CODEWITHSUNDEEP
kdb+k
egor7
egor7

Reputation: 4941

Variable scope propagation in k

I've seen a variable scope propagation to the inner function in previous versions of k. See eval: {[t;c]{x*t+y}/c} in http://www.math.bas.bg/bantchev/place/k.html

But if I try to do the same in modern k, I get an error:

KDB+ 3.6 2018.05.17 Copyright (C) 1993-2018 Kx Systems
q)\
  {[k]{x*x}k}3
9
  {[k]{x*x+k}k}3
'k
  [2]  k){x*x+k}
              ^
  )

So why this error happens? Is such variable scope propagation 'banned' in modern q?

Upvotes: 0

Views: 255

Answers (1)

Igor Korkhov
Igor Korkhov

Reputation: 8558

Indeed, k4, the most recent implementation of k by kx does not support closures. In fact, the article you refer to does mention that in a section called "Changes to the Language":

K4/q is a change over K3 in a number of significant ways, such as:

...

  • Nested functions in K4 and q cannot refer to surrounding function's local variables. (Often, the lack of this ability can be circumvented by making use of function projection.)

It turns out the lack of support of lexical scoping has not always been the case. Although the only officially documented language nowadays is q, one can still find a reference manual for k2, an implementation of k circa 1998, for example here: http://www.nsl.com/k/k2/k295/kreflite.pdf. Section "Local functions" on page 158 reads:

Local Functions

Suppose that the function g is defined within the body of another function f and uses the variable x in its definition, where x is local to f. Then x is a constant in g, not a variable, and its value is the current one when g is defined. For example, if:

  f:{b:3; g:{b}; b:4; g[]}

The value of f is the value of the local function g, which turns out to be 3, the value of b when g is defined, not the subsequent value 4.

  f[]
3

(I highly recommend reading the whole document, by the way).

I don't know why the support of closures was dropped but I think it was because of performance reasons, especially during interprocess communications.

Upvotes: 1

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