►Rhombus Reference
▼9 Object Protocols
On this page:
in
in
Membership  Testable
8.16.0.4
  top  contents    up   

9.7 Membership Tests🔗ℹ

A membership test uses the in operator to check whether an element is included in the value. Maps, lists, mutable lists, pair lists, arrays, sets, and ranges all support membership tests, as do instances of classes that implement MembershipTestable.

expression

elem_expr in expr

 

repetition

elem_repet in repet

 

expression

elem_expr !in expr

 

repetition

elem_repet !in repet

 

~order: equivalence

Checks whether the result of elem_expr is a member of the result of expr, where the latter is a value such as a map, list, mutable list, pair list, array, set, range, or MembershipTestable object that support membership tests. The result is #true for in if the element is present and #false otherwise. The operator combination !in inverts the result relative to in. Either form works as a repetition given repetitions for a member and a collection.

The in operator is also recognized by for and each as part of the syntax of iteration.

The use_static declaration constrains in to work only when the right-hand argument has static information indicating that it satisfies MembershipTestable.

> "a" in {"a", "b"}

#true

> "c" !in {"a", "b"}

#true

> "a" in {"a": 1, "b": 2}

#true

> "a" in ["a", "b"]

#true

> 1 in {"a": 1, "b": 2}

#false

interface

interface MembershipTestable

An interface that a class can implement (publicly or privately) to make instances of the class work with in. As an annotation, MembershipTestable matches all objects that support membership tests, not just instances of classes that publicly implement the MembershipTestable interface.

The interface has a single abstract method:

class Either(lst1, lst2):

  private implements MembershipTestable

  private override method contains(v):

    v in lst1 || v in lst2

> def lsts = Either([1, 2, 3], [-1, -2, -3])

> 2 in lsts

#true

> 4 !in lsts

#true

  top  contents    up