Run two commands in a row after an if statement in Clojure

Question

Why does the following Clojure program throw a NullPointerException?

user=> (defn x []  
       "Do two things if the expression is true."
       (if true ((println "first expr") (println "second expr")) false))

user=> (x)
first expr
java.lang.NullPointerException (NO_SOURCE_FILE:0)
second expr

This is a simplified version of my actual use case, where I want to execute maybe three statements (pull values from the DB) before returning a map - {:status 200, :body "Hello World"} inside of the branch.

possible duplicate of Nullpointer in clojure when running doseq with multiple expressions in the body — amalloy
– amalloy, Commented Sep 26, 2011 at 6:19

Kirk Broadhurst · Accepted Answer · 2011-09-26 02:22:06Z

36

It is trying to treat the result of the first println as a function to call on the second println function.

You need a do.

(defn x []  
   "Do two things if the expression is true."
   (if true (do (println "first expr") (println "second expr")) false))

(x)

The do special form (progn in CL, begin in Scheme) executes each of its arguments in sequence and returns the result of the last one.

edited Sep 26, 2011 at 2:22

Kirk Broadhurst

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answered Sep 26, 2011 at 1:24

Random832

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3 Comments

Random832 Over a year ago

Yes, generally. A few forms might imply it (Scheme cond implies it, for instance, but I don't think Clojure's version does since its arguments aren't in lists). ((foo ...) (bar ...)) generally implies taking the result of foo and using it as a function to call the rest of the arguments.

Kevin Burke Over a year ago

So does this mean that do will execute the statements sequentially? (it'll assume they have side effects)?

Hugh Over a year ago

do will indeed execute its statements sequentially.

Dave Ray · Accepted Answer · 2011-09-26 01:48:00Z

11

If nil is ok as a return value in the else case, consider using when which has an implicit do block:

(defn x []  
  "Do two things if the expression is true."
  (when true
    (println "first expr") 
    (println "second expr")))

answered Sep 26, 2011 at 1:48

Dave Ray

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Comments

bmillare · Accepted Answer · 2011-09-26 02:58:23Z

7

Not that it matters in your particular case, but do know the difference between (do ...) which will load each form in its own classloader, and an empty let form (let [] ...) which evaluates the whole form in a single classloader.

answered Sep 26, 2011 at 2:58

bmillare

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2 Comments

Sridhar Ratnakumar Over a year ago

What is the practical difference between loading each form in its own classloader vs otherwise? Maybe I don't understand the implications of classloader.

bmillare Over a year ago

One example, you are writing a macro that defines a custom deftype and multiple classes. As an optimization, you would like to use the same classloader so that less classloader instances are created.

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