The utilities.print-items Reference Manual

Table of Contents

The utilities.print-items Reference Manual

This is the utilities.print-items Reference Manual, version 0.2.0, generated automatically by Declt version 3.0 "Montgomery Scott" on Mon Apr 19 18:12:13 2021 GMT+0.

1 Introduction

#+TITLE:       utilities.print-items README
#+AUTHOR:      Jan Moringen
#+EMAIL:       jmoringe@techfak.uni-bielefeld.de
#+DESCRIPTION: Composable, unreadable printing of objects
#+KEYWORDS:    print-items, composable printing, print-object, utilities
#+LANGUAGE:    en

#+OPTIONS: num:nil

* Introduction

  The =utilities.print-items= system provides a protocol for flexible
  and composable printing, primarily unreadable printing.

  Why is this useful? Common Lisp has the generic function
  ~cl:print-object~, which is often used to print compact and
  intuitive yet unreadable (by ~cl:read~) representations of
  objects. Thanks to CLOS, considerable flexibility and composability
  would, in principle, be achievable. However, a common idiom is

  #+BEGIN_SRC lisp
    (defmethod print-object ((object CLASS) stream)
      (print-unreadable-object (stream :type t :id t)
        CODE))
  #+END_SRC

  which prevents the flexibility provided by CLOS from actually being
  used:

  + Calling the next method in a ~print-object~ method would either
    wrap the final result in multiple layers of
    ~print-unreadable-object~ or some of the methods would not be
    fully functional on their own.

  + Similarly, ~:before~ and ~:after~ methods on ~print-object~ would
    produce output parts outside of the prefix and suffix produced by
    ~print-unreadable-object~.

  #+ATTR_HTML: :alt "build status image" :title Build Status :align right
  [[https://travis-ci.org/scymtym/utilities.print-items][https://travis-ci.org/scymtym/utilities.print-items.svg]]

* Tutorial

  To illustrate the problem and the solution offered by the
  =utilities.print-items= system more concretely, consider the
  following example:

  #+BEGIN_SRC lisp :exports both :results silent
    (defclass name-mixin ()
      ((name :initarg :name :reader name)))

    (defclass value-mixin ()
      ((value :initarg :value :reader value)))

    (defclass binding (name-mixin value-mixin)
      ())
  #+END_SRC

** The Problem

   Due to the issues mentioned [[*Introduction][above]], ~print-object~-based solutions
   are not satisfactory:

   #+BEGIN_SRC lisp :exports both :results value
     (defmethod print-object ((object name-mixin) stream)
       (format stream "~S" (name object))
       (when (next-method-p)
         (call-next-method)))

     (defmethod print-object ((object value-mixin) stream)
       (format stream "= ~S" (value object))
       (when (next-method-p)
         (call-next-method)))

     (princ-to-string (make-instance 'binding :name "foo" :value 5))
   #+END_SRC

   #+RESULTS:
   : "foo"= 5#

   This is somewhat composable and the output has all expected parts,
   but the arrangement of the output parts is completely wrong as well
   as hard to control.

   #+BEGIN_SRC lisp :exports results :results silent
     (ignore-errors
      (remove-method #'print-object (find-method #'print-object '() (list (find-class 'name-mixin) (find-class 't)))))
     (ignore-errors
      (remove-method #'print-object (find-method #'print-object '() (list (find-class 'value-mixin) (find-class 't)))))
   #+END_SRC

   The opposite approach would be:

   #+BEGIN_SRC lisp :exports both :results value
     (defmethod print-object ((object binding) stream)
       (print-unreadable-object (object stream :type t :identity t)
         (format stream "~S = ~S" (name object) (value object))))

     (princ-to-string (make-instance 'binding :name "foo" :value 5))
   #+END_SRC

   #+RESULTS:
   : #

   This produces the expected result but is not composable at all
   since every user of ~name-mixin~ and ~value-mixin~ has to do all
   the printing itself.

