Subject: Re: doc: Add documentation for define-record-type*

From: Skyler Ferris <[email-masked]>
Date: Thu, 28 Dec 2023 01:23:21 +0000

I wrote this documentation by using the docstring in guix/record.scm as a
starting point and expanding on it/reformatting on it to be appropriate
for inclusion in a manual.

* doc/guix.texi: Add sections describing the typical
usage and API for define-record-type*

Change-Id: I19e7220553d10652c794e6e0172b2c9ee961f54f
---
# It looks like when I sent the last message the spaces in the diff got
# converted to non-breaking spaces, which makes it difficult to read in
# some clients. This version should be easier to read. The notes from
# the previous message are preserved below:
#
# This version of the patch updates the commit message to explain that I
# wrote the documentation by using the docstring in guix/records.scm as
# a starting point. This process seemed appropriate to me because it is
# being incorporated into the same project for the same purpose. Most of
# the commit is new content, but some of it is shared with the docstring
# - for example the "thing" struct definition and the explanation of
# struct fields. I think that it is useful for the manual and the
# docstrings to share content when appropriate to avoid accidentally
# communicating contradictory ideas to different audiences, but the
# relationship should still be acknowledged. If there is a more formal
# manner in which I should indicate this please let me know and I will
# be happy to update the commit. This version does not change anything
# other than the commit message.
  doc/contributing.texi |   1 -
  doc/guix.texi         | 274 ++++++++++++++++++++++++++++++++++++++++++
  2 files changed, 274 insertions(+), 1 deletion(-)

diff --git a/doc/contributing.texi b/doc/contributing.texi
index 7337f4bd58..d920b2589a 100644
--- a/doc/contributing.texi
+++ b/doc/contributing.texi
@@ -1313,7 +1313,6 @@ Data Types and Pattern Matching
  notably the fact that it is hard to read, error-prone, and a hindrance
  to proper type error reports.

-@findex define-record-type*
  @findex match-record
  @cindex pattern matching
  Guix code should define appropriate data types (for instance, using
diff --git a/doc/guix.texi b/doc/guix.texi
index e61a893af9..3cb15eeca3 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -12606,6 +12606,280 @@ G-Expressions
  @code{(*approximate*)}, but this may change.
  @end deffn

