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How many arguments should a function have?

One of the most interesting properties of the q language is that function application is itself a function. Apply takes a function on the left and a list of arguments on the right, and applies the function to the list, mapping each item of the list to the corresponding argument of the function.

Used properly, this is a wonderfully convenient feature, since functions can be designed to apply to many small, named things, which are themselves parts of a list or dictionary. Keeping the parts in a composite data structure is good for organizational reasons. Designing the function to see those parts as separate, named entities reduces code and improves readability.

For example, suppose that v is a list of five heterogeneous items, and f is a function which re-arranges those items.

v:(12;`xyz;"abc";1.1;10 20 30)
f:{[i;s;c;r;l] (s;c;i;l;r)}

Application of f to v serially maps the items of v to the arguments of f.

q)f . v
(`xyz;"abc"12;10 20 30;1.1)

Compare f with the unary list functions g and h:

g:{x[1 2 0 4 3]}        / what?

  i:x 0; s:x 1; c:x 2; r:x 3; l:x 5;
  (s;c;i;l;r)           / why?

Good function design is the outcome of taking multiple perspectives on functionality:

  • from within the function, we want the data already decomposed and ready for processing;
  • from outside, the function is a node in a network of calculation routines, and we strive for uniformity and simplicity.

Balance these competing forces.

Design functions with meaningful arguments.

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