12.8 Pretty Printing
In addition to the parameters defined in this section,
pretty-print conforms to the print-graph,
print-struct, print-hash-table,
print-vector-length, print-box, and
print-as-expression parameters.
The pretty printer detects structures that have the
prop:custom-write property and it calls the corresponding
custom-write procedure. The custom-write procedure can check the
parameter pretty-printing to cooperate with the
pretty-printer. Recursive printing to the port automatically uses
pretty printing, but if the structure has multiple recursively printed
sub-expressions, a custom-write procedure may need to cooperate more
to insert explicit newlines. Use port-next-location to
determine the current output column, use pretty-print-columns
to determine the target printing width, and use
pretty-print-newline to insert a newline (so that the
function in the pretty-print-print-line parameter can be
called appropriately). Use
make-tentative-pretty-print-output-port to obtain a port for
tentative recursive prints (e.g., to check the length of the output).
Like
pretty-print, except that it returns a string containing
the pretty-printed value, rather than sending the output to a port.
The optional argument columns argument is used to
parameterize pretty-print-columns.
Pretty-prints
v if
v is not
#<void>, or prints
nothing if
v is
#<void>. Pass this procedure to
current-print to install the pretty printer into the REPL run
by
read-eval-print-loop.
12.8.1 Basic Pretty-Print Options
A parameter that determines the default width for pretty printing.
If the display width is 'infinity, then pretty-printed output
is never broken into lines, and a newline is not added to the end of
the output.
Parameter that controls the default depth for recursive pretty
printing. Printing to depth means that elements nested more
deeply than depth are replaced with “...”; in particular, a
depth of 0 indicates that only simple values are printed. A depth of
#f (the default) allows printing to arbitrary
depths.
A parameter that determines how exact non-integers are printed. If
the parameter’s value is #t, then an exact non-integer with a
decimal representation is printed as a decimal number instead of a
fraction. The initial value is #f.
A parameter that controls the printing of the symbol whose print name
is just a period. If set to a true value, then such a symbol is
printed as only the period. If set to a false value, it is printed as
a period with vertical bars surrounding it.
A parameter that determines how inexact numbers are printed. If the
parameter’s value is #t, then inexact numbers are always
printed with a leading #i. The initial value is #f.
12.8.2 Per-Symbol Special Printing
A parameter that controls whether or not quote,
unquote, unquote-splicing, etc. are
abbreviated with ', ,, ,@, etc.
By default, the abbreviations are enabled.
See also pretty-print-remap-stylable.
Creates a new style table by extending an existing
style-table, so that the style mapping for each symbol of
like-symbol-list in the original table is used for the
corresponding symbol of symbol-list in the new table. The
symbol-list and like-symbol-list lists must have the
same length. The style-table argument can be #f, in
which case with default mappings are used for the original table (see
below).
The style mapping for a symbol controls the way that whitespace is
inserted when printing a list that starts with the symbol. In the
absence of any mapping, when a list is broken across multiple lines,
each element of the list is printed on its own line, each with the
same indentation.
The default style mapping includes mappings for the following symbols,
so that the output follows popular code-formatting rules:
| 'lambda 'case-lambda |
| 'define 'define-macro 'define-syntax |
| 'let 'letrec 'let* |
| 'let-syntax 'letrec-syntax |
| 'let-values 'letrec-values 'let*-values |
| 'let-syntaxes 'letrec-syntaxes |
| 'begin 'begin0 'do |
| 'if 'set! 'set!-values |
| 'unless 'when |
| 'cond 'case 'and 'or |
| 'module |
| 'syntax-rules 'syntax-case 'letrec-syntaxes+values |
| 'import 'export 'link |
| 'require 'require-for-syntax 'require-for-template 'provide |
| 'public 'private 'override 'rename 'inherit 'field 'init |
| 'shared 'send 'class 'instantiate 'make-object |
A parameter that controls remapping for styles and for the determination of
the reader shorthands.
This procedure is
called with each subexpression that appears as the first element in a
sequence. If it returns a symbol, the style table is used, as if that
symbol were at the head of the sequence. If it returns #f,
the style table is treated normally.
Similarly, when determining whether or not to abbreviate reader macros,
the parameter is consulted.
12.8.3 Line-Output Hook
Calls the procedure associated with the
pretty-print-print-line parameter to print a newline to
port, if
port is the output port that is redirected
to the original output port for printing, otherwise a plain newline is
printed to
port. The
width argument should be the
target column width, typically obtained from
pretty-print-columns.
