Confused "let" in Clojure

Question

I just started playing with Clojure and I wrote a little script to help me understand some of the features. It starts as follows:

(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 as follows:

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

The above code is working fine. However (read-string exstr), it repeats, so I tried to use letit to eliminate the repetition like this:

(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 NullPointerException. Why letdoes adding fail?

Answer 1

do. :

((do ...))

( ) do, do nil, println do nil.

. . . let do, . :

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)

Answer 2

, : ? , , Clojure:

• (exprs-to-test)
• q/println
• eval

API Clojure - REPL. , Vim, Emacs IDE, REPL. Clojure IDEs.

, , . -, eval. , , . -, , Clojure , "do". "do" , ( - println to * out *) , vars. vars, bindings, vars , concurrency.

, Clojure LISP, , , , Clojure. , Clojure, .

Answer 3

Brian has already answered your question, so I just want to give you general pointers for let-form:

• Special forms (see the section on let)
• Bindings