Hi--
The code in question is a solution to http://www.4clojure.com/problem/58
(write a function that does function composition, without using 'comp').
The solution I'm trying to understand is by amalloy:
(defn mycomp [& fs]
(reduce (fn [f g]
#(f (apply g %&)))
fs))
For example ((mycomp inc first reverse) [1 2 3 4]) would return the value 5.
Here are the notes I've written so far, my attempt to explain how this code
is evaluated, using ((mycomp inc first reverse) [1 2 3 4]) as the code to
be evaluated:
----- begin
; name the function used by reduce 'f1'
(defn f1 [f g]
#(f (apply g %&)))
; the collection used by reduce is:
[inc first reverse]
; first iteration of `reduce`; f = inc, g = first
; (f1 inc first) returns #(inc (apply first %&))
; the collection is now:
[first reverse]
; next iteration of reduce; f = #(inc (apply first %&)), g = reverse
; (f1 f g) returns:
#( #(inc (apply first %&)) (apply reverse %&) )
<------ function ----> <-- arg, sorta -->
; -- BTW, this is invalid as Clojure code
; now, apply this function to the data:
( #( #(inc (apply first %&)) (apply reverse %&) ) [1 2 3 4]) ; #1
( #(inc (apply first %&)) (apply reverse ([1 2 3 4]) )) ; #2
(inc (apply first ((apply reverse ([1 2 3 4]) )))) ; #3
(inc (apply first (( reverse [1 2 3 4]) ))) ; #4
(inc (apply first (( 4 3 2 1)))) ; #5
(inc ( first ( 4 3 2 1))) ; #6
(inc 4) ; #7
RESULT = 5
----- end
(FYI, I know that you can't nest anonymous functions. Consider the above to
be a thought experiment.)
Is this a good way to think of how the code is evaluated?
I have to admit, I'm not sure I'm thinking about the code correctly. The
code recurses until it gets to the rightmost function, then it is finally
able to execute (apply reverse ([1 2 3 4])) (see #3 above) and begin moving
"up and out of" the nested recursion.
When you look at the function given as the first argument to 'recurse', (fn
[f g] #(f (apply g %&))), how do you think about when '%&' is replaced by [1
2 3 4]? Does this happen only when 'recurse' has "consumed" all the items
in the collection it's been given (as the second argument)?
I'd appreciate any elaborations, observations, corrections, or alternate
explanations you would care to add.
Many thanks!
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