Context for styling
(if you haven't read the state section yet, read it; the context is started to be discussed there)
As we've already seen in states chapter, context is kind of object that stores the "layout instructions" of content that may be heavily dependent on outer states. These instructions are rendered later.
What is important to know is that the "outer states" I mention there include not just state (and counters, that are just special states for counting), but also styling.
What do I mean?
Well, see yourself:
Getting current style
Current font: #context text.font
We just got the current font that easily, and that works basically for any settable property! Isn't that neat?
See another example that demonstrates the properties of context better. Let's create a box that would be always the color of text:
#let colorful-rect = context box(stroke: text.fill, inset: 0.3em)[#repr(text.fill)]
Current color in box of the same color: #colorful-rect.
#set text(red)
Current color in box of the same color: #colorful-rect.
How to get things out of context?
That's the neat part, you don't!
Why? That's easy: for Typst the context block is a black box that can be opened only during rendering, when put inside the documents.
So if you want to get something, you should get it inside context.
Writing functions
Important fact: function, as any other content, may be context-depending without any declarations. And it is usually better to allow user to wrap them in context themselves instead of putting it in context.
Let's say you want to create a list that depends on some style (or maybe state) things. It would require context, so you can wrap it in context:
(Bad)
#let page-dimensions = context (page.width, page.height)
#page-dimensions, representation of that object is: #repr(page-dimensions)
That object would be almost useless. It's black box, so you can only put into the doc and that's all.
Hover, you can do this instead:
(Good!)
// To be context-dependent function it needs to be function, not just a fixed content
#let page-dimensions() = (page.width, page.height)
#context page-dimensions()
#context [
#let (x, y) = page-dimensions()
Half-width is: #(x/2), height is #y
]
So with context-dependent functions you allow user to put context anywhere they want.
Rules inside of context
As we've already discussed, context captures the outer state of the document, and doesn't see anything that happens inside it. So if you do
#context [
Text, color: #text.fill
#set text(blue)
Text, color: #text.fill
]
...right, the rules inside wouldn't affect style inside the context.