOCK

PPRINT-POP

FORMATTER

New FORMAT directives: ~W ~_ ~I ~:T ~/name/ ~<...~:>

The function WRITE is extended to accept four additional keyword arguments

:PPRINT-DISPATCH, :RIGHT-MARGIN, :LINES, and :MISER-WIDTH corresponding to

the four new variables.

The function FORMAT is extended so that it can accept a function instead of

a FORMAT string. (This change is also made in the other functions that

accept FORMAT strings such as ERROR and WARN.)

Examples: See attached document.

Rationale:

There ought to be a good user interface to the pretty printer.

This is the only proposal for which there is a portable implementation

that has seen extensive use and is being made freely available.

Current practice:

XP son of PP son of GPRINT son of #PRINT is the latest in a line of pretty

printers that goes back 13 years. All of these printers use essentially

the same basic algorithm and conceptual interface. Further, except for

#PRINT, which was implemented solely to satisfy the author's personal

needs, each of these printers has had extensive use. XP has been in

experimental use as the pretty printer in CMU Common Lisp for 6 months. PP

has been the pretty printer in DEC Common Lisp for the past 3 years. Prior

to three years ago, GPRINT was used for 2 years as the pretty printer in

DEC Common Lisp. In addition, GPRINT has been the pretty printer in

various generations of Symbolics Lisp for upwards of 5 years.

(See Waters R.C., "User Format Control in a Lisp Prettyprinter", ACM TOPLAS,

5(4):513--531, October 1983.)

Cost to Implementors: A fair amount of effort (several man-weeks at most).

Source code for XP is available to all comers from Dick Waters. It has

been arranged with MIT for anyone who wants to to get a non-exclusive

royalty-free license for XP from MIT. The system is documented in great

detail in:

Waters, Richard C., "XP: A Common Lisp Pretty Printing System",

Artificial Intelligence Laboratory Technical Memo 1102a,

Massachusetts Institute of Technology, Cambridge MA, July 1989.

Cost to Users: None (I think). This is an upward-compatible extension.

Cost of non-adoption: Continued inability for user print-functions

to interact with the pretty-printer in a useful and portable manner.

Performance impact: XP is claimed to be quite fast.

Benefits: User control of pretty-printing in a portable manner.

Aesthetics:

Using ~<...~:> may strike some as uncomfortably close in the syntactic

space of FORMAT directives to the existing ~<...~>. However, it is very

unlikely that both of these directives (pretty-print logical block and

columnar justification, respectively) will be used in the same call to

FORMAT. Previous versions of XP used ~!...~. instead of ~<...~:> but this

made FORMAT strings very difficult to read; it is preferable to have a

directive that looks like matching brackets of some sort.

Dan Pierson comments: You might mention that some people will undoubtedly

find piling more hair on FORMAT ugly (of course these same people may

well find FORMAT in general ugly :-)).

Discussion:

Zetalisp used ~:T to mean pixelwise tabulation, so the use of ~:T

suggested here may be a problem. If so, another suggestion for naming

this directive would be appreciated.

The ~/.../ directive is already in Zetalisp, and is not an idea new

to this proposal. However, it should be noted that the proposal for

~/.../ here is simpler than, and incompatible with, the current Zatalisp

practice.

Guy Steele and Dick Waters strongly support this proposal. (As an example,

Guy Steele has a portable simulator for Connection Machine Lisp, and would

like very much to have xappings and xectors pretty-print properly.)

---

Dan Pierson comments: You can add me to the list of strong supporters of

this proposal. While the proposal is long and complex, it is supported by

a long history of usage in several different Lisp environments.

The utility of *PRINT-LINES* becomes more obvious if it is pointed out

that Dick's pretty printers are implemented to print each line as it

is computed. This means that a small value for *PRINT-LINES* saves

significant time as well as output medium space.

The advantages of compiled format strings (format functions) should be

brought out as benefits in their own right. The current proposal just

mentions them as a minor feature of XP.

At first this struck me a very cute end run around the failure of

STREAM-INFO, then I realized that one of the problems with STREAM-INFO

may have been that it was a standard at the wrong level. STREAM-INFO

permitted people to use XP, but not to count on it. This proposal

makes it possible to write portable code whose new data structures and

language elements print correctly in whatever Common Lisp environment

they're run in. [End of comments by Pierson]

---

It has been noted by Guy Steele that some places in the initial document

where it says that circularity detection is handled correctly, this is

true a fortiori following the decision on PRINT-CIRCLE-STRUCTURE.

However, Waters notes that the vote on PRINT-CIRCLE-STRUCTURE said

nothing about the handling of *PRINT-LEVEL*. Therefore, the fact that

XP handles *PRINT-LEVEL* correctly is an improvement.

In addition, PRINT-CIRCLE-STRUCTURE is also silent on what is supposed

to happen if a user program decomposes a list itself (e.g., with DOLIST

or ~{~}) rather than calling a print function. Assumedly *PRINT-CIRCLE*

etc. is not handled in this case. In contrast, if one uses

PPRINT-LOGICAL-BLOCK or ~<~:>, then *PRINT-CIRCLE*, *PRINT-LEVEL*, and

*PRINT-LENGTH* are all automatically handled correctly.

For example, (format nil "-~1{~A ~A ~A ~A ~A ~}-" '#1=(1 #1# 2 . #1#))

produces "-1 #1=(1 #1# 2 . #1#) 2 1 #1=(1 #1# 2 . #1#) -"

even under PRINT-CIRCLE-STRUCTURE and

(format nil "-~1{~A ~}-" '#1=(1 #1# 2 . #1#))

causes infinite looping. However, in XP,

(format nil "-~:<~W ~W ~W ~W ~W~:>-" '#1=(1 #1# 2 . #1#))

produces "-#1=(1 #1# 2 . #1#)-".

This proves to be very useful when writing pretty printing functions for

things. Note also that ~<~:> supports *PRINT-LEVEL* and *PRINT-LENGTH*

correctly. All the same things can be said about the functional interface

and using PPRINT-LOGICAL-BLOCK rather than traversing a list yourself in

some fashion.

All in all, Waters claims that PRINT-CIRCLE-STRUCTURE covers at most 1/4

of what XP does in support of *PRINT-CIRCLE* and does not cover anything

of what XP does to support *PRINT-LEVEL*, *PRINT-LENGTH*, and

robustness in the face of malformed arguments. These are vital

features for writing printing functions that really work right all the time.

---

It has been suggested that objects should be looked up in pprint dispatch

tables by looking for the most specific type specifier that applies, rather

than looking for the highest priority type specifier that applies. There

are two problems with this. First, it is possible for two type specifiers

to apply without one being more specific than the other. For example,

consider the type specifiers (OR INTEGER FLOAT) and (OR INTEGER RATIONAL)

or the type specifiers (NOT COMPLEX) and (NOT INTEGER).

