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POHow can I avoid evaluation of a lazy-seq inside a collection
 

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  1. POHow can I avoid evaluation of a lazy-seq inside a collection
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    <p>I'm trying to write a debug macro that prints out expressions and their values. This lead to problems if I send in a lazy-seq, because if I turn it into a string (with str) the program hangs. It's easy to detect a lazy-seq if it's at the toplevel:</p> <pre><code>(def foo (cycle [1 2])) (= (type foo) clojure.lang.LazySeq) ;=&gt; true </code></pre> <p>But of course if it's nested inside another collection this doesn't work</p> <pre><code>(def bar (list (cycle [1 2]))) (= (type bar) clojure.lang.LazySeq) ;=&gt; false </code></pre> <p>To deal with this I would need one of two things:</p> <p>1: A function which checks a collection too see if it contains a lazy-seq nested somewhere.</p> <p>2: A function to turn a collection into a string without evaluating nested lazy-seqs, something like this: </p> <pre><code>(str2 {:inf (cycle [1 2])}) =&gt; "{:inf #clojure.lang.LazySeq@e9383}" </code></pre> <p>Using Michał Marczyk's answer I came up with this macro:</p> <pre><code>(defmacro dbg-print [&amp; rest] "Print out values or expressions in context" `(let [lazy-take# 5 ;when printing lazy-seq, how many elements to print symb-str# (map str '~rest) symb-evl# (reverse (binding [*print-length* 10] (loop [coll# (list ~@rest) retur# '()] (if (not (empty? coll#)) (recur (rest coll#) (cons (pr-str (first coll#)) retur#)) retur#)))) pairs# (map #(str %1 %2 %3 %4) symb-str# (repeat ":") symb-evl# (repeat " ")) str# (reduce str pairs#)] (println (format "%s\n" str#)))) </code></pre> <p>It works like this:</p> <pre><code>(dbg-print (+ 1 3) (cycle [1 2])) ;=&gt; (+ 1 3):4 (cycle [1 2]):(1 2 1 2 1 2 1 2 1 2 ...) </code></pre> <p>And can handle nested lazy-seqs:</p> <pre><code>(dbg-print (list (cycle [1 2]))) ;=&gt; (list (cycle [1 2])):((1 2 1 2 1 2 1 2 1 2 ...)) </code></pre>
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    1. COA practical counter-question: Why would you ever want to debug print a lazy-seq that is infinite? If you don't know about the infinity, it probably is the bug. If you do know about the infinity the more interesting part to debug is where you take something from it (sth. like (dbg-macro (first (partition-by (comp = second) inf-seq)))) . Just saying that a limit like your lazy-take# 5 would worry me more about generating bugs in my thinking while I do the debugging because the debug output would not represent the data that are processed in the program.
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      1. USLeon Grapenthin
    2. COWell you can figure it what it contains by taking something from it, but it's nice to get an immediate overview of the structure. For example I had a lazy-seq that was accidentally wrapped in one list to much. If I take the first of that list, the program will crash. It's nice in general not to have the macro crash because you accidentally fed it a lazy-seq.
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