Safe navigation operator

In object-oriented programming, the safe navigation operator (also known as optional chaining operator, safe call operator or null-conditional operator) is a binary operator that returns null if its first argument is null; otherwise it returns the second argument.

It is used to avoid sequential explicit null checks and assignments and replace them with method/property chaining. In programming languages where the navigation operator (e.g. ".") leads to an error if applied to a null object, the safe navigation operator stops the evaluation of a method/field chain and returns null as the value of the chain expression. It is currently supported in languages such as Apache Groovy,[1] Swift,[2] Ruby,[3] C#,[4] Kotlin,[5] CoffeeScript and others. There is currently no common naming convention for this operator, but safe navigation operator is the most widely used term.

The main advantage of using this operator is that it solves a problem commonly known as pyramid of doom. Instead of writing multiple nested ifs, programmers can just use usual chaining, but put question mark symbols before dots (or other characters used for chaining).

While the safe navigation operator and null coalescing operator are both null-aware operators, they are not operationally equivalent.

Examples

Groovy

Safe navigation operator:[6]

def name = article?.author?.name

Objective-C

Normal navigation syntax can be used in most cases without regarding NULLs, as the underlying messages, when sent to NULL, is discarded without any ill effects.

NSString *name = article.author[0].name;

Swift

Optional chaining operator:[7]

let name = article?.author?.name

Optional subscript operator:

let author = articles?[0].author

Ruby

Ruby supports the &. safe navigation operator (also known as the lonely operator) since version 2.3.0:[8]

name = article&.author&.name

C#

In C# 6.0 and above, basic null-conditional operators ?. and ?[]:[9]

String name = articles?[0]?.author?.name;

Kotlin

Safe call operator:[10]

val name = article?.author?.name

Perl 6

Safe method call:[11]

my $name = $article.?author.?name;

Python

The safe navigation operator is not currently supported in Python, but it is currently being proposed for inclusion with the following syntax:[12]

# Proposed syntax, not yet part of the language:
name = article?.author?.name;

CoffeeScript

Existential operator:[13]

zip = lottery.drawWinner?().address?.zipcode

Javascript

The safe navigation operator is not currently supported in Javascript, but it is currently being proposed for inclusion with the following syntax:[14] It is, however, unlikely that the current syntax will make the language, as some TC39 members are against the proposed syntax.

// Proposed syntax, not yet part of the language:
const name = article?.author?.name;

See also

References

  1. ↑ "6.1. Safe navigation operator". Retrieved 2016-01-28.
  2. ↑ "Optional Chaining". Retrieved 2016-01-28.
  3. ↑ "Ruby 2.3.0 Released". Retrieved 2016-01-28.
  4. ↑ "Null-conditional Operators (C# and Visual Basic)". Retrieved 2016-01-28.
  5. ↑ "Null Safety". Retrieved 2016-01-28.
  6. ↑ "6.1. Safe navigation operator". Retrieved 2016-01-28.
  7. ↑ "Optional Chaining". Retrieved 2016-01-28.
  8. ↑ "Ruby 2.3.0 Released". Retrieved 2016-01-28.
  9. ↑ "Null-conditional Operators (C# and Visual Basic)". Retrieved 2016-01-28.
  10. ↑ "Null Safety". Retrieved 2016-01-28.
  11. ↑ "Perl 6 Operators". Retrieved 2016-06-28.
  12. ↑ "PEP 505 -- None-aware operators". Retrieved 2018-08-27.
  13. ↑ "The Existential Operatior". Retrieved 2017-06-15.
  14. ↑ "Optional Chaining for JavaScript". Retrieved 2018-06-27.
  • PEP 505, discussing the possibility of safe navigation operators for Python
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