Point-free Style revisited

As it turns out you shouldn't listen to me that much. I messed up completely with my previous post „Pointfree vs. Pointless“. As Chouser points out: My claims about partial and comp creating classes are totally wrong. Here's why…

Macros' Revenge

What does partial actually do. It takes a function and some arguments and returns another function, which when called appends its arguments to the given ones and passes everything on to the given function. So this

(let [part-f (partial f :a :b)]
  (part-f :c :d))

is basically equivalent to

(f :a :b :c :d)

Another way to achieve this, is

(let [sharp-f #(f :a :b %1 %2)]
  (sharp-f :c :d))

So the #() reader macro can be used to implement partial.

(defn partial
  [f & args]
  #(apply f (concat args %&)))

Question: What is the difference between part-f and sharp-f? Right. part-f does not create a new class while sharp-f does.

The definition of partial creates actually two classes: one for partial itself and one for the returned function. However, the second class is always the same! For all calls to partial! So it actually does not create a new class when it's called, but only an instance of the second class.

Now the #() version on the other hand is actually a reader macro expanding into a fn form. This actually does create a new class each time it is used! So five #()s create five new classes, while five partials create only five instances of the same class.


Be humble. Understand before you start blogging.

Published by Meikel Brandmeyer on .