Safe navigation / null-conditional member access operator

[GreyCat]

Following the discussion in kaitai-io/kaitai_struct_webide#16, there is a proposal to introduce some sort of "safe navigation" or "null-conditional member access" operator into our expression language.

There's a Wikipedia article on this operator.

Actually, there should be probably at least 2 different operators - i.e. safe member access operator (a.b where a can be null) and safe collection indexing operator (a[x] where a can be null).

Known existing implementations in target languages:

• C# - ?. and ?[]
• Ruby - &.
• Swift - ?. and ?[]
• Dart - ?.
• Perl 6 - .?
• etc

Drafts:

• Python - ?. and ?.[]

Questions to decide:

1. What symbol shall we use for it? I'd propose to follow ?., as it seems to be used by majority of existing implementations.
2. Implementations in target languages — what would be exactly the substitute when it's not supported.

Cc @xavrr

[LogicAndTrick]

+1 for ?.

For C#: this connects a bit with #74 - the safe navigation operator was added in C# 6 which is the newest version (but not super new - mid 2015) - and is supported by all the major .NET implementations (.NET 4.5, Mono, dotnet core). My thoughts is to just use the ?. operator and target C# 6 as the minimum required version.

[GreyCat]

@LogicAndTrick It should be probably not that hard to:

• add config / CLI option (something like --csharp-version=6) to control it
• generate version-dependent code based on that option, i.e. workaround with ternary operator

[ghost]

Here is a quote from Python's PEP 505 referenced above:

The " null -coalescing" operator is a binary operator that returns its left operand if it is not null . Otherwise it returns its right operand.
The " null -aware member access" operator accesses an instance member only if that instance is non- null . Otherwise it returns null . (This is also called a "safe navigation" operator.)
The " null -aware index access" operator accesses an element of a collection only if that collection is non- null . Otherwise it returns null . (This is another type of "safe navigation" operator.)

Here are samples for PHP

// null -coalescing
$v = $a ?? 'foo'; // if $a is defined and not null, then use value of the $a as the result of the whole expression, otherwise use use the 'foo' as the result of the whole expression.

// null -aware member access
$v = isset($a) ? $a->bar() : null; // if $a is defined and not null, then use result of the $a->bar() as the result of the whole expression, otherwise use null as the result of the whole expression.

// null -aware index access
$v = isset($a) ? $a['bar'] : null; // (string indexes) if $a is defined and not null, then use result of the $a['bar'] as the result of the whole expression, otherwise use null as the result of the whole expression.
$v = isset($a) ? $a[2] : null; // (numeric indexes) if $a is defined and not null, then use result of the $a[2] as the result of the whole expression, otherwise use null as the result of the whole expression.