Derived attributes

Read-only values a construct computes while it renders — what publishes them, and why they cannot be substituted.

#What a derived attribute is

Some constructs compute something as they render — a fit's slope, an integral's area, the value a reader has dragged a parameter to. Those results are published as derived attributes: read-only, recomputed every frame, and never written by the author. They are listed in their own section on the reference page of any construct that has them.

#Reading one

By member access on the construct's id. In attribute position the binding is written directly; in prose it is wrapped in !value, because a reference written bare in a body compiles as literal text.

text(x: 0; y: 4; label: slope = #f.slope)     in an attribute — a binding
The fitted slope is !value:#f.slope(digits: 2). in prose — a micronode

What publishes them today:

fit — slope, intercept, r2. The slope is the fitted polynomial's derivative at zero: exact, and live under whichever degree the reader has chosen.
curve — max and min over the plotted domain.
integral — value, the computed area.
parameter — value, wherever the reader has left the slider.
contour and heatmap — min and max over the visible window.

#Why they refuse substitution

{{#fit.slope}} is a compile error, and deliberately so. A substitution is a snapshot spliced into the source before parsing; a derived value is whatever the render currently computes. Freezing one into the document would let the prose assert a number the picture beside it disagrees with — which is the exact failure this machinery exists to prevent.

The same rule covers animated attributes and reader-set parameters. If a value can change after compilation, it must be reached by a binding. Identifiers covers the distinction in full, and Honest numbers is the authoring rule that follows from it.

#What publishes them

The construct itself, from inside the renderer, once per frame. Nothing an author writes can add a derived attribute to a construct that does not declare one, and nothing can set one — an attempt to assign #f.slope is an error, not a silent override. Existence is still proven at compile time: a reference to a derived name the construct does not publish fails before anything renders.