Advent 2019 part 3, `every-pred` and `some-fn`

This post is part of Advent of Parens 2019 , my attempt to publish one blog post a day during the 24 days of the advent.

Ah clojure.core , it’s like an all you can eat hot-pot. Just when you think you’ve scooped up all it has to offer, you discover another small but delicious delicacy floating in the spicy broth.

In exactly the same way I recently became aware of two functions that until now had only existed on the periphery of my awareness. I’ve since enjoyed using them on several occasions, and keep finding uses for them.

These are every-pred , and some-fn . I’ll demonstrate them with an example. Say you have a discussion forum. People can be admins or moderators, and besides their username they can optionally set a nickname.

(def people [{:name   "Elsie"
              :admin? true
              :mod?   true}
             {:name     "Lin Shiwen"
              :nickname "Rocky"
              :mod?     true}])

If you want to grab people’s nickname if they have one, or fall back to their regular name otherwise, then this is pretty elegant with some-fn .

(map (some-fn :nickname :name) people)
;; => ("Elsie" "Rocky")

If on the other hand you want to find everyone who’s both an admin and a mod, then you can do so easily with every-pred .

(filter (every-pred :admin? :mod?) people)
;; => ({:name "Elsie", :admin? true, :mod? true})

Clojure has a bunch of functions with every and some in their names, which can be a bit confusing. In some? and some-> it refers to a value being something other than nil , whereas in some and some-fn it means “the first one that applies”.

I think this overloading is part of the reason why these two have eluded me so long. Only recently did I have the aha insight that while their names are very different they are really each other’s complement. One combines functions with a logical or , the other does so with a logical and .

(map #(or (:nickname %) (:name %)) people)
;; => ("Elsie" "Rocky")

(filter #(and (:admin? %) (:mod? %)) people)
;; => ({:name "Elsie", :admin? true, :mod? true})

So next time you find yourself writing an anonymous function like this, consider reaching for some-fn or every-pred instead.

The only difference is that every-pred coerces its result to a boolean, making it a little less general purpose. Say we had an every-fn instead, you could combine it with some to get for instance the name of the first admin in the list.

(some (every-fn :admin? :name) people)

The implementation of every-fn is left as an exercise to the reader. (hint: it’s just like every-pred , but without the calls to boolean ).

Comment on ClojureVerse