A much more serious problem is that the only way to tell whether one type

specifier is more specific than another is to use SUBTYPEP. Unfortunately,

even with the clarifications and extensions introduced by X3J13 with regard

to SUBTYPEP in particular and the type system in general, SUBTYPEP is not

very dependable. There are critical situations where it cannot reasonably

work in any implementation (e.g., when a type specifier contains SATISFIES)

and many other situations where it can vary from one implementation to

another. Unfortunately, pprint dispatch tables are expected to contain

type specifiers containing SATISFIES on a regular basis. In addition, the

variability in SUBTYPEP would reduce the portability of user-defined pprint

dispatch tables if dispatching through them depended on SUBTYPEP. All in

all, it seems much better for the pretty printing to rely on priorities

rather than on SUBTYPEP

---

It has been noted by Dave Moon that things would be much more elegant if

SET-PPRINT-DISPATCH could be expressed directly as a CLOS DEFMETHOD for

an appropriate generic function. Dick Waters agrees with this. However,

SET-PPRINT-DISPATCH depends on type specifiers that are more complex than

the ones CLOS deals with and ones that do not have clear subtype/supertype

relationships, compensating for the latter problem by supporting numerical

priorities to disambiguate things. (The defaulting behavior is a key

feature of the pretty printer.) At the very least, this means that

SET-PPRINT-DISPATCH will not fit into CLOS in a simple way.

Given the problems, Moon suggests that "it does seem that right now it

might be best to keep a separate SET-PPRINT-DISPATCH macro, with the

idea that the expansion is implementation-dependent at the moment, but

might some day be changed to be defined to expand into DEFMETHOD. I

haven't looked to see whether any syntactic changes would be appropriate

to make that transition smoother."

(Waters also worries that the overhead needed to locate the right CLOS

method would seriously degrade the pretty printer, because the printer

has to do this for every part of every object printed. This dispatching is

currently done by very fast code that is tuned to take advantage of the

observed distribution of kinds of objects that have special pretty

printers attached to them. Even with this special purpose code,

dispatching takes a significant part of the pretty printer's time.)

---

Dave Moon also comments that it is not good to have something that looks

like a type specifier (i.e., the extended form of the CONS type specifier

used by SET-PPRINT-DISPATCH) and yet is not a real type specifier. He

suggests that we should either amend Common Lisp to accept the extended

form of the CONS type specifier, or stop having SET-PPRINT-DISPATCH use it.

Many of the members of the ad hoc pretty printing subcommittee agree that

(CONS car-type cdr-type) should be made a full fledged type specifier. In

fact it has been suggested that there should be a much wider variety of

ways to talk about lists and their contents than currently exists.

However, the subcommittee feels that it would be going significantly beyond

our charter to change anything about the type system.

Therefore, we opted for the opposite tack of making it very clear in this

proposal that while the function SET-PPRINT-DISPATCH accepts the form

(CONS car-type cdr-type), this form is not a type specifier.

---

To a considerable extent, the design of the XP interface is completely

neutral about the issue of variable- versus fixed- width fonts. In

particular, most of the discussion of how the formating proceeds either

talks about absolute positions of zero or talks about something being

in the same horizontal position as something else. These statements are

all font-independent. (Further, although Waters' current implementation

does not support variable-width fonts, the algorithms used could be

extended to support them without radical changes.)

Nevertheless, there are 8 places where users specify explicit non-zero

lengths: the variables *PRINT-RIGHT-MARGIN* and *PRINT-MISER-WIDTH*,

the numeric arguments to ~T, ~I, and ~/pprint-tabular/ and their associated

functions PPRINT-TAB, PPRINT-INDENT, and PPRINT-TABULAR.

It is proposed that all of these lengths be in the same units, and that

this unit be ems (the length of an "m" in the font currently being used

to output characters to the relevant output stream at the moment that

the command is encountered or a variable is consulted).

It is further proposed that users and implementors be advised to set things

up so that explicit lengths do not have to be specified. For implementors,

this means making streams smart enough that they know how wide they are.

(This avoids the use of *PRINT-RIGHT-MARGIN*.) For users, this means

relying on streams knowing their own widths (which is a good idea for

adaptability in any case) and using ~:I (instead of just ~I) to specify

indentations wherever possible. Further, it should be noted that since

*PRINT-MISER-WIDTH* is essentially heuristic in nature, it does not

matter if its value is only an approximate length and users will only need

to change the value of *PRINT-MISER-WIDTH* in unusual situations.

This leaves only tabbing as an area where explicit lengths have to be

specified on a regular basis. Fortunately, approximate lengths are often

acceptable in this situation as well.

---

The currently proposed syntax for PPRINT-LOGICAL-BLOCK was suggested by

Dave Moon based on his experience with similar constructs at Symbolics.

Waters had originally suggested using a standard binding pair to specify

the underlying stream separately from the variable used to hold the special

stream used within the logical block. However, this invites user mistakes.

The problem is that peculiar results will be obtained if any I/O is sent

directly to the underlying stream from within the logical block. By

arranging the syntax as proposed, the same variable is always used for the

special stream within the logical block as is used for the underlying

stream outside of the block. As a result, it is syntactically impossible

to directly access the underlying stream (unless it is stored in two

different variables) and the user is painlessly saved from making mistakes.

Also, the PPRINT-LOGICAL-BLOCK macro is rendered more compact.

---

Another interesting issue is the interaction between pretty-printing and

*PRINT-READABLY*. Note first, that WITH-STANDARD-IO-SYNTAX binds

*PRINT-PRETTY* to NIL. In general one should expect that maximum machine

readability will be achieved when *PRINT-PRETTY* is nil. However, in

analogy with issue DATA-IO, it is reasonable to require that

PPRINT-DISPATCH functions must obey *PRINT-READABLY*. Note that XP

supports a number of features (e.g., automatic handling of malformed lists)

that are explicitly designed to make it easier to write pprint functions

that reliably produce machine readable output.

---

Steve Haflich comments that the macro FORMATTER provides an opportunity at

some later time to do something about making FORMAT control strings more

readable. Specifically, when using FORMATTER, one should not hesitate to

insert lots of newlines, because they will not waste any space since the

FORMAT string will be converted into a function in any case. Second, it

would be nice to add some feature to format strings that would allow

comments to appear in a FORMAT string. (This is what ~; probably should

have been reserved to mean.)

---

Walter van Roggen notes that the automatic handling of *PRINT-LEVEL* by XP

obsoletes the need for the third argument to print functions for

structures. (This argument was very seldom used in any case.) It might be

appropriate at some future time to get rid of the third argument to print

functions for structures. However, this would not be an upward compatible

change.

---

The pretty printer control interface is included in this proposal for two

key reasons:

(A) Something like the following is probably a key part of the argument

against the pretty-printer-control FORMAT interface in the minds of those

that oppose it. "Including the pretty-printer-control FORMAT interface

might be a gain or a loss for Common Lisp, but nothing could possibly be

lost by not including it. You can always just use FORMAT as it is now, and

you can get pretty printing control with the new functions." However, this

argument is not quite true.

