Chapter 21

pconvert.ss: Converted Printing

This library defines routines for printing Scheme values as evaluatable S-expressions rather than readable S-expressions. The print-convert procedure does not print values; rather, it converts a Scheme value into another Scheme value such that the new value pretty-prints as a Scheme expression that evaluates to the original value. For example, (pretty-print (print-convert `(9 ,(box 5) #(6 7))) prints the literal expression (list 9 (box 5) (vector 6 7)) to the current output port.

To install print converting into the read-eval-print loop, require pconvert.ss and call the procedure install-converting-printer.

In addition to print-convert, this library provides print-convert, build-share, get-shared, and print-convert-expr. The last three are used to convert sub-expressions of a larger expression (potentially with shared structure).

(abbreviate-cons-as-list [abbreviate?])      PROCEDURE

Parameter that controls how lists are represented with constructor-style conversion. If the parameter's value is #t, lists are represented using list. Otherwise, lists are represented using cons. The initial value of the parameter is #t.

(booleans-as-true/false [use-name?])      PROCEDURE

Parameter that controls how #t and #f are represented. If the parameter's value is #t, then #t is represented as true and #f is represented as false. The initial value of the parameter is #t.

(use-named/undefined-handler [use-handler])      PROCEDURE

This parameter that controls how values that have inferred names are represented. This parameter is passed a value. If the parameter returns #t, the named/undefined-handler is invoked to render that value. Only values that have inferred names but are not defined at the top-level are used with this handler.

The initial value of the parameter is (lambda (x) #f).

(use-named/undefined-handler [use-handler])      PROCEDURE

This parameter that controls how values that have inferred names are represented. This parameter is only called if use-named/undefined-handler returned true for some value. This parameter is passed that same value and the result of the parameter is used as the representation for the value.

The initial value of the parameter is (lambda (x) #f).

(build-share v)      PROCEDURE

Takes a value and computes sharing information used for representing the value as an expression. The return value is an opaque structure that can be passed back into get-shared or print-convert-expr.

(constructor-style-printing [use-constructors?])      PROCEDURE

Parameter that controls how values are represented after conversion. If this parameter is #t, then constructors are used, e.g., pair containing 1 and 2 is represented as (cons 1 2). Otherwise, quasiquote-style syntax is used, e.g. the pair containing 1 and 2 is represented as `(1 . 2). The initial value of the parameter is #f.

See also quasi-read-style-printing.

(current-build-share-hook [hook])      PROCEDURE

Parameter that sets a procedure used by print-convert and build-share to assemble sharing information. The procedure hook takes three arguments: a value v, a procedure basic-share, and a procedure sub-share; the return value is ignored. The basic-share procedure takes v and performs the built-in sharing analysis, while the sub-share procedure takes a component of v ands analyzes it. These procedures return void; sharing information is accumulated as values are passed to basic-share and sub-share.

A current-build-share-hook procedure usually works together with a current-print-convert-hook procedure.

(current-build-share-name-hook [hook])      PROCEDURE

Parameter that sets a procedure used by print-convert and build-share to generate a new name for a shared value. The hook procedure takes a single value and returns a symbol for the value's name. If hook returns #f, a name is generated using the form ``-n-'' (where n is an integer).

(current-print-convert-hook [hook])      PROCEDURE

Parameter that sets a procedure used by print-convert and print-convert-expr to convert values. The procedure hook takes three arguments -- a value v, a procedure basic-convert, and a procedure sub-convert -- and returns the converted representation of v. The basic-convert procedure takes v and returns the default conversion, while the sub-convert procedure takes a component of v and returns its conversion.

A current-print-convert-hook procedure usually works together with a current-build-share-hook procedure.

(current-read-eval-convert-print-prompt [str])      PROCEDURE

Parameter that sets the prompt used by install-converting-printer. The initial value is "|- ".

(get-shared share-info [cycles-only?])      PROCEDURE

The shared-info value must be a result from build-share. The procedure returns a list matching variables to shared values within the value passed to build-share. For example,

might return the list

The default value for cycles-only? is #f; if it is not #f, get-shared returns only information about cycles.

(install-converting-printer)      PROCEDURE

Sets the current print handler to print values using print-convert. The current read handler is also set to use the prompt returned by current-read-eval-convert-print-prompt.

(print-convert v [cycles-only?])      PROCEDURE

Converts the value v. If cycles-only? is not #f, then only circular objects are included in the output. The default value of cycles-only? is the value of (show-sharing).

(print-convert-expr share-info v unroll-once?)      PROCEDURE

Converts the value v using sharing information share-info previously returned by build-share for a value containing v. If the most recent call to get-shared with share-info requested information only for cycles, then print-convert-expr will only display sharing among values for cycles, rather than showing all value sharing.

The unroll-once? argument is used if v is a shared value in share-info. In this case, if unroll-once? is #f, then the return value will be a shared-value identifier; otherwise, the returned value shows the internal structure of v (using shared value identifiers within v's immediate structure as appropriate).

(quasi-read-style-printing [on?])      PROCEDURE

Parameter that controls how vectors and boxes are represented after conversion when the value of constructor-style-printing is #f. If quasi-read-style-printing is set to #f, then boxes and vectors are unquoted and represented using constructors. For example, the list of a box containing the number 1 and a vector containing the number 1 is represented as `(,(box 1) ,(vector 1)). If the parameter is #t, then #& and #() are used, e.g., `(#&1 #(1)). The initial value of the parameter is #t.

(show-sharing [show?])      PROCEDURE

Parameter that determines whether sub-value sharing is conserved (and shown) in the converted output by default. The initial value of the parameter is #t.

(whole/fractional-exact-numbers [whole-frac?])      PROCEDURE

Parameter that controls how exact, non-integer numbers are converted when the numerator is greater than the denominator. If the parameter's value is #t, the number is converted to the form (+ integer fraction) (i.e., a list containing '+, an exact integer, and an exact rational less than 1 and greater than -1). The initial value of the parameter is #f.