Add a "default" operator

[DartBot]

This issue was originally filed by @seaneagan

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Note: This is different than issue #41

The need to default a null value is quite common. Thus, many languages (see below) provide an operator which returns the LHS unless it is null, in which case it "defaults" to the RHS. This saves a temporary variable declaration when the LHS is an expression which needs to be evaluated, and is much shorter and less cryptic than the ternary expression which is otherwise needed. To illustrate, assume we want to assign a variable to an expression if it's not null, otherwise to a default value. Currently in Dart, this would look like:

var x = b.c();
a = x !== null ? x : d;

or possibly:

a = ((x) => x !== null ? x : d)(b.c())

With a "default" operator, which might be denoted "??", this would shorten to:

a = b.c() ?? d;

Prior art:

CoffeeScript http://coffeescript.org/#operators :

// might require Dart to replace ternary operator with if expression (see issue #55)
foo ? bar
foo ?= bar

Groovy "elvis" operator - http://groovy.codehaus.org/Operators :

foo ?: bar

C# "null-coalescing" operator - http://msdn.microsoft.com/en-us/library/ms173224.aspx :

foo ?? bar

EcmaScript strawman - http://wiki.ecmascript.org/doku.php?id=strawman:default_operator :

foo ?? bar
foo ??= bar

JavaScript:

// defaults for any falsey value :(
foo || bar

Ruby:

// defaults for any falsey value :(
foo || bar
foo ||= bar

[DartBot]

This comment was originally written by @seaneagan

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If extension methods are added, this extension method might do the trick as well:

T ifNull<T>(T defaultValue) => this === null ? this : defaultValue;

where T is the type of this

Then, we could write:

a = b.c().ifNull(d)

[DartBot]

This comment was originally written by @seaneagan

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Or if "self types" were supported, then this "ifNull" method could be defined on Object. Assuming "self" were used to denote a "self type" annotation, then it would look like:

self ifNull(self defaultValue) this === null ? this : defaultValue;

[DartBot]

This comment was originally written by @seaneagan

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Forgot the => :

self ifNull(self defaultValue) => this === null ? this : defaultValue;

[DartBot]

This comment was originally written by @seaneagan

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One more try, should be !== :

self ifNull(self defaultValue) => this !== null ? this : defaultValue;

[dgrove]

Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[gbracha]

Set owner to @gbracha.
Added Accepted label.

[gbracha]

Added this to the Later milestone.

[DartBot]

This comment was originally written by sra...@gmail.com

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+1 for this feature!

Porting (CoffeeScript) code to Dart it's a feature I sorely missed e.g. having to write something like:

  pager = Platform.environment['PAGER'];
  pager = pager == null ? 'less' : pager;

instead of this (C# '??' style syntax):

  pager = Platform.environment['PAGER'] ?? 'less';

is both verbose and wet (4 refs to the assigned variable instead
of just one).

[DartBot]

This comment was originally written by emilioplatz...@gmail.com

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More ideas in http://code.google.com/p/dart/issues/detail?id=6582

[gbracha]

Issue #6582 has been merged into this issue.

[DartBot]

This comment was originally written by @tomochikahara

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I already resolve this problem by option.dart in http://pub.dartlang.org/packages/option

pager = new Option(Platform.environment['PAGER']).getOrElse('less');

I believe Option should be in Dart core library.

[DartBot]

This comment was originally written by @seaneagan

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The main disadvantages I see with Option are:

* have to explicitly specify a type argument if you want to retain the type information (unless the analyzer can infer it somehow ?), which is not DRY.

• an extra object is created (probably not a deal breaker)

Using an | operator would make it slightly less verbose, and more familiar to JavaScripters:

pager = new Option<String>(Platform.environment['PAGER']) | 'less';

[DartBot]

This comment was originally written by @seaneagan

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Also the ??= shorthand form can't be supported by Option, so you have something like:

someFoo = new Option<Foo>(someFoo) | foo();

instead of:

someFoo ??= foo();

[kevmoo]

Issue #18045 has been merged into this issue.

[tfortes]

Sorry I missed this one when I filed my duplicate (18045), but mine had another operator suggestion too (verbatim from that bug):

Going one step further, we could also add the ??! operator that returns null if LHS is null, and RHS otherwise.

Element child = getChild();
Element parent = child ??! child.parent;

[The operator names are debatable of course]

[DartBot]

This comment was originally written by @simonpai

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I'd guess using ??! might be a little bit hard to read? If Dart is going introduce both operators, maybe we can use something like ?|| and ?&& to enhance symbol semantics? Just my 2 cents.

[DartBot]

This comment was originally written by @seaneagan

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@16: That particular example is what the safe-navigation operator (issue #41) is for:

Element child = getChild();
Element parent = child?.parent;

But maybe there are cases you want want to call a function only if the input is not null, but that's already pretty easy since you don't have to repeat the input, you can just use the null literal:

var result = input == null ? null : f(input);

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[kasperl]

Removed Oldschool-Milestone-Later label.

[sethladd]

There is a null-aware operator DEP that was approved: dart-archive/dart_enhancement_proposals#9

Implementations are underway.