Threading Macros in Phel: Thread-First vs. Thread-Last

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.

(-> 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.

(->> [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.