Nullpointer in clojure when running doseq with multiple expressions in the body

clojure

Solution

Well, you already figured out the solution, so just a few hints to explain the behavior:

In Clojure (just like in Lisp, Scheme, etc) everything is an expression and an expression is either an atom or a list. With regard to lists, the Clojure manual says

Non-empty Lists are considered calls to either special forms, macros, or functions. A call has the form (operator operands*).

In your example, the body `((println x) (println x))` is a list and the operator is itself an expression which Clojure has to evaluate to obtain the actual operator. That is, you're saying "evaluate the first expression and take its return value as a function to invoke upon the second expression". However, `println` returns, as you noticed, only `nil`. This leads to the `NullPointerException` if `nil` is interpreted as an operator.

Your code works with `(do (println x) (println x))` because `do` is a special form which evaluates each expression in turn and returns the value of the last expression. Here `do` is the operator and the expressions with `println` ar the operands.

To understand the usefulness of this behavior, note that functions are first-class objects in Clojure, e.g., you could return a function as a result from another function. For instance, take the following code:

(doseq [x '(1 2 3 4)] ((if (x > 2)
    (fn [x] (println (+ x 2)))
    (fn [x] (println (* x 3)))) x))

Here, I am dynamically figuring out the operator to invoke upon the element in the sequence. First, the `if`-expression is evaluated. If `x` is larger than two, the `if` evalutes to the function that prints `x + 2`, else it evaluates to the function that prints `x * 3`. This function is than applied to the `x` of the sequence.

Problem

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