blog: 2025-08-24-threading-macros

Author: Chemaclass

content/blog/2025-08-24-threading-macros.md

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+title = "Thread-first (->) vs Thread-last (->>)"
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+
+Phel includes two handy threading macros that let you express a sequence of transformations in a linear, readable style.
+
+* `->` (**thread-first**): inserts the previous value as the **first argument** of the next form.
+* `->>` (**thread-last**): inserts the previous value as the **last argument** of the next form.
+
+## Thread-first `->`
+
+Use `->` when the function you call expects the data as its first argument.
+
+```phel
+(-> 5
+    (+ 3)   # becomes (+ 5 3)
+    (* 2))  # becomes (* 8 2)
+=> 16
+```
+
+## Thread-last `->>`
+
+Use `->>` when the function expects the data as its last argument. This is common with functions that operate on sequences such as `map`, `filter` or `reduce`.
+
+```phel
+(->> [1 2 3 4]
+     (map inc)      # becomes (map inc [1 2 3 4])
+     (filter odd?)  # becomes (filter odd? [2 3 4 5])
+     (reduce +))    # becomes (reduce + [3 5])
+=> 8
+```
+
+## When to choose which
+
+Pick the macro that matches the position of the data argument in the next form:
+
+* Use `->` for functions like `inc`, `assoc` or your own functions where the data comes first.
+* Use `->>` for collection-processing functions where the data comes last.
+
+Knowing the difference keeps your pipelines clear and avoids confusing argument order.