Friday, December 30, 2016

Riddle 4 Answer

So apparently this is an annual series now…

Anyway, Dan Nugent and Akash were both on the right track, but neither had it exactly right: when a qsql query inside a function is executed, relative names used in it resolve against globals in the current namespace, not the namespace that was in effect when the function was created (i.e. the one returned by running {(get x). 3 0} on the function).

(Note that this actually applies to any type of global (e.g. an atom, a vector, etc.) referenced from a query inside a function, but for convenience, I’ll be writing this post assuming a function is what’s being referenced.)

I will admit to making this a bit of a trick question, as the way I constructed the example was designed around the most common case, which is entirely consistent with Dan’s and Akash’s answers. Here's a snippet showing the full behavior:
% q
KDB+ 3.3 2015.09.02 Copyright (C) 1993-2015 Kx Systems
m32/ 16()core 8192MB adavies aaron-daviess-mac-pro.local 192.168.1.151 NONEXPIRE  

q)f:{x+1}
q)\d .foo
q.foo)g:{select f a from x}
q.foo)\d .bar
q.bar).foo.g([]a:1 2 3)
{select f a from x}
'f
q.foo))\
q.bar)f:{x+2}
q.bar).foo.g([]a:1 2 3)
a
-
3
4
5
q.bar)
Compare to this snippet, where I reference the function from outside the qsql query:
% q
KDB+ 3.3 2015.09.02 Copyright (C) 1993-2015 Kx Systems
m32/ 16()core 8192MB adavies aaron-daviess-mac-pro.local 192.168.1.151 NONEXPIRE  

q)\d .foo
q.foo)f:{x+1}
q.foo)g:{f select a from x}
q.foo)\d .
q).foo.g([]a:1 2 3)
a
-
2
3
4
q)
This can become problematic if you try to create a group of functions in a namespace, some of which reference each other in queries, and then call those functions from outside that namespace. I’ve found two solutions for this, both unfortunately rather inelegant.
  1. You can “copy” the function you need from the global space to a local variable by referencing it from outside any qsql queries:
    % q
    KDB+ 3.3 2015.09.02 Copyright (C) 1993-2015 Kx Systems
    m32/ 16()core 8192MB adavies aaron-daviess-mac-pro.local 192.168.1.151 NONEXPIRE  
    
    q)\d .foo
    q.foo)f:{x+1}
    q.foo)g:{f0:f;select f0 a from x}
    q.foo)\d .
    q).foo.g([]a:1 2 3)
    a
    -
    2
    3
    4
    q)
  2. You can do the name resolution yourself, by leveraging the get results to automatically reference the correct namespace:
    % q
    KDB+ 3.3 2015.09.02 Copyright (C) 1993-2015 Kx Systems
    m32/ 16()core 8192MB adavies aaron-daviess-mac-pro.local 192.168.1.151 NONEXPIRE  
    
    q)\d .foo
    q.foo)f:{x+1}
    q.foo)g:{select((` sv`,((get .z.s). 3 0))`f)a from x}
    q.foo)\d .
    q).foo.g([]a:1 2 3)
    a
    -
    2
    3
    4
    q)
    This can be encapsulated in a utility function, but note that it must then itself be referenced by an absolute name, or the same problem will apply to it:
    % q
    KDB+ 3.3 2015.09.02 Copyright (C) 1993-2015 Kx Systems
    m32/ 16()core 8192MB adavies aaron-daviess-mac-pro.local 192.168.1.151 NONEXPIRE  
    
    q)\d .util
    q.util)r:{(` sv`,((get x). 3 0))y}
    q.util)\d .foo
    q.foo)f:{x+1}
    q.foo)g:{select((.util.r .z.s)`f)a from x}
    q.foo)\d .
    q).foo.g([]a:1 2 3)
    a
    -
    2
    3
    4
    q)

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1 Comments:

At December 31, 2016 at 8:09 AM , Blogger Dan Nugent said...

Frankly, a better approach is to literally never use the d command. :p

I stopped, ohhh, 3 or 4 years ago and debugging sessions have immeasurably improved. Q is a lot easier when you don't impose artificial constraints on the length of your variables and functions and use whitespace.

But you do have to be fine with being a heretic to the orthodoxy.

 

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