? Skeleton

prefix ?

The rules for ? have not been written yet. Everything below is extracted rather than authored: the official documentation supplies the classification, and the interpreters were asked directly about the rest.

What is established #

Classification #

? is parsed as a prefix at the symbolic unary precedence level, left-associative.

That is level 5 of 27 on the ladder, counting from the tightest. Operators sharing a level bind equally; grouping among them is decided by associativity alone.

Shares its precedence level with #

!, //, +, -, ~, |, +^, ~^, ?^, ^.

These all bind equally tightly, so an expression mixing them groups by associativity alone.

What the interpreter said #

Raku++ parses this spelling. Asked to compile:

my $a = 6; sink(? $a)

…it got through the parser. On its own that establishes very little: it says nothing about whether the construct executes, let alone whether the result is right. Where the behaviour table below exists, that is the stronger evidence — those expressions were actually run.

Covered upstream #

The official documentation has a section for this construct: docs.raku.org. Until the rules here are written, that is the better reference.

Observed behaviour #

Each expression below was executed by both interpreters when this page was built; the results are recorded, not written. The operands are chosen to cross the boundaries that catch people out: string against number, a list where a scalar was meant, a boolean, an undefined value, and an exact rational.

Raku++ and Rakudo agree on every row below.