+@node Records in Guix
+@section Records in Guix
+Guix uses @code{define-record-type*} to define structures with a lispy 
format.
+Packages, operating systems, etc are all defined with
+@code{define-record-type*} facilities. If one was using this facility to
+define preferences for a text editor, it might look like this:
+
+@lisp
+;; The only valid emulation modes are the symbol 'emacs, the symbol 
'vim, or
+;; the boolean #f. As a convenience to the user, if they pass in a string
+;; first convert it to a symbol and accept it if it is valid.
+(define (sanitize-emulation-mode value)
+  (let ((symbolized-value (cond ((not value)     #f)
+                                ((string? value) (string->symbol value))
+                                (#t              value))))
+    (unless (or (not symbolized-value)
+                (eq? symbolized-value 'emacs)
+                (eq? symbolized-value 'vim))
+      (throw 'bad-emulation-made
+             (format #f "Unrecognized emulation mode: ~s" value)))
+    symbolized-value))
+
+(define-record-type*
+  <editor-preferences> editor-preferences make-editor-preferences
+  editor-preferences? this-editor-preferences
+  (background-color editor-preferences-background-color
+                    (default "000000"))
+  (text-color       editor-preferences-text-color
+                    (default "FFFFFF"))
+  (emulation-mode   editor-preferences-emulation-mode
+                    (default #f)
+                    (sanitize sanitize-emulation-mode)))
+@end lisp
+
+A user could then define their preferences like this:
+
+@lisp
+(define my-preferences
+  (editor-preferences
+    (background-color "222222")
+    (emulation-mode   'vim)))
+@end lisp
+
+The value contained in @code{my-preferences} contains a custom
+@code{background-color} and @code{emulation-mode}, but keeps the default
+@code{text-color} (@code{"FFFFFF"}). If an invalid 
@code{emulation-mode} had
+been specified, for example if the user passed in @code{"vi"} instead of
+@code{"vim"}, @code{sanitize-emulation-mode} would immediately throw an 
error.
+
+The program can access values like this:
+
+@lisp
+(editor-preferences-background-color my-preferences)
+@result{} "222222"
+(editor-preferences-text-color my-preferences)
+@result{} "FFFFFF"
+(editor-preferences-emulation-mode my-preferences)
+@result{} 'vim
+@end lisp
+
+There is no way to define setters (all instances are immutable).
+
+@node Record Inheritance
+@subsection Record Inheritance
+It is also possible to inherit from previously defined instances when 
creating
+new ones. Continuing with the editor example, someone might want to 
base their
+preferences on their friend's preferences but customize a value:
+
+@lisp
+(define friends-preferences
+  (editor-preferences
+    (inherit my-preferences)
+    (emulation-mode 'emacs)))
+@end lisp
+
+This keeps the same @code{background-color} and @code{text-color} that are
+contained in @code{my-preferences} but changes the @code{emulation-mode} to
+be @code{'emacs} instead of @code{'vim}.
+
+Sometimes it does not make sense for a field to be inherited. Suppose 
that the
+@code{<editor-preferences>} type is updated to contain a username so that a
+friendly greeting can be displayed when the program starts up:
+
+@lisp
+;; Usernames must be strings. It would be strange to pass a username as a
+;; symbol, so throw an error in case the user meant to pass in a variable's
+;; value instead of a literal symbol.
+(define (sanitize-username value)
+  (unless (string? value)
+    (throw 'bad-username
+           (format #f "Usernames must be strings! Got: ~s" value)))
+  value)
+
+(define (sanitize-emulation-mode value)
+  (let ((symbolized-value (cond ((not value)     #f)
+                                ((string? value) (string->symbol value))
+                                (#t              value))))
+    (unless (or (not symbolized-value)
+                (eq? symbolized-value 'emacs)
+                (eq? symbolized-value 'vim))
+      (throw 'bad-emulation-made
+             (format #f "Unrecognized emulation mode: ~s" value)))
+    symbolized-value))
+
+(define-record-type*
+  <editor-preferences> editor-preferences make-editor-preferences
+  editor-preferences? this-editor-preferences
+  (username         editor-preferences-username
+                    (innate)
+                    (sanitize sanitize-username))
+  (background-color editor-preferences-background-color
+                    (default "000000"))
+  (text-color       editor-preferences-text-color
+                    (default "FFFFFF"))
+  (emulation-mode   editor-preferences-emulation-mode
+                    (default #f)
+                    (sanitize sanitize-emulation-mode)))
+@end lisp
+
+There are a couple of differences in the new @code{username} field 
compared to
+the fields we looked at earlier. It is marked as @code{innate}, which means
