Fields and reflection

Elements naturally contain data, such as the fields specified for them, as well as their names, unique IDs (eid), counters, and so on.

You can retrieve this data using the dedicated data-extraction functions made available by Elembic. They are all based around e.data, the main function which returns a data dictionary for lots of different types. Here are some of the most useful functions and tasks:

Accessing fields

You can use e.fields(instance). Note that the returned dictionary will be incomplete if the element was just created. It is only complete in show rules, when set rules and default fields have been resolved.

#import "@preview/elembic:1.1.1" as e

#show: e.set_(elem, field-c: 10)

#let instance = elem("abc", field-a: 5, field-b: 6)

// Field information incomplete: set rules not yet resolved
#assert.eq(e.fields(instance), (pos-field: "abc", field-a: 5, field-b: 6))

#show: e.show_(elem, it => {
  // Field information is complete in show rules
  assert.eq(e.fields(it), (some-non-required-field: "default value", field-c: 10, pos-field: "abc", field-a: 5, field-b: 6))
})

#instance

Accessing element ID and constructor

You can use e.eid(instance) or e.eid(elem) to obtain the corresponding unique element ID. This is always the same for types produced from the same element.

Similarly, e.func(instance) will give you the constructor used to create this instance.

However, it is not recommended to compare e.func because note that the constructor may change between versions of a package without changing the element ID. That is, e.eid(a) == e.eid(b) might hold (they come from the same element), but e.func(a) == e.func(b) might not, if a came from package version 0.0.1 and b, from version 0.0.2.

Therefore, to check if two element instances belong to the same element, write e.eid(a) == e.eid(b).

Checking if elements are equal

To check if two element instances a and b are equal (same type of element and have the same fields), it is recommended to use e.eq:

#let my-elem-instance = elem(field: 5)

// AVOID (in packages): my-elem-instance == elem(field: 5)
#if e.eq(my-elem-instance, elem(field: 5)) {
  [They are equal!]
} else {
  [Nope, not equal]
}

Without e.eq, if the two elements come from different versions of the same package, a == b will be false, even if they have the same eid and fields.