如何从Clojure中的first获取默认值？

Question

(> 1 (first [])) returns NullPointerException.

How can I make (first []) return a default value, such as 0, instead of nil?

Answer 1

You can use or to bypass the nil value

(> 1 (or (first []) 0))

Because in Clojure, nil is treated as a falsy value.

Answer 2

The or solution is good for this case. For cases where or is not sufficient, another option is to use the with-exception-default macro from the Tupelo library :

(with-exception-default default-val & body)
 "Evaluates body & returns its result.  In the event of an exception the
  specified default value is returned instead of the exception."

(with-exception-default 0
  (Long/parseLong "12xy3"))
;=> 0

Answer 3

Use fnil :

A function that replaces a nil second argument to > with 0 is ...

(fnil > nil 0)

So that, for instance,

((fnil > nil 0) 1 (first []))
=> true

Answer 4

imho you should clearly define the functional objects in your design

since you need a functionality: given a collection coll extract the first element or default to 0 then you should have a separate function for that.

eg

(defn first-or-zero [coll] 
 (if (seq coll)
   (first coll) 0))

Although a bit cumbersome to write (and the or macro does seem to get you there quicker you are missing out on the powerful concept that is FP.

a) doing this way you have a pure functional description of what you need to do b) you can test it either by proof or by unit-testing c) you can reuse it all over the place with minimum impact on change

A more FP way of doing it would be:

(defn first-or-default 
  ([coll] (first-or-default coll 0))
  ([coll dflt-val] 
   (if (seq coll)
      (first coll) dflt-val)))

Then just call: (< 1 (first-or-default coll 0))