   #+BEGIN_SRC lisp :exports results :results silent
     (ignore-errors
      (remove-method #'print-object (find-method #'print-object '() (list (find-class binding) (find-class 't)))))
   #+END_SRC

** The Solution

   When using the =utilities.print-items= system, ~print-object~
   methods are replaced by ~print-items:print-items~ methods (note the
   ~append~ method combination) for mixin classes:

   #+BEGIN_SRC lisp :exports both :results value
     (defclass name-mixin ()
       ((%name :initarg :name :reader name)))

     (defmethod print-items:print-items append ((object name-mixin))
       `((:name ,(name object) "~S")))

     (defclass value-mixin ()
       ((%value :initarg :value :reader value)))

     (defmethod print-items:print-items append ((object value-mixin))
       `((:value ,(value object) "= ~S")))

     (defclass binding (name-mixin value-mixin)
       ())

     (defmethod print-object ((object binding) stream)
       (print-unreadable-object (object stream :type t :identity t)
         (print-items:format-print-items stream (print-items:print-items object))))

     (princ-to-string (make-instance 'binding :name "foo" :value 5))
   #+END_SRC

   #+RESULTS:
   : #

   #+BEGIN_SRC lisp :exports results :results silent
     (ignore-errors
      (remove-method #'print-object (find-method #'print-object '() (list (find-class binding) (find-class 't)))))
   #+END_SRC

   This solves the problem of composability and getting all output
   parts between the prefix and suffix produced by
   ~print-unreadable-object~, but the arrangement of output parts is
   not ideal. We could improve the situation by tweaking the order of
   elements in the superclass list of ~binding~ but that would be
   intrusive and again not composable when, for example, subclasses of
   ~binding~ are defined. Furthermore, the ~print-object~ method does
   not do anything specific to ~binding~.

   The following adjustments solve both issues (changes in upper
   case):

   #+BEGIN_SRC lisp :exports both :results value
     (defmethod print-items:print-items append ((object value-mixin))
       `((:value ,(value object) " = ~S" ((:AFTER :NAME)))))

     (defclass binding (name-mixin value-mixin PRINT-ITEMS:PRINT-ITEMS-MIXIN)
       ())

     ;; no PRINT-OBJECT method for BINDING

     (princ-to-string (make-instance 'binding :name "foo" :value 5))
   #+END_SRC

   #+RESULTS:
   : #

   Constraints such as ~(:after :name)~ control the order of
   items. Constraints referring to absent items have no
   effect. Contradictory constraints cause an error to be signaled.

** Advanced Usage

   It is sometimes necessary to modify or suppress the print items
   produced for superclasses to get the desired printed
   representation. This can be achieved in two ways:

   1. By defining a ~print-items:print-items append~ method that
      returns replacements for the undesired items:

      #+BEGIN_SRC lisp :exports both :results value
        (defclass unnamed-binding (binding)
          ())

        (defmethod print-items:print-items append ((object unnamed-binding))
          `((:name nil "«unnamed»")))

        (princ-to-string (make-instance 'unnamed-binding :name nil :value 5))
      #+END_SRC

      #+RESULTS:
      : #

      #+BEGIN_SRC lisp :exports results :results silent
        (ignore-errors
         (remove-method #'print-items:print-items (find-method #'print-items:print-items '(append) (list (find-class 'unnamed-binding)))))
      #+END_SRC

   2. By defining a ~print-items:print-items :around~ method that
      explicitly modifies the complete item list:

      #+BEGIN_SRC lisp :exports both :results value
        (defclass unnamed-binding (binding)
          ())

        (defmethod print-items:print-items :around ((object unnamed-binding))
          (remove :name (call-next-method) :key #'first))

        (princ-to-string (make-instance 'unnamed-binding :name nil :value 5))
      #+END_SRC

      #+RESULTS:
      : #

      #+BEGIN_SRC lisp :exports results :results silent
        (ignore-errors
         (remove-method #'print-items:print-items (find-method #'print-items:print-items '(:around) (list (find-class 'unnamed-binding)))))
      #+END_SRC

* Reference

  The =utilities.print-items= system provides the following protocol
  for composable printing:

  * =print-items:print-items OBJECT [generic function]=

    Return a list of items that should appear in the printed
    representation of =OBJECT=.

