Threading Macros

Threading Macros: `->` and `->>`

Pipeline composition. Threading macros take a value and pass it through a series of transformations, eliminating deeply nested function calls.

`->` Thread-First

Inserts the threaded value as the first argument of each step:

(-> 5 (+ 3) (* 2))

(5 + 3) * 2   // → 16

The value 5 is threaded into (+ 3) as the first argument → (+ 5 3) → 8. Then 8 is threaded into (* 2) → (* 8 2) → 16.

Method Calls in Threading

Use (:method-name) to call a method on the threaded value:

(-> "hello" (:to-upper-case))

"hello".toUpperCase()   // → "HELLO"

A more complete example:

(-> user
  (get :name)
  (:to-upper-case)
  (:slice 0 3))

This reads: take user, get its :name, uppercase it, take the first 3 characters. Each step flows naturally from the previous one.

`->>` Thread-Last

Inserts the threaded value as the last argument. Used for collection pipelines where the collection comes last:

(->> items
  (:filter (fn (:number x) (> x 0)))
  (:map (fn (:number x) (* x 2))))

When to Use Which

-> for object/method chains — the subject comes first. ->> for collection pipelines — the collection comes last. This follows Clojure convention, where -> and ->> have the same semantics.

Threading macros are Clojure heritage. -> and ->> are standard Clojure forms, adopted by virtually every Clojure-influenced language. The semantics are identical — Lykn adds nothing and changes nothing.

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