On this page:
pretty-print
pretty-display
pretty-format
pretty-print-handler
12.8.1 Basic Pretty-Print Options
pretty-print-columns
pretty-print-depth
pretty-print-exact-as-decimal
pretty-print-.-symbol-without-bars
pretty-print-show-inexactness
12.8.2 Per-Symbol Special Printing
pretty-print-abbreviate-read-macros
pretty-print-style-table?
pretty-print-current-style-table
pretty-print-extend-style-table
pretty-print-remap-stylable
12.8.3 Line-Output Hook
pretty-print-newline
pretty-print-print-line
12.8.4 Value Output Hook
pretty-print-size-hook
pretty-print-print-hook
pretty-print-pre-print-hook
pretty-print-post-print-hook
12.8.5 Additional Custom-Output Support
pretty-printing
make-tentative-pretty-print-output-port
tentative-pretty-print-port-transfer
tentative-pretty-print-port-cancel
Version: 4.2.1

12.8 Pretty Printing

The bindings documented in this section are provided by the scheme/pretty and scheme libraries, but not scheme/base.

(pretty-print v [port])  void?
  v : any/c
  port : output-port? = (current-output-port)
Pretty-prints the value v using the same printed form as write, but with newlines and whitespace inserted to avoid lines longer than (pretty-print-columns), as controlled by (pretty-print-current-style-table). The printed form ends in a newline, unless the pretty-print-columns parameter is set to 'infinity.

In addition to the parameters defined in this section, pretty-print conforms to the print-graph, print-struct, print-hash-table, print-vector-length, and print-box parameters.

The pretty printer also 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).

(pretty-display v [port])  void?
  v : any/c
  port : output-port? = (current-output-port)
Same as pretty-print, but v is printed like display instead of like write.

(pretty-format v [columns])  string?
  v : any/c
  columns : exact-nonnegative-integer? = (pretty-print-columns)
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-print-handler v)  void?
  v : any/c
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

(pretty-print-columns)
  (or/c exact-positive-integer? 'infinity)
(pretty-print-columns width)  void?
  width : (or/c exact-positive-integer? 'infinity)
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.

(pretty-print-exact-as-decimal)  boolean?
(pretty-print-exact-as-decimal as-decimal?)  void?
  as-decimal? : any/c
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.

Returns #t if v is a style table for use with pretty-print-current-style-table, #f otherwise.

A parameter that holds a table of style mappings. See pretty-print-extend-style-table.

(pretty-print-extend-style-table style-table 
  symbol-list 
  like-symbol-list) 
  pretty-print-style-table?
  style-table : pretty-print-style-table?
  symbol-list : (listof symbol?)
  like-symbol-list : (listof symbol?)
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

(pretty-print-remap-stylable)
  (any/c . -> . (or/c symbol? #f))
(pretty-print-remap-stylable proc)  void?
  proc : (any/c . -> . (or/c symbol? #f))
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

(pretty-print-newline port width)  void?
  port : out-port?
  width : exact-nonnegative-integer?
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

(pretty-printing)  boolean?
(pretty-printing on?)  void?
  on? : any/c
A parameter that is set to #t when the pretty printer calls a custom-write procedure (see prop:custom-write) for output. When pretty printer calls a custom-write procedure merely to detect cycles, it sets this parameter to #f.

(make-tentative-pretty-print-output-port out 
  width 
  overflow-thunk) 
  output-port?
  out : output-port?
  width : exact-nonnegative-integer?
  overflow-thunk : (-> any)
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.

(tentative-pretty-print-port-transfer tentative-out    
  orig-out)  void?
  tentative-out : output-port?
  orig-out : output-port?
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.

(tentative-pretty-print-port-cancel tentative-out)  void?
  tentative-out : 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 #n=.