h“.z.pg:{.Q.trp[(0;)@value@;x;{(1;.Q.sbt y)}]}”
.z.pg is the message handler on the server
.Q.trp takes 3 arguments:
#1 the function: (0;)@value@ (the default value of .z.pg is just {value x}, how does this translate to that?)
#2 Argument: x (which is the user query)
#3 The binary function where x is the error string and y is the backtracing object
And why cannot we simply do this: h".z.pg:{.Q.trp[{value x};x;{(1;.Q.sbt y)}]}"
Thank you for all the help!
.Q.trp[(0;)@value@;x;{(1;.Q.sbt y)}]
means roughly the same as
.Q.trp[{(0;value x)};x;{(1;.Q.sbt y)}]
The only difference is that the second form uses a lambda, which appears as an additional level in the stack trace.
The @ at the end of an expression forces it to be a projection (since it’s a binary operator and we haven’t given it a right argument). But in order to compose this with the (0;) operation (which puts the result in the 2nd element of the list) we need to include another @ otherwise we would be composing the wrong functions.
.Q.trp[{value x};x;{(1;.Q.sbt y)}] works but it returns a different value in the success case (just the value instead of a list).