:: — 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.
•SIGNAL 'DOMAIN ERROR' raises an ordinary APL error. It is monadic and does not return a value. Existing :: guards catch it in the same way as an error from a primitive.
positive←{⍵≤0:•SIGNAL 'DOMAIN ERROR' ⋄ ⍵}
safe←{11::0 ⋄ positive ⍵}
safe ¯3 ⍝ 0
safe 4 ⍝ 4
The argument must be one of these uppercase names: SYNTAX ERROR, INDEX ERROR, RANK ERROR, LENGTH ERROR, VALUE ERROR, LIMIT ERROR, or DOMAIN ERROR. Their guard numbers are 2, 3, 4, 5, 6, 10 and 11 respectively. Numeric codes and other names are rejected with DOMAIN ERROR. Interrupts, timeouts and unsupported-feature errors cannot be signalled.