Attributes

In the simplest case, you can pass a map of attributes that you build yourself, such as when you want to tell draw-box to consume the space normally taken by two boxes:

The second argument to draw-box is an attribute expression. In this case we have told it that the :span attribute should have the value 2.

If you want to use one of the predefined sets of attributes, instead of a map you can pass the keyword that names them. For example, if you want to render some text in the same style normally used for drawing hexadecimal numbers, you can reference the attribute set named :hex as the second (attribute) argument to the text function:

When a keyword is found as an attribute expression, that keyword is looked up in the named-attributes table, and the corresponding value is used as the attributes to control the drawing.

In addition to the predefined values that start out in that table, you can add your own named attribute definitions by calling the defattrs function, perhaps in a shared include file.

Sometimes you want to combine multiple attribute specifications, perhaps starting with a named set of attributes and then augmenting or modifying it somehow. To express that you can pass a vector of attribute expressions, and each expression in turn will be evaluated into a map as described by this section, and the later maps will be merged into the earlier ones. The merge process allows later keys to replace earlier values, which is how you can override as well as adding on to the predefined attribute sets.

For example, here is an attribute expression that starts with the predefined :plain text attributes but establishes a lighter font weight:

And here is how to draw a box using the predefined :related (dotted) border styles, in addition to a purple background (assuming your diagram has called defattrs to set up the :bg-purple style), that takes up the space of four normal boxes:

If you don’t want to pass any attributes at all, you can either pass an empty map, {}, or nil which is interpreted in the same way. This tells the function to use its default values and behaviors. But most functions which accept an attribute parameter also allow you to call them with fewer parameters, and treat that as if you had passed nil for the attributes. You only need to pass the nil if you want to pass a value for parameter that comes later, but don’t care to send attributes.