Add functional versions of threading macros

CHANGELOG.md

@@ -9,6 +9,7 @@ All notable changes to this project will be documented in this file.
   These streams cannot be directly read to and written from, but can be passed to subprocesses.
 - Add `array/join`
 - Add `tuple/join`
+- Add `fn->` and `fn-?>`
 
 ## 1.35.2 - 2024-06-16
 - Add `bundle/add-bin` to make installing scripts easier. This also establishes a packaging convention for it.

src/boot/boot.janet

@@ -1371,6 +1371,15 @@
     (keep-syntax! n parts))
   (reduce fop x forms))
 
+(defn fn->
+  ``Threading function. Calls the first of the functions it's given with x as
+  the only argument, then calls the second of the functions with the result,
+  and so on. Useful for expressing pipelines of data.``
+  [x & funs]
+  (var x x)
+  (each fun funs (set x (fun x)))
+  x)
+
 (defmacro -?>
   ``Short circuit threading macro. Inserts x as the second value in the first form
   in `forms`, and inserts the modified first form into the second form
@@ -1403,6 +1412,20 @@
     ~(let [,sym ,last] (if ,sym ,(keep-syntax! n parts))))
   (reduce fop x forms))
 
+(defn fn-?>
+  ``Short circuit threading function. Calls the first of the functions it's
+  given with x as the only argument, then calls the second of the functions
+  with the result, and so on. The pipeline will return nil if an intermediate
+  value is nil. Useful for expressing pipelines of data.``
+  [x & funs]
+  (var x x)
+  (var index 0)
+  (def len (length funs))
+  (while (not (or (>= index len) (nil? x)))
+    (set x ((in funs index) x))
+    (++ index))
+  x)
+
 (defn- walk-ind [f form]
   (def ret @[])
   (each x form (array/push ret (f x)))