    Each method should return a list of items of the form

    #+BEGIN_EXAMPLE
      (KEY VALUE [FORMAT [(CONSTRAINT*)]]
    #+END_EXAMPLE

    where

    #+BEGIN_EXAMPLE
      KEY        ::= any Lisp object
      VALUE      ::= any Lisp object
      FORMAT     ::= nil or a format string (Default is \"~A\")

      CONSTRAINT ::= (:before | :after) KEY
    #+END_EXAMPLE

    When multiple items have =cl:eql= =KEY= s, items appearing closer
    to the beginning of the item list take precedence. This mechanism
    can be used by subclasses to replace print items produced by
    superclasses.

    When =FORMAT= is =nil=, the whole item is ignored. This mechanism
    can be used by subclasses to disable print items produced by
    superclasses.

  * =print-items:print-items-mixin [class]=

    This mixin class adds printing via =print-items= to classes.

    Subclasses can define methods on =print-items:print-items= to
    change or extend the printed representation.

  * =print-items:format-print-items STREAM ITEMS &optional COLON? AT? [function]=

    This utility function prints items in the format constructed by
    the =print-items= functions to a stream.

    It is used to implement the =cl:print-object= method for
    =print-items-mixin=.

2 Systems

The main system appears first, followed by any subsystem dependency.

2.1 utilities.print-items

Maintainer

Jan Moringen <jmoringe@techfak.uni-bielefeld.de>

Author

Jan Moringen <jmoringe@techfak.uni-bielefeld.de>

License

LLGPLv3

Description

A protocol for flexible and composable printing.

Version

0.2.0

Dependency

alexandria

Source

utilities.print-items.asd (file)

Components

• src (module)
• readme.org (file)

3 Modules

Modules are listed depth-first from the system components tree.

3.1 utilities.print-items/src

Parent

utilities.print-items (system)

Location

src/

Components

• package.lisp (file)
• util.lisp (file)
• protocol.lisp (file)

4 Files

Files are sorted by type and then listed depth-first from the systems components trees.

4.1 Lisp

4.1.1 utilities.print-items.asd

Location

utilities.print-items.asd

Systems

utilities.print-items (system)

4.1.2 utilities.print-items/src/package.lisp

Parent

src (module)

Location

src/package.lisp

Packages

utilities.print-items

4.1.3 utilities.print-items/src/util.lisp

Dependency

package.lisp (file)

Parent

src (module)

Location

src/util.lisp

Internal Definitions

• cycle-error (condition)
• cycle-error-path (method)
• item-< (function)
• make-node (function)
• node (structure)
• node-edges (function)
• (setf node-edges) (function)
• node-object (function)
• node-state (function)
• (setf node-state) (function)
• sort-with-partial-order (function)
• topological-sort (function)

4.1.4 utilities.print-items/src/protocol.lisp

Dependency

util.lisp (file)

Parent

src (module)

Location

src/protocol.lisp

Exported Definitions

• format-print-items (function)
• print-items (generic function)
• print-items (method)
• print-items-mixin (class)

4.2 Static

4.2.1 utilities.print-items/README.org

Parent

utilities.print-items (system)

Location

README.org

5 Packages

Packages are listed by definition order.

5.1 utilities.print-items

Utilities for composable printing.

The interface consists of

* ‘print-items’ [generic function]

Methods extend the printed representation of an object in a composable manner.

* ‘print-items-mixin’ [class]

Can be used to make subclasses the print items mechanism when passed to ‘print-object’.

* ‘format-print-items’ [function]

Utility function for formatting print items onto a stream.

Source

package.lisp (file)

Nickname

print-items

Use List

• alexandria
• common-lisp

Exported Definitions

• format-print-items (function)
• print-items (generic function)
• print-items (method)
• print-items-mixin (class)

Internal Definitions

• cycle-error (condition)
• cycle-error-path (generic function)
• cycle-error-path (method)
• item-< (function)
• make-node (function)
• node (structure)
• node-edges (function)
• (setf node-edges) (function)
• node-object (function)
• node-state (function)
• (setf node-state) (function)
• sort-with-partial-order (function)
• topological-sort (function)

6 Definitions

Definitions are sorted by export status, category, package, and then by lexicographic order.

