Functional Programming in q

Introduction

Functional programming is a paradigm that emphasizes the use of pure functions and immutable data structures. It promotes code reusability, modularity, and readability. While kdb+ is primarily known for its vectorized operations, it also offers powerful functional programming capabilities. This chapter explores the functional aspects of kdb+ and how to leverage them for efficient and elegant code.

Pure Functions

A pure function is a function that always produces the same output for a given input and has no side effects. This means it doesn't modify any external state or rely on external variables.

Code snippet

f:{[x] x*2}  / Pure function to double a value
f[3]        / Output: 6

In this example, f is a pure function because it always returns double the input value, and it doesn't modify any external variables.

Higher-Order Functions

Higher-order functions are functions that take other functions as arguments or return functions as results.

Code snippet

apply:{[f;x] f[x]}  / Apply function f to x
apply[f;4]          / Output: 8

Here, apply is a higher-order function that takes a function f and a value x as arguments, applies f to x, and returns the result.

Recursion

Recursion is a technique where a function calls itself directly or indirectly.

Code snippet

factorial:{[n] if[n<=1; 1; n*factorial[n-1]]}
factorial[5]  / Output: 120

The factorial function calculates the factorial of a number recursively.

Lambda Functions

Lambda functions are anonymous functions defined inline.

Code snippet

{x*2}[5]  / Output: 10

This lambda function squares the input value and applies it to 5.

Functional Operators

kdb+ provides several functional operators that make it easy to apply functions to lists:

• each: Applies a function to each element of a list.Code snippet

  {x*2} each 1 2 3  / Output: 2 4 6

• over: Applies a function pairwise to two lists.Code snippet

  {x+y} over (1 2 3; 4 5 6)  / Output: 5 7 9

• where: Filters a list based on a predicate function.Code snippet

  {x>2} where 1 2 3 4  / Output: 3 4

Functional Composition

Functional composition involves combining multiple functions into a single function.

Code snippet

f:{[x] x*2}
g:{[x] x+1}
h:{[x] g[f[x]]}
h[3]  / Output: 7

The h function composes f and g, applying f first and then g.

Advanced Topics

• Currying: Creating new functions by partially applying arguments.

• Monads: Structures that represent computations with side effects in a pure context.

• Functional Reactive Programming (FRP): A programming paradigm for managing stateful systems in a declarative way.

Conclusion

Functional programming offers a powerful and expressive way to write code in kdb+. By understanding and applying these concepts, you can write cleaner, more maintainable, and often more efficient code. While kdb+ is primarily known for its vectorized operations, combining functional programming with vectorization can lead to even greater performance gains.