As can be seen in the example below, pretty printer control information has

to be highly intertwined with the rest of the output functions. As a

practical matter, this means that if there is no pretty-printer-control

FORMAT interface you cannot make much if any use of FORMAT when specifying

output that contains pretty-printer-control information. (For instance in

the 20 line example below, there is not even one place where there are two

printing operations in a row that could be combined into a call on FORMAT

if there were no pretty-printer-control FORMAT interface. This means that

there would be no benefit to using FORMAT anywhere in this example.)

The consequence of the above is that, even though pretty printer control

and FORMAT inherently have nothing to do with each other, adding pretty

printer control facilities without adding a pretty-printer-control FORMAT

interface, effectively requires people to choose between FORMAT and pretty

printer control. In order to allow people to separately decide whether

they do or do not want to user FORMAT and do or do not want to get involved

with controlling the pretty printer, you have to provide a

pretty-printer-control FORMAT interface.

This issue is intensified given a pretty printer such as XP that is

within epsilon of just as fast as the ordinary non-pretty printer,

because many people choose to set *PRINT-PRETTY* to T all of the time.

They are therefore naturally led to wanting to specify at least some

pretty printing control information whenever defining a print function

for a structure, or an error message, or in fact essentially any kind

of output. If there is no pretty-printer-control FORMAT interface,

then this would effectively bar the use of FORMAT for these users.

In summary, rather than pushing FORMAT into new territory, the

pretty-printer-control FORMAT interface is required to prevent FORMAT from

sliding into obsolescence.

(B) Although it probably is not convincing to those that do not like

FORMAT, a additional reason for having a pretty-printer-control FORMAT

interface is exactly the same as the reason for having FORMAT at all:

compactness. For example, as shown in the main proposal, the format string:

"~:<~W~^ ~:<~@{~:<~@{~W~^ ~_~}~:>~^ ~:_~}~:>~1I~@{~^ ~_~W~}~:>"

is the same as the expression:

(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)))))

It is of course also true that a line of FORMAT control string is a

lot harder to read than a line of Lisp code. However, FORMAT has

become a permanent fixture of Common Lisp, because a line of FORMAT

control string is a lot easier to read and work with than 20 lines of

Lisp code.

up made the FORMAT interface look a lot more complex than it is. Only six

things are being added. Two of these just fix problems with FORMAT and do

not have anything to do with pretty printing. Of the remaining four, three

are very simple. Only ~<...~:> could be called complex.

~A and ~S both force the value of *PRINT-ESCAPE*. ~W is needed in

order to write FORMAT strings that obey *PRINT-ESCAPE*.

~/.../ restores (in a simplified form) a feature of FORMAT that was

lost in the first version of Common Lisp. A number of people have

commented that this is something that they like, having nothing to do

with pretty printing. (Note that ~/PPRINT-LINEAR/, ~/PPRINT-FILL/,

and ~/PPRINT-TABULAR/ are not new directives, they are just a

consequence of the fact that ~/.../ can call the functions

PPRINT-LINEAR, PPRINT-FILL, and PPRINT-TABULAR.)

========================= ``Attached Document'' =========================

Proposal (PRETTY-PRINT-INTERFACE:XP):

Full description of XP accompanying version 5 of the proposal

--------------------

Pretty Printing

Richard C. Waters

Pretty printing has traditionally been a black box process, displaying

program code using a set of fixed layout rules. Its utility can be greatly

enhanced by opening it up to user control.

By providing direct access to the mechanisms within the pretty printer that

make dynamic decisions about layout, the macros and functions

PPRINT-LOGICAL-BLOCK, PPRINT-NEWLINE, and 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 function SET-PPRINT-DISPATCH makes it

possible to associate a user-defined pretty printing function with any type

of object. Together, these facilities enable users to redefine the way

code is displayed and allow the full power of pretty printing to be applied

to complex combinations of data structures.

Pretty Printing Control Variables

*PRINT-RIGHT-MARGIN* [variable]

A primary goal of pretty printing is to keep the output between a pair of

margins. The left margin is set at the column where the output begins. If

this cannot be determined, the left margin is set to zero.

When *PRINT-RIGHT-MARGIN* is not NIL, it specifies the right margin to use

when making layout decisions. When *PRINT-RIGHT-MARGIN* is NIL (the

initial value), the right margin is set at the maximum line length that can

be displayed by the output stream without wraparound or truncation. If

this cannot be determined, the right margin is set to an implementation

dependent value.

To allow for the possibility of variable-width fonts, *PRINT-RIGHT-MARGIN*

is interpreted in terms of ems---the length of an "m" in the font being

used to display characters on the relevant output stream at the moment when

the variables are consulted.

*PRINT-MISER-WIDTH* [variable]

If *PRINT-MISER-WIDTH* is not NIL, the pretty printer switches to a compact

style of output (called miser style) whenever the width available for

printing a substructure is less than or equal to *PRINT-MISER-WIDTH* ems.

The initial value of *PRINT-MISER-WIDTH* is implementation-dependent.

*PRINT-LINES* [variable]

When given a value other than its initial value of NIL, *PRINT-LINES*

limits the number of output lines produced when something is pretty

printed. If an attempt is made to go beyond *PRINT-LINES* lines, "

.." is printed at the end of the last line followed by all of the

suffixes (closing delimiters) that are pending to be printed.

(let ((*print-right-margin* 25) (*print-lines* 3))

(pprint '(progn (setq a 1 b 2 c 3 d 4))))

(PROGN (SETQ A 1

B 2

C 3 ..))

(The symbol ".." is printed out to ensure that a reader error will occur

if the output is later read. A symbol different than "..." is used to

indicate that a different kind of abbreviation has occurred.)

*PRINT-PPRINT-DISPATCH* [variable]

When *PRINT-PRETTY* is not NIL, printing is controlled by the `pprint

dispatch table' stored in the variable *PRINT-PPRINT-DISPATCH*. The

initial value of *PRINT-PPRINT-DISPATCH* is implementation dependent and

causes traditional pretty printing of Lisp code. The last section of this

proposal explains how the contents of this table can be changed.

The function WRITE accepts keyword arguments :PPRINT-DISPATCH,

:RIGHT-MARGIN, :LINES, and :MISER-WIDTH corresponding to

*PRINT-PPRINT-DISPATCH*, *PRINT-RIGHT-MARGIN*, *PRINT-LINES*, and

*PRINT-MISER-WIDTH*.

Dynamic Control of the Arrangement of Output

The following functions and macros support precise control of what should

be done when a piece of output is too large to fit in the space available.

Three concepts underlie the way these operations work---`logical blocks',

`conditional newlines', and `sections'. Before proceeding further, it is

important to define these terms.

