2.6. Object system¶

This chapter describes the object system.

2.6.1. Values¶

A value is a unit of data, such as a number, a function or a container of values. For example, each line of this example produces a value.

'Foo'
42
{(:Num) Num * 2 }

Value has a variable mapping, and optionally intrinsic properties.

Intrinsic properties of a value are any data contained in the value not via the variable mapping. The examples are the identity of a value, a character sequence of a string value, and bytes of a number value. Intrinsic properties cannot be accessed directly from Kink programs, but can be used by the runtime, functions implemented in the platform language (typically Java), and so on. Due to existence of intrinsic properties, values which have the same variable mapping may perform differently.

2.6.2. Variables¶

A value has a variable mapping, which is a finite partial mapping from strings to values.

A variable is a pair of a value and a string. If a variable V is a pair (O, S) where O is a value and S is a string, O is said to be the owner of the variable V, and S is said to be the symbol of the variable V.

For simplicity, a variable V whose symbol is S can be referred as “variable S,” when it is clear which value is the owner.

When a symbol S is defined in the domain of the variable mapping of a value O, O is said to have a variable (O, S), and the variable is said to be nonempty.

When a symbol S is mapped to a value T by the variable mapping of a value O, T is said to be the target of the variable (O, S).

From definitions above, a variable mapping can also be represented as a set of pairs of symbols and targets. For example, {(s₁, t₁), (s₂, t₂), ... (s_n, t_n)}, where a string s_i is mapped to a value t_i by the variable mapping.

2.6.3. Assignment¶

A value T_n can be assigned to a variable (O, S). Such action is called assignment.

Let O have a variable mapping M_o. After assignment, O gets to have a new variable mapping M_n, which can be different from M_o.

It must be distinguished whether S is defined in the domain of M_o.

When S is defined in the domain of M_o

Let S be mapped to a value T_o by M_o, and M_o be a mapping {(S, T_o), (s₁, t₁), (s₂, t₂), ... (s_n, t_n)}.

In this case, M_n is the mapping {(S, T_n), (s₁, t₁), (s₂, t₂), ... (s_n, t_n)}.

When S is not defined in the domain of M_o

Let M_o be a mapping {(s₁, t₁), (s₂, t₂), ... (s_n, t_n)}.

In this case, M_n is the mapping {(S, T_n), (s₁, t₁), (s₂, t₂), ... (s_n, t_n)}.

Example

The program below assigns numbers to variables of a value Point, and gets the targets from them.

# (1)
:Point <- new_val

# (2)
Point:X <- 10
Point:Y <- 20

stdout.print_line(
  [Point:X.nonempty? Point:Y.nonempty? Point:Z.nonempty?].repr
) # => [true true false]
stdout.print_line(
  [Point.X Point.Y].repr
) # => [10 20]

# (3)
Point:Y <- 200
Point:Z <- 300

stdout.print_line(
  [Point:X.nonempty? Point:Y.nonempty? Point:Z.nonempty?].repr
) # => [true true true]
stdout.print_line(
  [Point.X Point.Y Point.Z].repr
) # => [10 200 300]

On (1), a value is created by new_val function, and it is assigned to the local variable Point. The variable mapping of the created value is:

Symbol	Target
repr	a function which returns a string representation of the receiver.

Note that the value already has variable repr. See V.repr in kink/CORE for details.

On (2), the number 10 is assigned to the variable X, and 20 to Y. The variable mapping here is:

Symbol	Target
X	10
Y	20
repr	a function

On (3), the number 200 is assigned to the existing variable Y, and 300 to the new variable Z. The variable mapping here is:

Symbol	Target
X	10
Y	200
Z	300
repr	a function

2.6.4. Variable references¶

A variable reference, or a varref is a value which represents a variable. Operations such as assignment or existence checking are done through variable references.

Variable references can be typically created by local reference expressions (local_ref) or attributional reference expressions (attr_ref). For example, this expression creates a reference of a variable Data, whose owner is Owner.

Owner:Data

This expression creates a reference of a variable fun, whose owner is Owner.

Owner:fun