:: — Error guardcodes::handler catches matching later errors in a dfn. 0:: catches ordinary APL errors; a vector selects several error numbers.
{0::42 ⋄ 1÷⍵}0 ⍝ 42
{11::0 ⋄ ÷⍵}0 ⍝ 0
The handler sees local bindings restored to guard installation. Outer writes and output remain. The selected guard is inactive in its handler.
{x←1 ⋄ 0::x ⋄ x←2 ⋄ 1÷0}0 ⍝ 1
Cancellation and unsupported-feature errors bypass guards.