6.1 Exported definitions

6.1.1 Functions

Function: format-print-items STREAM ITEMS &optional COLON? AT?

Print ITEMS onto STREAM.

ITEMS is a list of items of the form

(KEY VALUE [FORMAT [(CONSTRAINT*)]]

where

KEY ::= any Lisp object
VALUE ::= any Lisp object
FORMAT ::= ‘nil’ or a format string (Default is "~A")

CONSTRAINT ::= (:before | :after) KEY

Package

utilities.print-items

Source

protocol.lisp (file)

6.1.2 Generic functions

Generic Function: print-items OBJECT

Return a list of items that should appear in the printed representation of OBJECT.

Each method should return a list of items of the form

(KEY VALUE [FORMAT [(CONSTRAINT*)]]

where

KEY ::= any Lisp object
VALUE ::= any Lisp object
FORMAT ::= ‘nil’ or a format string (Default is "~A")

CONSTRAINT ::= (:before | :after) KEY

When multiple items have ‘eql’ KEYs, items appearing closer to the beginning of the item list take precedence. This mechanism can be used by subclasses to replace print items produced by superclasses.

When FORMAT is ‘nil’ the whole item is ignored. This mechanism can be used by subclasses to disable print items produced by superclasses.

Package

utilities.print-items

Source

protocol.lisp (file)

Method Combination

append (short method combination)

Options: :most-specific-first

Methods

Method: print-items OBJECT append

6.1.3 Classes

Class: print-items-mixin ()

This mixin class adds printing via ‘print-items’ to classes.

Package

utilities.print-items

Source

protocol.lisp (file)

Direct superclasses

standard-object (class)

Direct methods

print-object (method)

6.2 Internal definitions

6.2.1 Functions

Function: item-< LEFT RIGHT

Return non-nil if the left item should be placed before the right.

Package

utilities.print-items

Source

util.lisp (file)

Function: make-node OBJECT

Package

utilities.print-items

Source

util.lisp (file)

Function: node-edges INSTANCE

Function: (setf node-edges) VALUE INSTANCE

Package

utilities.print-items

Source

util.lisp (file)

Function: node-object INSTANCE

Package

utilities.print-items

Source

util.lisp (file)

Function: node-state INSTANCE

Function: (setf node-state) VALUE INSTANCE

Package

utilities.print-items

Source

util.lisp (file)

Function: sort-with-partial-order SEQUENCE PREDICATE

Package

utilities.print-items

Source

util.lisp (file)

Function: topological-sort NODES

Package

utilities.print-items

Source

util.lisp (file)

6.2.2 Generic functions

Generic Function: cycle-error-path CONDITION

Package

utilities.print-items

Methods

Method: cycle-error-path (CONDITION cycle-error)

Source

util.lisp (file)

6.2.3 Conditions

Condition: cycle-error ()

Package

utilities.print-items

Source

util.lisp (file)

Direct superclasses

error (condition)

Direct methods

cycle-error-path (method)

Direct slots

Slot: %path

Initargs

:path

Readers

cycle-error-path (generic function)

Direct Default Initargs

Initarg	Value
:path	(error "missing required initarg for class ~s: ~s" (quote utilities.print-items::cycle-error) :path)

6.2.4 Structures

Structure: node ()

Package

utilities.print-items

Source

util.lisp (file)

Direct superclasses

structure-object (structure)

Direct slots

Slot: object

Readers

node-object (function)

Writers

(setf node-object) (function)

Slot: edges

Type

list

Initform

(quote nil)

Readers

node-edges (function)

Writers

(setf node-edges) (function)

Slot: state

Type

(member :new :in-progress :done)

Initform

:new

Readers

node-state (function)

Writers

(setf node-state) (function)

Appendix A Indexes

A.1 Concepts

A.2 Functions

A.3 Variables

A.4 Data types