The first line of Figure 1 shows a schematic piece of output. The

characters in the output are represented by "-"s. The positions of

conditional newlines are indicated by digits. The beginnings and ends of

logical blocks are indicated by "<" and ">" respectively.

The output as a whole is a logical block and the outermost section. This

section is indicated by the 0's on the second line of Figure 1. Logical

blocks nested within the output are specified by the macro

PPRINT-LOGICAL-BLOCK. Conditional newline positions are specified by calls

on 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 Figure 1,

the first conditional newline is immediately contained in the section

marked with 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.

<-1---<--<--2---3->--4-->->

000000000000000000000000000

11 111111111111111111111111

22 222

333 3333

44444444444444 44444

Figure 1: 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.

PPRINT-NEWLINE kind &OPTIONAL (stream *STANDARD-OUTPUT*) [Function]

STREAM (which defaults to *STANDARD-OUTPUT*) follows the standard

conventions for stream arguments to printing functions (i.e., NIL stands

for *STANDARD-OUTPUT* and T stands for *TERMINAL-IO*). The KIND argument

specifies the style of conditional newline. It must be one of :LINEAR,

:FILL, :MISER, or :MANDATORY. An error is signalled if any other value is

supplied. If STREAM is a pretty printing stream created by

PPRINT-LOGICAL-BLOCK, a line break is inserted in the output when the

appropriate condition below is satisfied. Otherwise, PPRINT-NEWLINE has no

effect. The value NIL is always returned.

If KIND is :LINEAR, it specifies a `linear-style' conditional newline. A

line break is inserted if and only if the immediately containing section

cannot be printed on one line. The effect of this is that line breaks are

either inserted at every linear-style conditional newline in a logical

block or at none of them.

If KIND is :MISER, it specifies a `miser-style' conditional newline. A

line break is inserted if and only if the immediately containing section

cannot be printed on one line and miser style is in effect in the

immediately containing logical block. The effect of this is that

miser-style conditional newlines act like linear-style conditional

newlines, but only when miser style is in effect. Miser style is in effect

for a logical block if and only if the starting position of the logical

block is less than or equal to *PRINT-MISER-WIDTH* from the right margin.

If KIND is :FILL, it specifies a `fill-style' conditional newline. A line

break is inserted if and only if either (a) the following section cannot be

printed on the end of the current line, (b) the preceding section was not

printed on a single line, or (c) the immediately containing section cannot

be printed on one line and miser style is in effect in the immediately

containing logical block. If a logical block is broken up into a number of

subsections by fill-style conditional newlines, the basic effect is that

the logical block is printed with as many subsections as possible on each

line. However, if miser style is in effect, fill-style conditional

newlines act like linear-style conditional newlines.

If KIND is :MANDATORY, it specifies a `mandatory-style' conditional

newline. A line break is always inserted. This implies that none of the

containing sections can be printed on a single line and will therefore

trigger the insertion of line breaks at linear-style conditional newlines

in these sections.

When a line break is inserted by any type of conditional newline, any

blanks that immediately precede the conditional newline are omitted from

the output and indentation is introduced at the beginning of the next line.

By default, the indentation causes the following line to begin in the same

horizontal position as the first character in the immediately containing

logical block. (The indentation can be changed via PPRINT-INDENT.)

There are a variety of ways unconditional newlines can be introduced into

the output (e.g., via TERPRI or by printing a string containing a newline

character). As with mandatory conditional newlines, this prevents any of

the containing sections from being printed on one line. In general, when

an unconditional newline is encountered, it is printed out without

suppression of the preceding blanks and without any indentation following

it. However, if a per-line prefix has been specified (see

PPRINT-LOGICAL-BLOCK), this prefix will always be printed no matter how a

newline originates.

PPRINT-LOGICAL-BLOCK (stream-symbol list [Macro]

&KEY :prefix :per-line-prefix :suffix)

&BODY body

This macro causes printing to be grouped into a logical block. The value

NIL is always returned.

STREAM-SYMBOL must be a symbol. If it is NIL, it is treated the same as

if it were *STANDARD-OUTPUT*. If it is T, it is treated the same as if

it were *TERMINAL-IO*. The run-time value of STREAM-SYMBOL must be a

stream (or NIL standing for *STANDARD-OUTPUT* or T standing for *TERMINAL-IO*).

The logical block is printed into this destination stream.

The BODY can contain any arbitrary Lisp forms. Within the BODY,

STREAM-SYMBOL (or *STANDARD-OUTPUT* if STREAM-SYMBOL is NIL, or

*TERMINAL-IO* if STREAM-SYMBOL is T) is bound to a `pretty printing' stream

that supports decisions about the arrangement of output and then forwards

the output to the destination stream. All the standard printing functions

(e.g., WRITE, PRINC, TERPRI) can be used to print output the pretty

printing stream created by PPRINT-LOGICAL-BLOCK. All and only the output

sent to this pretty printing stream is treated as being in the logical

block.

PPRINT-LOGICAL-BLOCK and the pretty printing stream it creates have dynamic

extent. It is undefined what happens if output is attempted outside of

this extent to the pretty printing stream created. It is unspecified what

happens if, within this extent, any output is sent directly to the

underlying destination stream.

The :SUFFIX, :PREFIX, and :PER-LINE-PREFIX must all be expressions that (at

run time) evaluate to strings. :SUFFIX (which defaults to the null string)

specifies a suffix that is printed just after the logical block. The

:PREFIX and :PRE-LINE-PREFIX arguments are mutually exclusive. If neither

:PREFIX or :PER-LINE-PREFIX is specified, a :PREFIX of the null string is

assumed. :PREFIX specifies a prefix to be printed before the beginning of

the logical block. :PER-LINE-PREFIX specifies a prefix that is printed

before the block and at the beginning of each new line in the block. An

error is signalled if :PREFIX and :PRE-LINE-PREFIX are both used or if the

:SUFFIX, :PREFIX, or :PER-LINE-PREFIX do not evaluate to strings.

LIST is interpreted as being a list that BODY is responsible for

printing. (See PPRINT-EXIT-IF-LIST-EXHAUSTED and PPRINT-POP.) If

LIST does not (at run time) evaluate to a list, it is printed using

WRITE. (This makes it easier to write printing functions that are

robust in the face of malformed arguments.) If *PRINT-CIRCLE* (and

possibly *PRINT-SHARED*) is not NIL and LIST is a circular (or shared)

reference to a cons, then an appropriate #n# marker is printed. (This

makes it easy to write printing functions that provide full support

for circularity and sharing abbreviation.) If *PRINT-LEVEL* is not

NIL and the logical block is at a dynamic nesting depth of greater

than *PRINT-LEVEL* in logical blocks, # is printed. (This makes easy

to write printing functions that provide full support for depth

abbreviation.)

