Clojure variadic functions, multiple bindings

Question

Given this example:

(defn foo [a & [b c :as args]](prn args))

Is there any way I can add a fourth, optional argument after b and c?

I've tried this and a few other variations with no success (CompilerException java.lang.RuntimeException: Unexpected parameter):

(defn foo [a & [b c :as args d]](prn d))

..another example (CompilerException java.lang.RuntimeException: Unable to resolve symbol: d in this context)

(defn foo [a & [b c :as args & d]](prn d))

Am I hosed after :as args if I want to add more arguments?

EDIT: This was suggested in IRC, and it works, I just wonder if there's a syntactically simpler way...

(defn foo [a & rst] (let [[b c & d] rst args [b c]] {:a a :b b c :args args :rst rst}))

T.Gounelle · Accepted Answer · 2015-04-11 13:19:27Z

If the pair (b, c) has a special relationship, why not put them together in a nested vector []? this will allow to define a specific binding for it (but add a constraint on the input), e.g.

(defn bar [a & [[b c :as args] & rest :as all]]
  {:a a :b b :c c :args args :rest rest :all all})

Then:

> (bar 1)
{:a 1, :b nil, :c nil, :args nil, :rest nil, :all nil}

when you enter the vector [b c] you get the binding with args:

> (bar 1 [2 3])
{:a 1, :b 2, :c 3, :args [2 3], :rest nil, :all ([2 3])}

you can have a vector with one element

> (bar 1 [2] 4)
{:a 1, :b 2, :c nil, :args [2], :rest (4), :all ([2] 4)}

or no element at all

> (bar 1 [] 4 5 6)
{:a 1, :b nil, :c nil, :args [], :rest (4 5 6), :all ([] 4 5 6)}

and with all parameters set

> (bar 1 [2 3] 4 5 6)
{:a 1, :b 2, :c 3, :args [2 3], :rest (4 5 6), :all ([2 3] 4 5 6)}

Leon Grapenthin · Accepted Answer · 2015-04-10 18:00:12Z

You can't control the amount of elements in a destructored rest sequence by destructoring it further.

E. g.

(defn foo [a & [b c :as args]] args)
(foo 1 2 3 4 5)
;; => (2 3 4 5)

Binding the first elements makes no difference. It is the same as if your binding vector was [a & args], only that you have additionally bound the first two elements (b and c) of args by destructoring it further. It is not affected by it.

If you need args to have only the first two variadic args, b and c, and wish to bind the third as an optional d one approach would be:

(defn foo [a & [b c d :as args]]
  (let [args (seq (take 2 args))]
    ;; ...
    ))

Joost Diepenmaat · Accepted Answer · 2015-04-10 16:31:46Z

0

(defn foo [a & [b c d :as args]](prn args))

answered Apr 10, 2015 at 16:31

Joost Diepenmaat

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2 Comments

Calvin Froedge Over a year ago

But now d is part of args, that's not what I want. I want to be able to bind b and c to one thing, and d to something else.

Calvin Froedge Over a year ago

Someone suggested this in IRC: (defn foo [a & rst] (let [[b c & d] rst args [b c]] {:a a :b b :c c :args args :rst rst})), which works, but is quite a bit more complicated = )

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