The Q Tips book (Nick Psaris) shows the following function (Chapter 10):
q)merge:`time xdesc upsert
As it is stated, it corresponds to function composition. I see the pattern: the
expression supplies a function that takes both arguments for upsert and then
uses its result to feed time xdesc. However the syntax feels weird, since
I would expect upsert to be the second argument of the xdesc invocation.
Aiming at simplifying the expression, I could see that the very same scenario applies here:
q)f:1+*
q)f[2;3]
7
If we show its result, we can clearly see that f behaves as expected:
q)f
+[1]*
However, If we slightly modify the function, the meaning of the expression is completely different:
q)g:+[1;]*
q)g[2;3]
'rank
[0] g[2;3]
^
In fact, +[1;] is passed as first argument to the * operator instead,
leading us to a rank error:
q)g
*[+[1;]]
I could also notice that the pattern breaks when the first function is "monadic":
q)h:neg *
q)h[2;3]
'rank
[0] h[2;3]
^
Also here:
q)i:neg neg
'type
[0] i:neg neg
^
At this point, my intuition is that this pattern only applies when we are interested on composing dyadic standard (vs user-defined) operators that exploit infix notation. Am I getting it right? Is this syntactic sugar actually more general? Is there any documentation where the pattern is fully described? Thanks!
There are some documented ways to achieve what you wish:
https://code.kx.com/q/ref/apply/#composition
You can create a unary train using @
q)r:neg neg@
q)r 1
1
https://code.kx.com/q/ref/compose/
You can use ' to compose a unary value with another of rank >=1
q)f:('[1+;*])
q)f[2;3]
7
Likely the behaviour you are seeing is not officially there to be exploited by users in q so should not be relied upon. This link may be of interest: