Add "safe navigation"/existential operator

[DartBot]

This issue was originally filed by todd.chamb...@gmail.com

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I didn't find the "safe navigation"/existential operator in the specification. Please consider including something like what's available in

Coffeescript (http://jashkenas.github.com/coffee-script/#operators),

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

compiles to

    var zip, _ref;
    zip = typeof lottery.drawWinner === "function" ? (_ref = lottery.drawWinner().address) != null ? _ref.zipcode : void 0 : void 0;

Groovy (http://groovy.codehaus.org/Operators#Operators-SafeNavigationOperator%28%3F.%29)

The Safe Navigation operator is used to avoid a NullPointerException. Typically when you have a reference to an object you might need to verify that it is not null before accessing methods or properties of the object. To avoid this, the safe navigation operator will simply return null instead of throwing an exception, like so:

def user = User.find( "admin" ) //this might be null if 'admin' does not exist
def streetName = user?.address?.street //streetName will be null if user or user.address is null - no NPE thrown

Ruby, etc.

Null checks are tedious and clutter code-- really, anything is better than if( foo != null && ....)

[DartBot]

This comment was originally written by todd.chambery...@gmail.com

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This was entered as a Defect in error, please change to Enhancement.

[DartBot]

This comment was originally written by jat@google.com

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Removed Type-Defect label.
Added Type-Enhancement label.

[DartBot]

This comment was originally written by drfibonacci@google.com

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Added Area-Language, Triaged labels.

[gbracha]

Interesting idea. We'll think about it.

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Set owner to @gbracha.

[gbracha]

Issue #66 has been merged into this issue.

[DartBot]

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

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A replacement of || for null-checks would also be quite useful, as || now works only on booleans. C# uses ?? for this purpose.

foo = mightBeNull || default; //in JS

becomes

foo = mightBeNull ?? default; //in C# or hopefully future Dart?

[DartBot]

This comment was originally written by demis.b...@gmail.com

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+1 Dart really needs to copy CoffeeScript's existential as this is one of the biggest friction points I'm facing (moving from CoffeeScript).

Both these snippets are obvious, more readable, require less boilerplate and make it effortless to write safer code:

zip = lottery.drawWinner?().address?.zipcode;
foo = mightBeNull ?? default;

[anders-sandholm]

Added apr30-triage label.

[anders-sandholm]

Removed apr30-triage label.

[anders-sandholm]

Added triage1 label.

[anders-sandholm]

Added this to the Later milestone.
Removed triage1 label.

[DartBot]

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

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∃ and ∄ would be appropriate to use for this. As our current static keyboard layouts fail to provide a usable input mechanism for such symbols, an ascii alias might be usefull ;)

[munificent]

Issue #5630 has been merged into this issue.

[DartBot]

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

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How about making this more like as in Objective C where you can send messages to NIL that then will be ignored?

In case of chaining multiple messages send to a given object and objects returned thereof, each message sent to a NIL objects would return a NIL again so that the chained calls would not fail.

[DartBot]

This comment was originally written by demis.b...@gmail.com

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I like Obj-C approach as well but for ease of determining the source of errors, it may be a good idea to be explicit when it doesn't matter.

The existential operator is a convenient choice to opt-in for this.

[DartBot]

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

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I use in Javascript when recibe info in Json files with optative fields.

((student.preferences || {}).hobies || {}).bricollage

I prefer any sugar sintax not LISP (ie: without the intrusive parenthesis).

[DartBot]

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

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For the ease of discussion, I'll add that the ?? operator in C# is called the null-coalescing operator.

Here's an example I've used in C# where this operator has come in handy:

if ((stringMightBeNull ?? "").Length == 0) {
    // string is empty or null
}

[sethladd]

I miss this from Ruby, fwiw.

[sethladd]

Met with external customers today, and they asked for this, or something like it. They'd be fine with C#'s null-coalescing operator.