Literals

Literals in Selector Header

Declarations of Literals

Unlike in the substance program, within a Style selector block, we can declare objects of literal types. Here is an example:

style

forall Number n {}
forall String s {}

substance

P(1)
P(-1)
Q("Hello")
Q("World")

domain

predicate P(Number)
predicate Q(String)

Here, the first Style block (forall Number n {}) will be invoked twice with n -> 1 and n -> -1, and the second Style block (forall String s {}) will be invoked twice with s -> "Hello" and s -> "World.

Uses of Literals

If an argument of the predicate expects an object of literal type, then for convenience, in the selector, the style variable for that argument does not need to be explicitly declared (though it can). In this concrete example,

style

forall Set s
where Has(s, n) {}

domain

type Set
predicate Has(Set, Number)

the Style variable n is automatically inferred to have type Number since per Domain, the second parameter of predicate Has expects a Number type. Hence, explicitly declaring Number n is not necessary in the Style header.

Furthermore, literal expressions can also appear directly in Style selectors. A literal expression in style selector will only match the same literal expression in Substance. For example,

style

forall Set s
where Has(s, 1) {}

substance

Set s1, s2
Has(s1, 1)
Has(s2, 2)
Has(s3, 1)

domain

type Set
predicate Has(Set, Number)

The style selector block will only match twice, since only s = s1 and s = s3 satisfy the relation Has(s, 1).

Literals in Selector Body

If a style variable x refers to some Substance literal, then within the Style block, accessing x directly gives the value of the Substance literal.

For example, consider the following set of programs:

style

forall Number n
where P(n) {
    Circle {
        r: n
    }
}
forall String s
where Q(s) {
    Text {
        string: s
    }
}

substance

P(123)
P(456)
Q("Hello")
Q("World")

domain

predicate P(Number)
predicate Q(String)

the above program will render the following shapes:

• a circle of radius 123 (for mapping n -> 123) and a circle of radius 2 (for mapping n -> 456)
• a text of string "Hello" (for mapping s -> "Hello") and a text of string "World" (for mapping s -> "World").

If a variable xs is a collection of substance literals (for now, only literal numbers), then accessing xs gives a vector of the corresponding values, without requiring a Collection Access expression.

If x refers to a Substance literal, then nameof x just returns a string representation of that literal.