On this page:
preferences: get
preferences: set
preferences: add-callback
preferences: set-default
preferences: set-un/ marshall
preferences: restore-defaults
preferences: register-save-callback
preferences: unregister-save-callback
exn: make-unknown-preference
exn: unknown-preference?
exn: struct: unknown-preference
preferences: low-level-put-preferences
preferences: low-level-get-preference
preferences: snapshot?
preferences: restore-prefs-snapshot
preferences: get-prefs-snapshot

27 Preferences, Textual

 (require framework/preferences)

(preferences:get symbol)  any/c
  symbol : symbol?

preferences:get returns the value for the preference symbol. It raises exn:unknown-preference if the preference’s default has not been set.
(preferences:set symbol value)  void?
  symbol : symbol?
  value : any/c
Sets the preference symbol to value. It should be called when the users requests a change to a preference.

preferences:set immediately writes the preference value to disk. It raises exn:unknown-preference if the preference’s default has not been set.

(preferences:add-callback p f [weak?])  (-> void?)
  p : symbol?
  f : 
(let ([procedure-with-arity2? (λ (x) (and (procedure? x) (procedure-arity-includes? x 2)))])
  weak? : boolean? = #f
This function adds a callback which is called with a symbol naming a preference and its value, when the preference changes. preferences:add-callback returns a thunk, which when invoked, removes the callback from this preference.

If weak? is true, the preferences system will only hold on to the callback weakly.

The callbacks will be called in the order in which they were added.

If you are adding a callback for a preference that requires marshalling and unmarshalling, you must set the marshalling and unmarshalling functions by calling preferences:set-un/marshall before adding a callback.

This function raises exn:unknown-preference exn:unknown-preference if the preference has not been set.
(preferences:set-default symbol 
  [#:aliases aliases 
  #:rewrite-aliases rewrite-aliases]) 
  symbol : symbol?
  value : any/c
  test : (any/c . -> . any)
  aliases : (listof symbol?) = '()
  rewrite-aliases : (listof (-> any/c any))
   = (map (lambda (x) values) aliases)
This function must be called every time your application starts up, before any call to preferences:get or preferences:set (for any given preference).

If you use preferences:set-un/marshall, you must call this function before calling it.

This sets the default value of the preference symbol to value. If the user has chosen a different setting, the user’s setting will take precedence over the default value.

The test argument is used as a safeguard. That function is called to determine if a preference read in from a file is a valid preference. If test returns #t, then the preference is treated as valid. If test returns #f then the default is used.

The aliases and rewrite-aliases arguments aids in renaming preferences. If aliases is present, it is expected to be a list of symbols that correspond to old versions of the preferences. It defaults to '(). If rewrite-aliases is present, it is used to adjust the old values of the preferences when they are present in the saved file.
(preferences:set-un/marshall symbol    
  unmarshall)  void?
  symbol : symbol?
  marshall : (any/c . -> . printable/c)
  unmarshall : (printable/c . -> . any/c)
preferences:set-un/marshall is used to specify marshalling and unmarshalling functions for the preference symbol. marshall will be called when the users saves their preferences to turn the preference value for symbol into a printable value. unmarshall will be called when the user’s preferences are read from the file to transform the printable value into its internal representation. If preferences:set-un/marshall is never called for a particular preference, the values of that preference are assumed to be printable.

If the unmarshalling function returns a value that does not meet the guard passed to preferences:set-default for this preference, the default value is used.

The marshall function might be called with any value returned from read and it must not raise an error (although it can return arbitrary results if it gets bad input). This might happen when the preferences file becomes corrupted, or is edited by hand.

preferences:set-un/marshall must be called before calling preferences:get,preferences:set.

See also serialize and deserialize.
(preferences:restore-defaults) restores the users’ configuration to the default preferences.
(preferences:register-save-callback callback)  symbol?
  callback : (-> boolean? any)
Registers callback to run twice for each call to preferences:setonce before the preferences file is written, with #t, and once after it is written, with #f. Registration returns a key for use with preferences:unregister-save-callback. Caveats:
  • The callback occurs on whichever thread happened to call preferences:set.

  • Pre- and post-write notifications are not necessarily paired; unregistration may cancel the post-write notification before it occurs.

Unregisters the save callback associated with key.
(exn:make-unknown-preference message 
  message : string?
  continuation-marks : continuation-mark-set?
Creates an unknown preference exception.
(exn:unknown-preference? exn)  boolean?
  exn : any/c
Determines if a value is an unknown preference exn.
The struct type for the unknown preference exn.
  ((listof symbol?) (listof any/c) . -> . any)
(preferences:low-level-put-preferences put-preferences)  void?
  put-preferences : ((listof symbol?) (listof any/c) . -> . any)
This parameter’s value is called to save preference the preferences file. Its interface should be just like mzlib’s put-preferences.

The default value calls put-preferences and, if there is an error, then starts using a hash-table to save the preferences instead. See also
  (->* (symbol?) [(-> any)] any)
(preferences:low-level-get-preference get-preference)  void?
  get-preference : (->* (symbol?) [(-> any)] any)
This parameter’s value is called to get a preference from the preferences file. Its interface should be just like get-preference.

The default value calls get-preferences and, if there is an error, then starts using a hash-table to save the preferences instead.
(preferences:snapshot? arg)  boolean?
  arg : any/c
Determines if its argument is a preferences snapshot.

Restores the preferences saved in snapshot.

Caches all of the current values of the preferences and returns them. For any preference that has marshalling and unmarshalling set (see preferences:set-un/marshall), the preference value is copied by passing it through the marshalling and unmarshalling process. Other values are not copied, but references to them are instead saved.

See also preferences:restore-prefs-snapshot.