Identity, Null

When the generic null is applied to another value, it is the Identity function.

Indexing with the generic null has the same effect.

:: Identity

Return a value unchanged

Applying null to a value

(::) x     ::[x]

Where x is any value, returns x.

q)(::)1
1

Applying multiple functions to the same data, with one of the operations as “do nothing”.

q)(::;avg)@\:1 2 3
1 2 3
2f

Applying a value to null

x ::      x[::]

Identity can also be achieved via indexing.

q)1 2 3 ::
1 2 3

and used in variants thereof for e.g. amends

q)@[til 10;(::;2 3);2+]
2 3 6 7 6 7 8 9 10 11

When prefix notation is used, x does not have to be an applicable value.

q)q:3[::]       / not an applicable value
'type
  [0]  q:3[::]
         ^
q)q:3 ::
q)q~3
1b

:: Null

Q does not have a dedicated null type. Instead :: is used to denote a generic null value. For example, functions that ‘return no value’, actually return ::.

q)enlist {1;}[]
::

We use enlist above to force display of a null result – a pure :: is not displayed.

When a unary function is called with no arguments, :: is passed in.

q)enlist {x}[]
::

Use :: to prevent a mixed list changing type.

Since :: has a type for which no vector variant exists, it is useful to prevent a mixed list from being coerced into a vector when all items happen to be of the same type. (This is important when you need to preserve the ability to add non-conforming items later.)

q)x:(1;2;3)
q)x,:`a
'type

but

q)x:(::;1;2)
q)x,:`a  / ok

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null