2.1 KiB
Specter is useful for navigating through nested data structures, and then returning (selecting) or transforming what it finds. Specter can also work its magic recursively. Many of Specter's most important and powerful use cases in your codebase will require you to use Specter's recursive features. However, just as recursion can be difficult to grok at first, using Specter recursively can be challenging. This guide is designed to help you feel at home using Specter recursively.
A Review of Recursion
Most simple functions do not call themselves. For example:
(defn square [x] (* x x))
Instead of calling square, itself, square calls another function, * or multiply. This, of course, serves the purpose of squaring the input. You don't always need recursion. But sometimes you need a function to call itself - to recur. To take another example from mathematics, you need recursion to implement factorials. Factorials are equal to the product of every positive number equal to or less than the number. For example, the factorial of 5 ("5!") is 5 * 4 * 3 * 2 * 1 = 120.
Here is a simple implementation of factorials in Clojure:
=> (defn factorial [x]
(if (< x 2)
1
(* x (factorial (dec x)))))
#'playground.specter/factorial
=> (factorial 5)
120
Here, the function definition calls itself with the symbol factorial. You still supply input - the value x - but producing the function's return value requires calling the function itself.
We can see that there are up to three elements to basic recursion:
- a name or symbol for the recursion point or function (defn function-name)
- optional input arguments
- the body of the recursion - what you do recursively - and, at some point, calling the function by its name
Using Specter Recursively
Using Specter recursively uses these two elements: a name (self-sym or self-symbol), and arguments (params or parameters). But it adds a third element: the path that the
This is actually similar to the function body.
requires three elements: a name (self-sym), arguments