Threading macros

Threading macros rewrite a stack of nested calls into a top-to-bottom pipeline. You write the steps in reading order, and the macro plumbs each result into the next call as if you'd written (g (f (h x))) by hand. The two most common are -‌>clojure.core/->Thread the value through forms by inserting it as the first arg.view on clojuredocs → ("thread first") and -‌>‌>clojure.core/->>Thread the value through forms by inserting it as the last arg.view on clojuredocs → ("thread last").

Minimal example

-‌>clojure.core/->Thread the value through forms by inserting it as the first arg.view on clojuredocs → inserts the running value as the first argument of the next form. Useful when chaining map/struct operations:

-‌>‌>clojure.core/->>Thread the value through forms by inserting it as the last arg.view on clojuredocs → inserts the value as the last argument. Useful with sequence operations, because the collection traditionally goes last:

Practical example: choosing the right thread

Pick -‌>clojure.core/->Thread the value through forms by inserting it as the first arg.view on clojuredocs → for "I'm transforming a thing" code, and -‌>‌>clojure.core/->>Thread the value through forms by inserting it as the last arg.view on clojuredocs → for "I'm transforming a collection" code. Mixing them in one pipeline usually means you want a‌s‌-‌>clojure.core/as->Thread with an explicit binding name for flexible insertion.view on clojuredocs →, which gives the running value an explicit name you can put anywhere:

Conditional threading

s‌o‌m‌e‌-‌>clojure.core/some->Threading that short-circuits on nil.view on clojuredocs → short-circuits the moment a step returns nil. cond-> lets you apply transformations only when a predicate holds. Together they handle the "optional pipeline" pattern without nested i‌fclojure.core/ifConditional: (if test then else?).view on clojuredocs →s.

Why this matters

• Without threading, the same logic stacks parentheses inside-out, forcing you to read backwards.
• Threading turns a value-transformation pipeline into a sequence of small, inspectable steps — pause anywhere, you know what's in the "wire."

Check yourself

Exercise

Rewrite the nested expression below using -‌>‌>clojure.core/->>Thread the value through forms by inserting it as the last arg.view on clojuredocs →. It should compute the sum of the squares of the odd numbers up to 10.