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kdb

Using trap with KDB and non-declared variables?

Say for example I'm trying to do try-catch where x does not exist:

@[x;::;"error"]

I get an 'x error, rather than "error". What am I doing wrong?

Solution

  • Have a look at the documentation here https://code.kx.com/q/ref/apply/#trap "In the ternary, if evaluation of the function fails, the expression is evaluated"

    The function x cannot be evaluated as it does not exist. The function is not failing here it just cannot be called. The hint here is in evaluated - we can use value to check if the function exists.

    q)f:{x+x}
q)@[value;`f;"error"]
{x+x}
q)@[value;`p;"error"]
"error"

    The returns of values on the defined function f vs something that is not defined p >

    q)value f
0x6161410003
,`x
`symbol$()
,`
5 3 4 2 2
"..f"
""
-1
"{x+x}"
q)value p
'p
  [0]  value p
             ^