How do I get the stacktrace of a Q script

Community Support
1

When debugging, I want to insert something into a function and see when this function gets called. 

2

0N! is one way to go.

3

Another way is to use “show”.
The end result of using show is arguably identical to using ON!, but the implementations appear to be different.

Implementation of “show”

q)show

k){1 .Q.s x;}

q).Q.s

k){$[(::)~x;“”;`/:$[10h=@r:@[S[(.“\c”)-2 1;0];x;::];,-3!x;r]]}

Implementation of 0N!

q)0N!

![0N]

4

I lean towards 0N! to reveal the data and structure.
e.g.
q)0N!enlist s#0 1; ,s#0 1
q)show enlist `s#0 1;
0 1
If I’m interested in the data only and am confident of the structure, I might use show.

For debugging, my colleague Pierre cooked up the following wrapper a while back

L:{('[{.[x;y;{0N!(string x;y;z);'z}[x;y]]};enlist])}

sample usage:

q)f:L{x+g[y;z]}
q)g:L{x*y}
q)f[1;“a”;2]
(“{x*y}”;(“a”;2);“type”)
(“{x+g[y;z]}”;(1;“a”;2);“type”)
{.[x;y;{0N!(string x;y;z);'z}[x;y]]}
'type

which shows the stack of wrapped functions on error.

A small tweak shows the func+args as they are called
q)L:{('[{0N!(x;y);x . y};enlist])}
q)f:L{x+g[y;z]}
q)g:L{x*y}
q)f[1;2;3]
({x+g[y;z]};1 2 3)
({x*y};2 3)
7

Aaron also cooked up the helpful “what the function”
http://www.q-ist.com/search?q=wtf

5

Thanks Charles.
On second look, 0N! and show are quite different.

In particular, (from the wiki):

0N! returns the right hand side after printing its unformatted text representation to console. This is useful for debugging, or avoiding formatting which may obscure the data’s structure.

q)2+0N!335

show only writes to the console; it does not return the formatted result:

q)a:show til 50 1 2 3 4q)a / a is null