In q, there are several ways to apply a function and index a variable. Underlying them all is the ur-function Apply/Index

.

pronounced dot. Application of the rank-n function f to a list of arguments v of count n is expressed:

f . v

The only place you can fall from
is where it’s at; and when you’re down,
you’re not there – that’s why.
— Alastair Howard Robertson

Application of the unary function g to w is expressed:

g . enlist w

sugar for which is

g@w

Even more simply:

g w

Brackets are more sugar:

f[first v;…;last v]
g[w]

Maximize readability by using the simplest syntax available.

All functions can be applied with bracket notation. Infix and prefix syntax is usually clearer and preferable.

+[2;2]     2+2
til[5]     til 5

Not always, particularly when composing the left argument of an infix.

Short expressions within brackets or parentheses are clearer than long expressions. Bracket notation can avoid parenthesizing a longer expression.

+/[foo goo v]             / less clear
(+/) foo goo v            / more clear

bar[(+//)m; goo v]        / less clear
((+//)m) bar goo v        / clearer?
+//[m] bar goo v          / clear

Minimize the distance between opening and closing brackets and parentheses.