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Threading Macros

Nested calls can become hard to manage. Threading macros help us take control. In this chapter, we'll learn about the concept of threading macros and study three commonly-used versions.

Threading Macros#

The term thread has got nothing to do with parallel programming. Thread is an expression for a linear series of operations, like pearls on a thread.

As your functions grow in complexity, the readability of your code is affected. Suppose you have a list of books with various attributes, and you want to find the number of books related to each unrestricted book:

The complexity of that map operation can easily grow beyond control. What happens if we want the publishing year of all related books?:

We performed the following operations:

  • filtered all books that are restricted

  • called publish-year-by-id function on each element of :related vector

Threading macros help decomplect nested function calls.

thread-last macro (->>)#

The thread-last macro makes nested function calls easier to express. A function call:

can be expressed as:

In thread-last macro expansion, the result of a form is passed as the last argument to the next form. Let's rewrite our publish-year-by-id with the thread-last macro:

Notice how we are able to use the first function and :publish-year keyword without parentheses. You can add parentheses if you wish to, but Clojure assumes parentheses in cases where a callable form is passed. Most developers find forms using threading macros more readable.

Let's rewrite the other operation using the thread-last macro too:

 

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