Data.AppSettings

A library to deal with application settings. This library deals with read-write application settings. You will have to specify the settings that your application uses, their name, types and default values. Setting types must implement the Read and Show typeclasses.

The settings are saved in a file in an INI-like key-value format (without sections).

Reading and updating settings is done in pure code, the IO monad is only used to load settings and save them to disk. It is advised for the user to create a module in your project holding settings handling.

You can then declare settings:

 fontSize :: Setting Double
 fontSize = Setting "fontSize" 14
 
 dateFormat :: Setting String
 dateFormat = Setting "dateFormat" "%x"
 
 backgroundColor :: Setting (Int, Int, Int)
 backgroundColor = Setting "backcolor" (255, 0, 0)

Optionally you can declare the list of all your settings:

 defaultConfig :: DefaultConfig
 defaultConfig = getDefaultConfig $ do
     setting fontSize
     setting dateFormat
     setting backgroundColor

If you do it, saveSettings will also save settings which have not been modified, which are still at their default value in the configuration file, in a commented form, as a documentation to the user who may open the configuration file. So for instance if you declare this default configuration and have set the font size to 16 but left the other settings untouched, the configuration file which will be saved will be:

 fontSize=16
 # dateFormat="%x"
 # backcolor=(255,0,0)

If you did not specify the list of settings, only the first line would be present in the configuration file.

Once we declared the settings, we can read the configuration from disk (and your settings module should export your wrapper around the function offered by this library):

 readResult <- try $ readSettings (AutoFromAppName "test")
 case readResult of
 	Right (conf, GetSetting getSetting) -> do
 		let textSize = getSetting textSizeFromWidth
 		saveSettings getDefaultConfig (AutoFromAppName "test") conf
 	Left (x :: SomeException) -> error "Error reading the config file!"

AutoFromAppName specifies where to save the configuration file. And we've already covered the getSetting in this snippet, see the readSettings documentation for further information.

Documentation

type Conf = Map String SettingInfo

The in-memory configuration data.

data DefaultConfig

Information about the default configuration. Contains all the settings (that you declare using getDefaultConfig) and their default values. It is useful when you save a configuration file, if you give this information to saveSettings, it will save default options in the configuration file in a commented form, as a form of documentation to a user who would edit the configuration file. However this is completely optional, you can give emptyDefaultConfig if you don't want this behaviour.

data Setting a

The type of a setting. It contains the setting name (key in the configuration file) and its default value.

It is advised to have a module in your project handling settings. In this module, you'd have all the settings declared at the toplevel, and exported. The rest of the application can then do

 getSetting <setting>
 setSetting <conf> <setting> <value>

and so on.

Constructors

Setting

Fields

name :: String

defaultValue :: a

newtype GetSetting

Constructors

GetSetting (forall a. Read a => Setting a -> a)

Instances

Show GetSetting

setting :: Show a => Setting a -> State Conf ()

see the getDefaultConfig documentation.

getDefaultConfig :: State Conf () -> DefaultConfig

Used in combination with setting to register settings. Registering settings is optional, see DefaultConfig.

 defaultSettings :: DefaultConfig
 defaultSettings = getDefaultConfig $ do
     setting <setting1>
     setting <setting2>

emptyDefaultConfig :: DefaultConfig

Default configuration containing no options. It's fine to give that to saveSettings if you don't want default settings being written to the configuration file in commented form (see DefaultConfig)

data FileLocation

Where to look for or store the configuration file.

Constructors

AutoFromAppName String

Automatically build the location based on the application name. It will be ~/.<app name>/config.ini.

Path FilePath

Absolute path to a location on disk.

readSettings :: FileLocation -> IO (Conf, GetSetting)

Read settings from disk. Because it is doing file I/O it is smart to wrap the call with a try, as I/O exceptions can be thrown. Also, the function will throw a ParseException if the file is not properly formatted. NOTE that if the file is properly formatted in general, but a value is stored in an invalid format (for instance "hello" for a Double), you will get no error and get the default value for that setting when you attempt to read it.

This function returns a pair. The first element is the configuration itself, which you can use to save back or modify the configuration. The second element is a function wrapped in the GetSetting newtype. This function allows you to read a configuration option simply by giving that option (without that callback you'd have to call getSetting settings <setting>, so the callback lets you save a parameter). There is no such shortcut for setSetting though, as it's normally used less often and in other contexts, it is probably OK to have that extra parameter for the setSetting.

Example of use:

 readResult <- try $ readSettings (Path "my.config")
 case readResult of
 	Right (conf, GetSetting getSetting) -> do
 		let textSize = getSetting textSizeFromWidth
 		saveSettings getDefaultConfig (Path "my.config") conf
 	Left (x :: SomeException) -> error "Error reading the config file!"

data ParseException

The configuration file is in an invalid format.

Instances

Show ParseException

Typeable ParseException

Exception ParseException

saveSettings :: DefaultConfig -> FileLocation -> Conf -> IO ()

It is advised to run the save within a try call because it does disk I/O, otherwise the call is straightforward.

setSetting :: Show a => Conf -> Setting a -> a -> Conf

Change the value of a setting. You'll have to call saveSettings so that the change is written to disk.