Add 'in' infix operator

[rakudrama]

An 'in' operator would make Set membership and Map definedness tests look nicer.

'in' is slightly unusual in that the receiver would be the right hand operand.

class MySet<T> implements Set<T> {
  bool operator in(T candidate) => contains(candidate);
}

if (mySet.contains(50)) ...
-->
if (50 in mySet) ...

The subexpressions would still be evaluated left-to-right,
so
    f() in g()
calls f, calls g, and then invokes the 'in' operator of the result of g(), passing the result of f().

'in' is already a keyword from for-in

[gbracha]

I think we want to keep the number of constructs in the language small. This does not seem particularly necessary or valuable. It just does not pass the bar in terms of utility or power to weight ratio.

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Set owner to @gbracha.
Added WontFix label.

[DartBot]

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

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Among other things 'in' combined with list literals is a very nice way to write clear and concise code in many circumstances. Couldn't this just be given a low priority for 'maybe' doing 'one day' instead of being rejected?

[floitschG]

Issue #3979 has been merged into this issue.