+that it will not be inherited. For example, consider what would happen 
if we
+tried to define new instances like this:
+
+@lisp
+(define my-preferences
+  (editor-preferences
+    (username         "my-username")
+    (background-color "222222")
+    (emulation-mode   'vim)))
+
+(define friends-preferences
+  (editor-preferences
+    (inherit my-preferences)
+    (emulation-mode 'emacs)))
+@end lisp
+
+While the @code{friends-preferences} instance still inherits the values for
+@code{background-color} and @code{text-color}, it will not inherit the 
value
+for @code{username}. Furthermore, as the @code{username} field does not 
define
+a default value the attempted creation of @code{friends-preferences} will
+actually throw an error. Instead, we could do this:
+
+@lisp
+(define my-preferences
+  (editor-preferences
+    (username         "my-username")
+    (background-color "222222")
+    (emulation-mode   'vim)))
+
+(define friends-preferences
+  (editor-preferences
+    (inherit        my-preferences)
+    (username       "friends-username")
+    (emulation-mode 'emacs)))
+@end lisp
+
+@node @code{define-record-type*} Reference
+@subsection @code{define-record-type*} Reference
+@defmac define-record-type* name syntactic-constructor constructor 
predicate this-identifier fields ...
+
+Define a new record type and associated helpers.
+
+@table @var
+@item name
+A symbol used to name the type, as would normally be provided to a plain
+@code{define-record-type} form. For example, @code{<package>}.
+
+@item syntactic-constructor
+A symbol that will be used to define the user-facing constructor. For 
example,
+the symbol @code{package} is the syntactic constructor for the 
@code{<package>}
+structure.
+
+@item constructor
+A symbol that will be used to define the traditional constructor. It is 
used in
+the implementation of the syntactic constructor, but will not typically 
be used
+elsewhere. The traditional @code{make-name} (for example, 
@code{make-package})
+is a fine value to use here.
+
+@item predicate
+A symbol that will be used to test if a value is an instance of this 
record.
+For example, @code{package?}.
+
+@item this-identifier
+This symbol can be used when defining fields that need to refer to the 
struct
+that contains them. For an example of this, see the @code{thunked} field
+property, below.
+
+@item fields
+A set of field specifiers which take the following form:
+
+@lisp
+(field-name field-getter properties ...)
+@end lisp
+
+Each of the properties must have one of the following forms:
+
+@table @code
+@item (default @var{value})
+Defines the default value for the field, if the user does not specify 
one using
+the syntactic constructor.
+
+@item (innate)
+Fields marked as innate will not be inherited from parent objects (see
+Instantiating Records, below, for details of object inheritance).
+
+@item (sanitize @var{proc})
+The value given by the user will be passed into @var{proc} before being 
stored
+in the object. For example, consider this struct definition:
+
+@lisp
+(define-record-type* <thing> thing make-thing
+  thing?
+  this-thing
+  (name  thing-name
+         (sanitize (lambda (value)
+                     (cond ((string? value) value)
+                           ((symbol? value) (symbol->string value))
+                           (else (throw 'bad! value)))))))
+@end lisp
+
+When creating @code{thing} instances either a string or a symbol can be
+supplied but it will always be stored as a string:
+
+@lisp
+(string? (thing-name (thing (name "some-name"))))
+@result{} #t
+(string? (thing-name (thing (name 'some-name))))
+@result{} #t
+(thing (name 1994))
+@result{} Throw to key `bad!' with args `(1994)'.
+@end lisp
+
+@item (thunked)
+Fields marked as @code{thunked} will actually compute the field's value 
in the
+current dynamic extent which is useful when referring to fluids in a 
field's
+value. Furthermore, that thunk can access the record it belongs to via the
+@code{this-identifier}. For example:
+
+@lisp
+(define-record-type* <rectangle> rectangle make-rectangle
+  rectangle?
+  this-rectangle
+  (width  rectangle-width)
+  (height rectangle-height)
+  (area rectangle-area (thunked)
+                       (default (* (rectangle-width this-rectangle)
+                                   (rectangle-height this-rectangle)))))
+
+(define base-rectangle
+  (rectangle
+    (width  2)
+    (height 4)))
+
+(define derived-rectangle
+  (rectangle
+    (inherit base)
+    (width   6)))
+
+(rectangle-area base-rectangle)
+@result{} 8
+
+(rectangle-area derived-rectangle
+@result{} 24
+@end lisp
+
+@item (delayed)
+Fields marked as @code{delayed} are similar to @code{thunked} fields, 
except
+that they are effectively wrapped in a @code{(delay @dots{})} form. 
Note that
+delayed fields cannot use @code{this-identifier}.
+@end table
+@end table
+@end defmac
+
  @node Invoking guix repl
  @section Invoking @command{guix repl}

-- 
2.41.0