If either of the three conditions above occurs, the indicated output is

printed on STREAM-SYMBOL and the BODY is skipped along with the printing of

the :PREFIX and :SUFFIX. (If the BODY is not responsible for printing a

list, then the first two tests above can be turned off by supplying NIL for

the LIST argument.)

In addition to the LIST argument of PPRINT-LOGICAL-BLOCK, the arguments of

the standard printing functions such as WRITE, PRINT, PPRINT, PRINT1, and

PPRINT, as well as the arguments of the standard FORMAT directives such as

~A, ~S, (and ~W) are all checked (when necessary) for circularity and

sharing. However, such checking is not applied to the arguments of the

functions WRITE-LINE, WRITE-STRING, and WRITE-CHAR or to the literal text

output by FORMAT. A consequence of this is that you must use one of the

latter functions if you want to print some literal text in the output that

is not supposed to be checked for circularity or sharing. (See the

examples below.)

----------------------------------------

Implementation note: detection of circularity and sharing is supported by

the pretty printer by in essence performing requested output twice.

On the first pass, circularities and sharing are detected and the

actual outputting of characters is suppressed. On the second pass, the

appropriate #n= and #n# markers are inserted and characters are output.

A consequence of this two-pass approach to the detection of circularity and

sharing is that the BODY of a PPRINT-LOGICAL-BLOCK must not perform any

side-effects on the surrounding environment. This includes not modifying

any variables that are bound outside of its scope. Obeying this

restriction is facilitated by using PPRINT-POP, instead of an ordinary POP

when traversing a list being printed by the BODY of a

PPRINT-LOGICAL-BLOCK.)

----------------------------------------

PPRINT-EXIT-IF-LIST-EXHAUSTED [Macro]

PPRINT-EXIT-IF-LIST-EXHAUSTED tests whether or not the LIST passed to

PPRINT-LOGICAL-BLOCK has been exhausted (see PPRINT-POP). If this list has

been reduced to NIL, PPRINT-EXIT-IF-LIST-EXHAUSTED terminates the execution

of the immediately containing PPRINT-LOGICAL-BLOCK except for the printing

of the suffix. Otherwise PPRINT-EXIT-IF-LIST-EXHAUSTED returns NIL. An

error message is issued if PPRINT-EXIT-IF-LIST-EXHAUSTED is used anywhere

other than syntactically nested within a call on PPRINT-LOGICAL-BLOCK. It

is undefined what happens if PPRINT-IF-LIST-EXHAUSTED is executed outside

of the dynamic extent of this PPRINT-LOGICAL-BLOCK.

PPRINT-POP [Macro]

PPRINT-POP pops elements one at a time off the LIST passed to

PPRINT-LOGICAL-BLOCK obeying *PRINT-LENGTH*, *PRINT-CIRCLE*, and

*PRINT-SHARED*. An error message is issued if it is used anywhere

other than syntactically nested within a call on PPRINT-LOGICAL-BLOCK.

It is undefined what happens if PPRINT-POP is executed outside of the

dynamic extent of this PPRINT-LOGICAL-BLOCK.

Each time PPRINT-POP is called, it pops the next value off the LIST

passed to PPRINT-LOGICAL-BLOCK and returns it. However, before doing

this, it performs three tests. If the remaining list is not a list

(i.e., a cons or NIL), ". " is printed followed by the remaining list.

(This makes it easier to write printing functions that are robust in

the face of malformed arguments.) If *PRINT-LENGTH* is NIL and

PPRINT-POP has already been called *PRINT-LENGTH* times within the

immediately containing logical block, "..." is printed. (This makes

it easy to write printing functions that properly handle

*PRINT-LENGTH*.) If *PRINT-CIRCLE* (and possibly *PRINT-SHARED*) is

not NIL, and the remaining list is a circular (or shared) reference,

then ". " is printed followed by an appropriate #n# marker. (This

catches instances of cdr circularity and sharing in lists.)

If either of the three conditions above occurs, the indicated output is

printed on the pretty printing stream created by the immediately containing

PPRINT-LOGICAL-BLOCK and the execution of the immediately containing

PPRINT-LOGICAL-BLOCK is terminated except for the printing of the suffix.

If PPRINT-LOGICAL-BLOCK is given a LIST argument of NIL---because it is not

processing a list---PPRINT-POP can still be used to obtain support for

*PRINT-LENGTH* (see the example function PPRINT-VECTOR below). In this

situation, the first and third tests above are disabled and

PPRINT-POP always returns NIL.

PPRINT-INDENT relative-to n &OPTIONAL (stream *STANDARD-OUTPUT*) [Function]

PPRINT-INDENT specifies the indentation to use in a logical block. STREAM

(which defaults to *STANDARD-OUTPUT*) follows the standard conventions for

stream arguments to printing functions. N specifies the indentation in

ems. If RELATIVE-TO is :BLOCK, the indentation is set to the horizontal

position of the first character in the block plus N ems. If RELATIVE-TO is

:CURRENT, the indentation is set to the current output position plus N ems.

(For robustness in the face of variable-width fonts, it is advisable to use

:CURRENT with an N of zero whenever possible.)

N can be negative; however, the total indentation cannot be moved left of

the beginning of the line or left of the end of the rightmost per-line

prefix. Changes in indentation caused by PPRINT-INDENT do not take

effect until after the next line break. In addition, in miser mode all

calls on PPRINT-INDENT are ignored, forcing the lines corresponding to the

logical block to line up under the first character in the block.

An error is signalled if a value other than :BLOCK or :CURRENT is supplied

for RELATIVE-TO. If STREAM is a pretty printing stream created by

PPRINT-LOGICAL-BLOCK, PPRINT-INDENT sets the indentation in the innermost

dynamically enclosing logical block. Otherwise, PPRINT-INDENT has no

effect. The value NIL is always returned.

PPRINT-TAB kind colnum colinc &OPTIONAL (stream *STANDARD-OUTPUT*) [function]

PPRINT-TAB specifies tabbing as performed by the standard FORMAT directive

~T. STREAM (which defaults to *STANDARD-OUTPUT*) follows the standard

conventions for stream arguments to printing functions. The arguments

COLNUM and COLINC correspond to the two parameters to ~T and are in terms

of ems. The KIND argument specifies the style of tabbing. It must be one

of :LINE (tab as by ~T) :SECTION (tab as by ~T, but measuring horizontal

positions relative to the start of the dynamically enclosing section),

:LINE-RELATIVE (tab as by ~@T), or :SECTION-RELATIVE (tab as by ~@T, but

measuring horizontal positions relative to the start of the dynamically

enclosing section). An error is signalled if any other value is supplied

for KIND. If STREAM is a pretty printing stream created by

PPRINT-LOGICAL-BLOCK, tabbing is performed. Otherwise, PPRINT-TAB has no

effect. The value NIL is always returned.

