Question

I just started playing with Clojure, and I wrote a small script to help me understand some of the functions. It begins like this:

(def *exprs-to-test* [  
    "(filter #(< % 3) '(1 2 3 4 3 2 1))"
    "(remove #(< % 3) '(1 2 3 4 3 2 1))"
    "(distinct '(1 2 3 4 3 2 1))"
])

Then it goes through *exprs-to-test*, evaluates them all, and prints the output like this:

(doseq [exstr *exprs-to-test*]
    (do 
        (println "===" (first (read-string exstr)) "=========================")
        (println "Code: " exstr)
        (println "Eval: " (eval (read-string exstr)))
    )
)

The above code is all working fine. However, (read-string exstr) is repeated so I tried to use let to eliminate the repetition like so:

(doseq [exstr *exprs-to-test*]
    (let [ex (read-string exstr)] (
        (do 
            (println "===" (first ex) "=========================")
            (println "Code: " exstr)
            (println "Eval: " (eval ex))
        )
    ))
)

But this works once for the first item in *exprs-to-test*, then crashes with a NullPointerException. Why is the addition of let causing the crash?

Answer 1

You have an extra set of parentheses around the do form. Your code is doing this:

((do ...))

It’s trying to execute (as a function call) the value of the entire do form, but do is returning nil, because the last println in the do form returns nil.

Note, your indentation style is non-standard. You shouldn’t put the closing parens on their own lines. And let has an implicit do so you don’t need one there. Try this:

user> (doseq [exstr *exprs-to-test*]
        (let [ex (read-string exstr)] 
          (println "===" (first ex) "=========================")
          (println "Code: " exstr)
          (println "Eval: " (eval ex))))
=== filter =========================
Code:  (filter #(< % 3) '(1 2 3 4 3 2 1))
Eval:  (1 2 2 1)
=== remove =========================
Code:  (remove #(< % 3) '(1 2 3 4 3 2 1))
Eval:  (3 4 3)
=== distinct =========================
Code:  (distinct '(1 2 3 4 3 2 1))
Eval:  (1 2 3 4)