The slip operator |

Flatten a list into the surrounding list, in place.

Prefix | turns a list into a Slip — a list that dissolves into its container. Where a nested list would stay nested, a slipped one merges its elements into the enclosing list.

Flattening in place #

my @a = 1, |(2, 3), 4;
say @a;
say (1, |(2, 3), 4).elems;

[1 2 3 4]
4

Without the |, (2, 3) would be one nested element; slipped, its 2 and 3 sit directly in the list, so the total length is 4.

Notes #

• | is also how you pass a list as separate arguments to a call: f(|@args) spreads @args into f's parameters (the reverse of a slurpy *@).
• A Slip is a real value (slip(2, 3) or the Slip type) — returning one from a map block lets a single element expand into several results.
• It's the list-level analogue of the | in signatures/captures, and distinct from the numeric/junction | (an any junction on numbers).