PPRINT-FILL STREAM LIST &OPTIONAL (COLON? T) ATSIGN? [function]

PPRINT-LINEAR STREAM LIST &OPTIONAL (COLON? T) ATSIGN? [function]

PPRINT-TABULAR STREAM LIST &OPTIONAL (COLON? T) ATSIGN? (TABSIZE 16) [function]

These three functions specify particular ways of pretty printing lists.

STREAM follows the standard conventions for stream arguments to printing

functions. Each function prints parentheses around the output if and only

if COLON? (default T) is not NIL. Each function ignores its ATSIGN?

argument and returns NIL. (These two arguments are included in this way so

that these functions can be used via ~/.../ and as SET-PPRINT-DISPATCH

functions as well as directly.) Each function handles abbreviation and the

detection of circularity and sharing correctly, and uses WRITE to print

LIST when given a non-list argument.

The function PPRINT-LINEAR prints a list either all on one line, or with

each element on a separate line. The function PPRINT-FILL prints a list

with as many elements as possible on each line. The function

PPRINT-TABULAR is the same as PPRINT-FILL except that it prints the

elements so that they line up in columns. This function takes an

additional argument TABSIZE (default 16) that specifies the column

spacing in ems.

---

As an example of the interaction of logical blocks, conditional newlines,

and indentation, consider the function SIMPLE-PPRINT-DEFUN below. This

function prints out lists whose cars are DEFUN in the standard way assuming

that the list has exactly length 4.

(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))))

Suppose that one evaluates the following:

(simple-pprint-defun *standard-output* '(defun prod (x y) (* x y)))

If the line width available is greater than or equal to 26, then all of the

output appears on one line. If the line width available is reduced to 25,

a line break is inserted at the linear-style conditional newline before the

expression (* X Y), producing the output shown. The (PPRINT-INDENT :BLOCK 1)

causes (* X Y) to be printed at a relative indentation of 1 in the logical block.

(DEFUN PROD (X Y)

(* X Y))

If the line width available is 15, a line break is also inserted at the

fill style conditional newline before the argument list. The call on

(PPRINT-INDENT :CURRENT 0) causes the argument list to line up under the

function name.

(DEFUN PROD

(X Y)

(* X Y))

If *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.

(DEFUN

PROD

(X Y)

(* X Y))

---

As an example of a per-line prefix, consider that evaluating the following

produces the output shown with a line width of 20 and *PRINT-MISER-WIDTH*

of NIL.

(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))

---

As a more complex (and realistic) example, consider the function PPRINT-LET

below. This specifies how to print a LET in the standard 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 symbol LET. The outermost

PPRINT-LOGICAL-BLOCK handles the printing of the input list as a whole and

specifies that parentheses should be printed in the output. The second

PPRINT-LOGICAL-BLOCK handles the list of binding pairs. Each pair in the list

is itself printed by the innermost PPRINT-LOGICAL-BLOCK. (A LOOP is used

instead of merely decomposing the pair into two elements 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 are placed after

each pair except the last. The loop at the end of the topmost

PPRINT-LOGICAL-BLOCK prints out the forms in the body of the LET separated by

spaces and linear-style conditional newlines.

(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)))))

Suppose that one evaluates the following with *PRINT-LEVEL* 4, and

*PRINT-CIRCLE* T.

(pprint-let *standard-output*

'#1=(let (x (*print-length* (f (g 3)))

(z . 2) (k (car y)))

(setq x (sqrt z)) #1#))

If the line length is greater than or equal to 77, the output produced

appears on one line. However, if the line length is 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 X is printed readably even though it fails to be a list; a

depth abbreviation marker is printed in place of (G 3); the binding pair

(Z . 2) is printed readably even though it is not a proper list; and

appropriate circularity markers are printed.

#1=(LET (X (*PRINT-LENGTH* (F #)) (Z . 2) (K (CAR Y)))

(SETQ X (SQRT Z))

#1#)

If the line length is reduced to 35, a line break is inserted at one of the

fill-style conditional newlines separating the binding pairs.

#1=(LET (X (*PRINT-PRETTY* (F #))

(Z . 2) (K (CAR Y)))

(SETQ X (SQRT Z))

#1#)

Suppose that the line length is further reduced to 22 and *PRINT-LENGTH* is

set to 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 prevents the binding pair (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.

(LET (X

(*PRINT-LENGTH*

(F #))

(Z . 2) ...)

(SETQ X (SQRT Z))

...)

---

The function PPRINT-TABULAR could be defined as follows.

(defun pprint-tabular (s list &optional (colon? T) atsign? (tabsize nil))

(declare (ignore atsign?))

(when (null tabsize) (setq tabsize 16))

(pprint-logical-block (s list :prefix (if colon? "(" "")

:suffix (if colon? ")" ""))

(pprint-exit-if-list-exhausted)

(loop (write (pprint-pop) :stream s)

(pprint-exit-if-list-exhausted)

(write-char #\space s)

(pprint-tab :section-relative 0 tabsize s)

(pprint-newline :fill s))))

Evaluating the following with a line length of 25 produces the output shown.

(princ "Roads ")

(pprint-tabular *standard-output* '(elm main maple center) nil nil 8)

Roads ELM MAIN

MAPLE CENTER

---

The function below prints a vector using #(...) notation.

(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))))))

Evaluating the following with a line length of 15 produces the output shown.

(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)

Format Directive Interface

The primary interface to operations for dynamically determining the

arrangement of output is provided through the functions above. However, an

additional interface is provided via a set of new format directives.

This is done, because as shown by the examples in this section and the

next, FORMAT strings are typically a much more compact way to specify

pretty printing. In addition, without such an interface, one would have to

abandon the use of FORMAT when interacting with the pretty printer.

~W [format directive]

WRITE -- An arg, any Lisp object, is printed obeying every printer control

variable (as by WRITE). In addition, ~W interacts correctly with depth

abbreviation, by not resetting the depth counter to zero. ~W does not

accept parameters. If given the colon modifier, ~W binds *PRINT-PRETTY* to

T. If given the atsign modifier, ~W binds *PRINT-LEVEL* and *PRINT-LENGTH*

to NIL.

~W provides automatic support for the detection of circularity and

sharing. If *PRINT-CIRCLE* (and possibly *PRINT-SHARED*) is not NIL

and ~W is applied to an argument that is a circular (or shared)

reference, an appropriate #n# marker is inserted in the output instead

of printing the argument.

~_ [format directive]

CONDITIONAL-NEWLINE -- Without any modifiers, ~_ is the same as

(PPRINT-NEWLINE :LINEAR). ~@_ is the same as (PPRINT-NEWLINE :MISER).

~:_ is the same as (PPRINT-NEWLINE :FILL). ~:@_ is the same as

(PPRINT-NEWLINE :MANDATORY).