A parameter that determines a procedure for printing the newline
separator between lines of a pretty-printed value. The procedure is
called with four arguments: a new line number, an output port, the old
line’s length, and the number of destination columns. The return value
from proc is the number of extra characters it printed at the
beginning of the new line.
The proc procedure is called before any characters are
printed with 0 as the line number and 0 as the old
line length; proc is called after the last character for a
value is printed with #f as the line number and with the
length of the last line. Whenever the pretty-printer starts a new
line, proc is called with the new line’s number (where the
first new line is numbered 1) and the just-finished line’s
length. The destination-columns argument to proc is always
the total width of the destination printing area, or
'infinity if pretty-printed values are not broken into lines.
The default proc procedure prints a newline whenever the line
number is not 0 and the column count is not
'infinity, always returning 0. A custom
proc procedure can be used to print extra text before each
line of pretty-printed output; the number of characters printed before
each line should be returned by proc so that the next line
break can be chosen correctly.
The destination port supplied to proc is generally not the
port supplied to pretty-print or pretty-display (or
the current output port), but output to this port is ultimately
redirected to the port supplied to pretty-print or
pretty-display.
12.8.4 Value Output Hook
A parameter that determines a sizing hook for pretty-printing.
The sizing hook is applied to each value to be printed. If the hook
returns #f, then printing is handled internally by the
pretty-printer. Otherwise, the value should be an integer specifying
the length of the printed value in characters; the print hook will be
called to actually print the value (see
pretty-print-print-hook).
The sizing hook receives three arguments. The first argument is the
value to print. The second argument is a Boolean: #t for
printing like display and #f for printing like
write. The third argument is the destination port; the port
is the one supplied to pretty-print or
pretty-display (or the current output port). The sizing hook
may be applied to a single value multiple times during
pretty-printing.
A parameter that determines a print hook for pretty-printing. The
print-hook procedure is applied to a value for printing when the
sizing hook (see
pretty-print-size-hook) returns an integer
size for the value.
The print hook receives three arguments. The first argument is the
value to print. The second argument is a boolean: #t for
printing like display and #f for printing like
write. The third argument is the destination port; this port
is generally not the port supplied to pretty-print or
pretty-display (or the current output port), but output to
this port is ultimately redirected to the port supplied to
pretty-print or pretty-display.
A parameter that determines a hook procedure to be called just before
an object is printed. The hook receives two arguments: the object and
the output port. The port is the one supplied to
pretty-print
or
pretty-display (or the current output port).
A parameter that determines a hook procedure to be called just after
an object is printed. The hook receives two arguments: the object and
the output port. The port is the one supplied to
pretty-print
or
pretty-display (or the current output port).
12.8.5 Additional Custom-Output Support
A parameter that is set to
#t when the pretty printer calls a
custom-write procedure (see
prop:custom-write) for output in
a mode that supports line breaks. When pretty printer calls a
custom-write procedure merely to detect cycles or to try to print on a
single line, it sets this parameter to
#f.
Produces an output port that is suitable for recursive pretty printing
without actually producing output. Use such a port to tentatively
print when proper output depends on the size of recursive
prints. After printing, determine the size of the tentative output
using
file-position.
The out argument should be a pretty-printing port, such as
the one supplied to a custom-write procedure when
pretty-printing is set to true, or another tentative output
port. The width argument should be a target column width,
usually obtained from pretty-print-columns, possibly
decremented to leave room for a terminator. The
overflow-thunk procedure is called if more than
width items are printed to the port; it can escape from the
recursive print through a continuation as a short cut, but
overflow-thunk can also return, in which case it is called
every time afterward that additional output is written to the port.
After tentative printing, either accept the result with
tentative-pretty-print-port-transfer or reject it with
tentative-pretty-print-port-cancel. Failure to accept or
cancel properly interferes with graph-structure printing, calls to
hook procedures, etc. Explicitly cancel the tentative print even when
overflow-thunk escapes from a recursive print.
Causes the data written to
tentative-out to be transferred as
if written to
orig-out. The
tentative-out argument
should be a port produced by
make-tentative-pretty-print-output-port, and
orig-out should be either a pretty-printing port (provided to
a custom-write procedure) or another tentative output port.
Cancels the content of
tentative-out, which was produced by
make-tentative-pretty-print-output-port. The main effect of
canceling is that graph-reference definitions are undone, so that a
future print of a graph-referenced object includes the defining
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