.z.vs:f

Where f is a binary function, .z.vs is evaluated after a value is set globally in the default namespace (e.g. a, a.b): x is the symbol of the variable that is being modified and y is the index. This is not triggered for function-local variables, nor globals that are not in the default namespace (e.g. those prefixed with a dot such as .a.b) .

The following example sets .z.vs to display the symbol, the index and the value of the variable.

q).z.vs:{0N!(x;y;value x)}
q)m:(1 2;3 4)
(`m;();(1 2;3 4))
q)m[1;1]:0
(`m;1 1;(1 2;3 0))