~<...~:> [format directive]

LOGICAL BLOCK -- If ~:> is used to terminate a ~<...~>, the directive

is equivalent to a call on PPRINT-LOGICAL-BLOCK. The FORMAT argument

corresponding to the ~<...~:> directive is treated in the same way as

the LIST argument to PPRINT-LOGICAL-BLOCK, thereby providing automatic

support for non-list arguments and the detection of circularity,

sharing, and depth abbreviation. The portion of the FORMAT control

string nested within the ~<...~:> specifies the :PREFIX (or

:PER-LINE-PREFIX), :suffix}, and body of the PPRINT-LOGICAL-BLOCK.

The FORMAT string portion enclosed by ~<...~:> can be divided into

segments ~<prefix~;body~;suffix~:> by ~; directives. If the first

section is terminated by ~@;, it specifies a per-line prefix rather

than a simple prefix. The prefix and suffix cannot contain FORMAT

directives. An error is signalled if either the prefix or suffix

fails to be a constant string or if the enclosed portion is divided

into more than three segments.

If the enclosed portion is divided into only two segments, the suffix

defaults to the null string. If the enclosed portion consists of only

a single segment, both the prefix and the suffix default to the null

string. If the colon modifier is used (i.e., ~:<...~:>), the prefix

and suffix default to "(" and ")" (respectively) instead of the null

string.

The body segment can be any arbitrary FORMAT control string. This

FORMAT control string is applied to the elements of the list

corresponding to the ~<...~:> directive as a whole. Elements are

extracted from this list using PPRINT-POP, thereby providing automatic

support for malformed lists, and the detection of circularity,

sharing, and length abbreviation. Within the body segment, ~^ acts

like PPRINT-EXIT-IF-LIST-EXHAUSTED.

~<...~:> supports a feature not supported by PPRINT-LOGICAL-BLOCK. If

~:@> is used to terminate the directive (i.e., ~<...~:@>), then a

fill-style conditional newline is automatically inserted after each

group of blanks immediately contained in the body (except for blanks

after a ~<newline> directive). This makes it easy to achieve the

equivalent of paragraph filling.

If the atsign modifier is used with ~<...~:>, the entire remaining argument

list is passed to the directive as its argument. All of the remaining

arguments are always consumed by ~@<...~:>, even if they are not all used

by the FORMAT string nested in the directive. Other than the difference in

its argument, ~@<...~:> is exactly the same as ~<...~:> except that

circularity detection is not applied if ~@<...~:> is encountered at top

level in a {\cd format} string. This ensures that circularity detection is

applied only to data lists, not to {\cd format} argument lists.

" . #n#" is printed if circularity or sharing has

to be indicated for its argument as a whole.

To a considerable extent, the basic form of the directive ~<...~> is

incompatible with the dynamic control of the arrangement of output by

~W, ~_, ~<...~:>, ~I, and ~:T. As a result, an error is signalled if

any of these directives is nested within ~<...~>. Beyond this, an

error is also signalled if the ~<...~:;...~> form of ~<...~> is used

in the same FORMAT string with ~W, ~_, ~<...~:>, ~I, or ~:T.

~I [format directive]

INDENT -- ~nI is the same as (PPRINT-INDENT :BLOCK N).

~n:I is the same as (PPRINT-INDENT :CURRENT N). In both cases, N defaults

to zero, if it is omitted.

~:T [format directive]

TABULATE -- If the colon modifier is used with the ~T directive, the

tabbing computation is done relative to the horizontal position where the

section immediately containing the directive begins, rather than with

respect to a horizontal position of zero. The numerical parameters are

both interpreted as being in units of ems and both default to 1.

~n,m:T is the same as (PPRINT-TAB :SECTION N M).

~n,m:@T is the same as (PPRINT-TAB :SECTION-RELATIVE N M).

~/name/ [format directive]

CALL FUNCTION -- User defined functions can be called from within a FORMAT

string by using the directive ~/name/. The colon modifier, the atsign

modifier, and arbitrarily many parameters can be specified with the ~/name/

directive. NAME can be any arbitrary string that does not contain a "/".

All of the characters in NAME are treated as if they were upper case. If

NAME contains a ":" or "::", then everything up to but not including the

first ":" or "::" is taken to be a string that names a package. Everything

after the first ":" or "::" (if any) is taken to be a string that names a

symbol. The function corresponding to a ~/name/ directive is obtained by

looking up the symbol that has the indicated name in the indicated package.

If NAME does not contain a ":" or "::", then the whole name string is

looked up in the USER package.

When a ~/name/ directive is encountered, the indicated function is called

with four or more arguments. The first four arguments are: the output

stream, the FORMAT argument corresponding to the directive, the value T if

the colon modifier was used (NIL otherwise), and the value T if the atsign

modifier was used (NIL otherwise). The remaining arguments consist of any

parameters specified with the directive. The function should print the

argument appropriately. Any values returned by the function are ignored.

The three functions PPRINT-LINEAR, PPRINT-FILL, and PPRINT-TABULAR are

specifically designed so that they can be called by ~/.../ (i.e.,

~/PPRINT-LINEAR/, ~/PPRINT-FILL/, and ~/PPRINT-TABULAR/). In particular

they take colon and atsign arguments.

---

As examples of the convenience of specifying pretty printing with FORMAT

strings, consider that the first two functions used as examples in the last

section can be compactly defined as follows. The function PPRINT-VECTOR

cannot be defined using FORMAT, because the data structure it traverses is

not a list. The function PPRINT-TABULAR is inconvenient to define using

FORMAT, because of the need to pass its TABSIZE argument through to a ~:T

directive nested within an iteration over a list.

(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~^ ~:<~@{~:<~@{~W~^ ~_~}~:>~^ ~:_~}~:>~1I~@{~^ ~_~W~}~:>" list))

Compiling Format Control Strings

The control strings used by FORMAT are essentially programs that perform

printing. The macro FORMATTER provides the efficiency of using a compiled

function for printing without losing the compactness of FORMAT control

strings.

FORMATTER control-string [macro]

CONTROL-STRING must be a literal string. An error is signalled if

CONTROL-STRING is not a valid FORMAT control string. The macro FORMATTER

expands into an expression of the form (FUNCTION (LAMBDA (STREAM &REST

ARGS) ...)) that does the printing specified by CONTROL-STRING. The

LAMBDA created accepts an output stream as its first argument and zero or

more data values as its remaining arguments. The value returned by the

LAMBDA is the tail (if any) of the data values that are not printed out by

CONTROL-STRING. (E.g., if the CONTROL-STRING is "~A~A" the CDDR (if any)

of the data values is returned.)

For instance: (formatter "~%~2@{~S, ~}") is equivalent to

#'(lambda (stream &rest args)

(terpri stream)

(dotimes (n 2)

(if (null args) (return nil))

(prin1 (pop args) stream)

(write-string ", " stream))

args)

In support of the above, FORMAT is extended so that it accepts functions as

its second argument as well as strings. When a function is provided, it

must be a function of the form created by FORMATTER. The function is called

with the appropriate output stream as its first argument and the data

arguments to FORMAT as its remaining arguments. The function should

perform whatever output is necessary and return the unused tail of the

arguments (if any). The directives ~? and ~{~} with no body are also

extended so that they accept functions as well as control strings.

