.Q.trp[f;x;g]

Where

• f is a unary function
• x is its argument
• g is a binary function

extends Trap (@[f;x;g]) to collect backtrace: g gets called with arguments:

1. the error string
2. the backtrace object

You can format the backtrace object with .Q.sbt.

q)f:{`hello+x}
q)           / print the formatted backtrace and error string to stderr
q).Q.trp[f;2;{2@"error: ",x,"\nbacktrace:\n",.Q.sbt y;-1}]
error: type
backtrace:
  [2]  f:{`hello+x}
                ^
  [1]  (.Q.trp)

  [0]  .Q.trp[f;2;{2@"error: ",x,"\nbacktrace:\n",.Q.sbt y;-1}]
       ^
-1
q)

.Q.trp can be used for remote debugging.

q)h:hopen`::5001   / f is defined on the remote
q)h"f `a"
'type              / q's IPC protocol can only get the error string back
  [0]  h"f `a"
       ^
q)                 / a made up protocol: (0;result) or (1;backtrace string)
q)h".z.pg:{.Q.trp[(0;)@value@;x;{(1;.Q.sbt y)}]}"
q)h"f 3"
0                  / result
,9 9 9
q)h"f `a"
1                  / failure
"  [4]  f@:{x*y}\n            ^\n  [3..
q)1@(h"f `a")1;    / output the backtrace string to stdout
  [4]  f@:{x*y}
            ^
  [3]  f:{{x*y}[x;3#x]}
          ^
  [2]  f `a
       ^
  [1]  (.Q.trp)

  [0]  .z.pg:{.Q.trp[(0;)@enlist value@;x;{(1;.Q.sbt y)}]}
              ^

Since V3.5 2017.03.15.