Reduce a set of functions over a value?

Question

Hi,

I'm looking for a clean, idiomatic way to do a "backwards reduce" in Clojure.

I've got

(def fns '(fn1 fn2 fn3))
(def val 42)

I'd like to obtain (fn3 (fn2 (fn1 val))), and I'm not picky about the order. So I'd like to consecutively apply a sequence of functions to a value, rather than consecutively a sequence of values to a function.

Suggestions? Thanks!

Answer 1

I'm not a very advanced Clojure user either, so the language of the API description tends to look a bit terse and cryptic to me. But given my limited understanding, the "->" macro struck my eye. It "threads" a value x through a series of "forms," which looks to me suspiciously similar to what you're after.

Since it's a macro, I'm not sure if you can use (apply) to push it inside your list of fn's, but I'd give it a shot!

Answer 2

how about ...

(defn back-reduce
([firstfunction restfunctions val]
    (if (= restfunctions nil)
        (firstfunction val)
        (back-reduce (car restfunctions) (cdr restfunctions) (firstfunction val))  
    )
))

Answer 3

This is just reduce. Remember functions are objects too. Also, you are still applying your functions in order f1, then f2, etc. Don't be confused with the fact that it reads (f4 (f3 ...

So just use a regular reduce.

(reduce #(%2 %1) val functions)

Answer 4

Yet another way to think about it is as a composition of functions:

user=> ((apply comp [inc dec inc]) 42)
43

Answer 5

Simplest way to write it down in Clojure would be:

((comp fn3 fn2 fn1) 42)