Basic attributes interface

Attributes of an object can be get, set and constructed. For types descending from GObject, properties are encoded in attributes, although attributes are slightly more general (every property of a GObject is an attribute, but we can also have attributes for types not descending from GObject).

If you're wondering what the possible attributes of a GObject are, look at the list of properties in the documentation, e.g. the Properties heading of the docs for Button lists properties such as image and relief. Parent classes may also introduce properties, so since a Button is an instance of IsActionable, it inherits properties like actionName from Actionable too.

As an example consider a button widget and a property (of the Button class, or any of its parent classes or implemented interfaces) called "label". The simplest way of getting the value of the button is to do

value <- getButtonLabel button

And for setting:

setButtonLabel button label

This mechanism quickly becomes rather cumbersome, for example for setting the "window" property in a DOMDOMWindow in WebKit:

win <- getDOMDOMWindowWindow dom

and perhaps more importantly, one needs to chase down the type which introduces the property:

setWidgetSensitive button False

There is no setButtonSensitive, since it is the Widget type that introduces the "sensitive" property.

Overloaded attributes

A much more convenient overloaded attribute resolution API is provided by this module. Getting the value of an object's attribute is straightforward:

value <- get button _label

The definition of _label is basically a Proxy encoding the name of the attribute to get:

_label = fromLabelProxy (Proxy :: Proxy "label")

These proxies can be automatically generated by invoking the code generator with the -l option. The leading underscore is simply so the autogenerated identifiers do not pollute the namespace, but if this is not a concern the autogenerated names (in the autogenerated GI/Properties.hs) can be edited as one wishes.

In addition, for ghc >= 8.0, one can directly use the overloaded labels provided by GHC itself. Using the OverloadedLabels extension, the code above can also be written as

value <- get button #label

The syntax for setting or updating an attribute is only slightly more complex. At the simplest level it is just:

set button [ _label := value ]

or for the WebKit example above

set dom [_window := win]

However as the list notation would indicate, you can set or update multiple attributes of the same object in one go:

set button [ _label := value, _sensitive := False ]

You are not limited to setting the value of an attribute, you can also apply an update function to an attribute's value. That is the function receives the current value of the attribute and returns the new value.

set spinButton [ _value :~ (+1) ]

There are other variants of these operators, see AttrOp below. :=> and :~> are like := and :~ but operate in the IO monad rather than being pure.

Attributes can also be set during construction of a GObject using new

button <- new Button [_label := "Can't touch this!", _sensitive := False]

In addition for value being set/get having to have the right type, there can be attributes that are read-only, or that can only be set during construction with new, but cannot be set afterwards. That these invariants hold is also checked during compile time.

Nullable atributes

Whenever the attribute is represented as a pointer in the C side, it is often the case that the underlying C representation admits or returns NULL as a valid value for the property. In these cases the get operation may return a Maybe value, with Nothing representing the NULL pointer value (notable exceptions are GList and GSList, for which NULL is represented simply as he empty list). This can be overriden in the introspection data, since sometimes attributes are non-nullable, even if the type would allow for NULL.

For convenience, in nullable cases the set operation will by default not take a Maybe value, but rather assume that the caller wants to set a non-NULL value. If setting a NULL value is desired, use clear as follows

clear object _propName