Every other standard function that takes a FORMAT string as an argument

(e.g., ERROR and WARN) are also extended so that they can accept functions

of the form above instead.

Pretty Print Dispatch Tables

When *PRINT-PRETTY* is not NIL, the pprint dispatch table in the variable

*PRINT-PPRINT-DISPATCH* controls how objects are printed. The information

in this table takes precedence over all other mechanisms for specifying how

to print objects. In particular, it overrides user-defined PRINT-OBJECT

methods and print functions for structures. However, if there is no

specification for how to pretty print a particular kind of object, it is then

printed using the standard mechanisms as if *PRINT-PRETTY* were NIL.

Pprint dispatch tables are mappings from keys to pairs of values. The keys

are type specifiers. The values are functions and numerical priorities.

Basic insertion and retrieval is done based on the keys with the equality

of keys being tested by EQUAL. The function to use when pretty printing an

object is chosen by finding the highest priority function from

*PRINT-PPRINT-DISPATCH* that is associated with a type specifier that

matches the object.

COPY-PPRINT-DISPATCH &optional (table *PRINT-PPRINT-DISPATCH*) [function]

A copy is made of TABLE, which defaults to the current pprint dispatch

table. If TABLE is NIL, a copy is returned of the initial value of

*PRINT-PPRINT-DISPATCH*.

PPRINT-DISPATCH object &optional (table *PRINT-PPRINT-DISPATCH*) [function]

This retrieves the highest priority function from a pprint table that is

associated with a type specifier in the table that matches OBJECT. The

function is chosen by finding all the type specifiers in TABLE that match

the object and selecting the highest priority function associated with any

of these type specifiers. If there is more than one highest priority

function, an arbitrary choice is made. If no type specifiers match the

object, a function is returned that prints object with *PRINT-PRETTY* bound

to NIL.

As a second return value, PPRINT-DISPATCH returns a flag that is T if a

matching type specifier was found in TABLE and NIL if not.

TABLE (which defaults to *PRINT-PPRINT-DISPATCH*) must be a pprint dispatch

table. TABLE can be NIL, in which case retrieval is done in the initial

value of *PRINT-PPRINT-DISPATCH*.

When *PRINT-PRETTY* is T, (WRITE OBJECT :STREAM S) is equivalent to

(FUNCALL (PPRINT-DISPATCH OBJECT) S OBJECT).

SET-PPRINT-DISPATCH type-specifier function [function]

&optional (priority 0) (table *PRINT-PPRINT-DISPATCH*)

This puts an entry into a pprint dispatch table and returns NIL.

TYPE-SPECIFIER must be a valid type specifier and is the key of the entry.

The first action of SET-PPRINT-DISPATCH is to remove any pre-existing entry

associated with TYPE-SPECIFIER. This guarantees that there will never be

two entries associated with the same type specifier in a given pprint

dispatch table. Equality of type specifiers is tested by EQUAL.

Two values are associated with each type specifier in a pprint dispatch

table: a function and a priority. FUNCTION must accept two arguments: the

stream to send output to and the object to be printed. FUNCTION should

pretty print the object on the stream. FUNCTION can assume that object

satisfies TYPE-SPECIFIER. Function must obey *PRINT-READABLY* (see issue

DATA-IO). Any values returned by FUNCTION are ignored.

PRIORITY (which defaults to 0) must be a non-complex number. This

number is used as a priority to resolve conflicts when an object

matches more than one entry. An error is signalled if priority fails

to be a non-complex number.

TABLE (which defaults to *PRINT-PPRINT-DISPATCH*) must be a pprint dispatch

table. The specified entry is placed in this table.

It is permissible for FUNCTION to be NIL. In this situation, there will be

no TYPE-SPECIFIER entry in TABLE after SET-PPRINT-DISPATCH is evaluated.

To facilitate the use of pprint dispatch tables for controlling the pretty

printing of Lisp code, the TYPE-SPECIFIER argument of the function

SET-PPRINT-DISPATCH is allowed to contain constructs of the form

(CONS car-type cdr-type)

This signifies that the corresponding object must be a cons cell whose car

matches the type specifier CAR-TYPE and whose cdr matches the type specifier

CDR-TYPE. The CDR-TYPE can be omitted in which case it defaults to T.

The initial value of *PRINT-PPRINT-DISPATCH* is implementation dependent.

However, the initial entries all use a special class of priorities that

have the property that they are less than every priority that can be

specified using SET-PPRINT-DISPATCH. The benefit of this is that it

guarantees that any pretty printing functions users specify will override

everything in the initial value of *PRINT-PPRINT-DISPATCH*.

----------------------------------------------------------------------

Implementation note: The restriction above is very useful to users

without actually limiting what Common Lisp implementors can do. It is

possible for implementors to set up any kind of pretty printing they

desire using the range of priorities available to them.

----------------------------------------------------------------------

Consider the following examples. The first form restores

*PRINT-PPRINT-DISPATCH* to its initial value. The next two forms then set

up a special way to pretty print ratios. Note that the more specific type

specifier has to be associated with a higher priority.

(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)))

(pprint '(1/3 -2/3)) prints: (#.(/ 1 3) #.(- (/ 2 3)))

The following two forms illustrate the definition of pretty printing

functions for types of Lisp code. The first form illustrates how to

specify the traditional method for printing quoted objects using "'"

syntax. Note the care taken to ensure that data lists that happen to begin

with QUOTE will be printed readably. The second form specifies that lists

beginning with the symbol MY-LET should print the same way that lists

beginning with LET print when the initial pprint dispatch table is in effect.

(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))

The next example specifies a default method for printing lists that do not

correspond to function calls. Note that, as shown in the definition of

PPRINT-TABULAR above, PPRINT-LINEAR, PPRINT-FILL, and PPRINT-TABULAR are

all defined with optional COLON? and ATSIGN? arguments so that they can be

used as pprint dispatch functions as well as ~/.../ functions.

(set-pprint-dispatch '(cons (not (and symbol (satisfies fboundp))))

#'pprint-fill -5)

with a line length of 9, (pprint '(0 b c d e f g h i j k)) prints:

(0 b c d

e f g h

i j k)

This final example shows how to define a pretty printing function for a

user defined data structure.

(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))))

The pretty printing function for the structure 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 *PRINT-LEVEL*, *PRINT-LENGTH*, *PRINT-LINES*,

*PRINT-CIRCLE*, *PRINT-SHARED* and *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 25,

*PRINT-PRETTY* T, *PRINT-ESCAPE* NIL, and a malformed KIDS list.

(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>)

Note that a pretty printing function for a structure is different from the

structure's print function. While print functions are permanently

associated with a structure, pretty printing functions are stored in 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 print dispatch table, the print function (if any) is used instead.