defining an optional with a default

Question

I'm trying to define a function with optional parameters and each optional also has a default if it is not passed in. but i cannot get it to compile, and cannot find anything on the web to suggest something that will work.

here is what i have

defn test-optional-default-params [manA manB manC & [optA optB optC optD optE]
                                   :or {[optA optAdef
                                        optB optBdef
                                        optC optCdef
                                        optD optDdef
                                        optE optEdef]}]

(prn (str "manB->" manB ", mnC->" manC ", optA->" optA ", optB->" optB ", optC->" optC ", optD->" optD ", optE->" optE)))

Joseph Yourine · Accepted Answer · 2017-01-06T21:03:35.430

2

You can use

(defn f [a b & {:keys [c d e] :or {c defaultc d defaultd e defaulte}}]
  blabla)

But you will have to do

(f 1 2 :c 3 :d 4 :e 5)

So you can maybe do something like this (I tried with a basic function : addition) :

(defn add-rest* [c d e]
  (+ c d e))

(def add-rest*
  (fnil add-rest* 0 0 0))

(defn add
  [a b & [c d e]]
  (+ a b (add-rest c d e)))

(add 1 1) ;; 2
(add 1 1 1) ;; 3
(add 1 1 1 1) ;; 4

edited Jan 06 '17 at 21:03

answered Jan 06 '17 at 09:12

Joseph Yourine

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is the * mean optional all params? – f1wade Jan 06 '17 at 09:45
I think i prefer the first option of 'f ab {:c c etc}' – f1wade Jan 06 '17 at 09:51
No it is just a 'random' name – Joseph Yourine Jan 06 '17 at 21:03

score 1 · Answer 2 · edited May 23 '17 at 12:09

You can use multi-arity functions, so example with addition with default values of 0 becomes:

(defn f+
  ([a b]        (f+ a b 0 0 0))
  ([a b c]      (f+ a b c 0 0))
  ([a b c d]    (f+ a b c d 0))
  ([a b c d e]  (+ a b c d e)))

Replace + above with desired function of five variables. Usage of the above becomes:

(f+ 1 2)         ;; 3
(f+ 1 2 3)       ;; 6
(f+ 1 2 3 4)     ;; 10
(f+ 1 2 3 4 5)   ;; 15

Note: also discussed previously with additional options

defining an optional with a default

2 Answers2