6.38 Listables

8.12

6.38 Listables🔗ℹ

A listable value can be converted to a list using a to_list operation such as Listable.to_list. Lists are themselves listable, where List.to_list just returns the same list. Pair lists, arrays, and instances of classes that implemement Listable are also listable.

List-splicing contexts generally allow listables, including in the construction of a list. As a result, if expr produces a listable, another way to convert it to a list is [& expr], while PairList[& expr] would convert it to a pair list.

Typically, a listable object is also sequence and directly indexable, but listable does not imply indexable or working as a sequence.

interface

interface Listable

An interface that a class can implement (publicly or privately) to make instances of the class convertable to a list. As an annotation, Listable matches all listable objects, not just instances of classes that publicly implement the Listable interface.

The interface has a single abstract method:

to_list() — produces a list with the same elements as the object in the same order.

class Posn(x, y):
  private implements Listable
  private override method to_list():
    [x, y]

> def l :: Listable = Posn(1, 2)
> l.to_list()
[1, 2]

function

fun Listable.to_list(v :: Listable) :: List

Converts any listable value to a list.

> Listable.to_list([1, 2, 3])
[1, 2, 3]
> Listable.to_list(PairList[1, 2, 3])
[1, 2, 3]
> Listable.to_list(Array(1, 2, 3))
[1, 2, 3]

annotation

Listable.to_list

A converter annotation that is satified by any listable value and converts it to a List.

> def sizes :: Listable.to_list = Array("small", "medium", "large")
> sizes
["small", "medium", "large"]

fun avg(ns :: (Listable.to_list && NonemptyList.of(Number))):
math.sum(& ns) / ns.length()

> avg([1, 2, 3])
2
> avg(Array(10, 20, 30, 40))
25
> avg(Array("small", "medium", "large"))
avg: argument does not satisfy annotation
argument: Array("small", "medium", "large")
annotation: (Listable.to_list && NonemptyList.of(Number))

1	Rhombus Essentials
2	Collections and Iteration
3	Syntax Objects and Macros
4	Classes and Interfaces
5	Static Information and Binding
6	Core Reference
7	Meta and Macros Reference
8	Rhombus Static by Default
9	Libraries
10	Naming Conventions

6.1	General Forms
6.2	Definitions
6.3	Namespaces
6.4	Import
6.5	Export
6.6	Functions
6.7	Operators
6.8	Classes and Interfaces
6.9	Veneers
6.10	Mutable Variables and Assignment
6.11	Repetitions
6.12	Block
6.13	Conditionals
6.14	Matching
6.15	Iteration
6.16	Recursion
6.17	Context Parameters
6.18	Exceptions
6.19	Continuations
6.20	Implicit Forms
6.21	Static and Dynamic Lookup
6.22	Modules and Submodules
6.23	Booleans
6.24	Numbers
6.25	Characters
6.26	Strings
6.27	Keywords
6.28	Symbols
6.29	Byte Strings
6.30	Lists
6.31	Pairs and Pair Lists
6.32	Mutable Lists
6.33	Arrays
6.34	Maps
6.35	Sets
6.36	Boxes
6.37	Indexables
6.38	Listables
6.39	Sequences
6.40	Appendables
6.41	Comparables
6.42	Paths
6.43	Source Locations
6.44	Void
6.45	Equality
6.46	Dot
6.47	Multiple Values
6.48	Annotations
6.49	Enumerations
6.50	Input and Output
6.51	Eval
6.52	Syntax Objects
6.53	Syntax Classes
6.54	Simple Expression Macros
6.55	Unit Testing