How to convert lazy sequence to non-lazy in Clojure

Question

I tried the following in Clojure, expecting to have the class of a non-lazy sequence returned:

(.getClass (doall (take 3 (repeatedly rand))))

However, this still returns clojure.lang.LazySeq. My guess is that doall does evaluate the entire sequence, but returns the original sequence as it's still useful for memoization.

So what is the idiomatic means of creating a non-lazy sequence from a lazy one?

I'm surprised nobody has asked why you are concerned about the actual type of the return value of `doall` — tar, Nov 01 '17 at 17:14
You could convert to a vector: `(vec (take 3 (repeatedly rand)))` — Kris, Jan 23 '18 at 12:33

score 180 · Accepted Answer · edited Oct 02 '22 at 08:30

180

doall is all you need. Just because the seq has type LazySeq doesn't mean it has pending evaluation. Lazy seqs cache their results, so all you need to do is walk the lazy seq once (as doall does) in order to force it all, and thus render it non-lazy. seq does not force the entire collection to be evaluated.

edited Oct 02 '22 at 08:30

Magnar

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answered Oct 29 '09 at 10:41

Rich Hickey

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I've changed this to the accepted answer. On a related note, by what means can you determine if a LazySeq has previously been evaluated? – Tim Clemons Oct 29 '09 at 14:16
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I believe you just call `realized?`. – toofarsideways Feb 01 '12 at 02:40
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There should probably be a `realize` operation to match `realized?`. – Reut Sharabani Feb 01 '17 at 14:07
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This is all very nice. But since some functions like `contains?` don't care whether you realized the lazy seq or not, this answers to the specific question as asked, but less so to the title of the question. – matanster May 15 '17 at 22:29

Arthur Ulfeldt · Answer 2 · 2009-10-29T17:27:25.877

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This is to some degree a question of taxonomy. a lazy sequence is just one type of sequence as is a list, vector or map. So the answer is of course "it depends on what type of non lazy sequence you want to get:
Take your pick from:

an ex-lazy (fully evaluated) lazy sequence (doall ... )
a list for sequential access (apply list (my-lazy-seq)) OR (into () ...)
a vector for later random access (vec (my-lazy-seq))
a map or a set if you have some special purpose.

You can have whatever type of sequence most suites your needs.

edited Oct 29 '09 at 17:27

answered Oct 29 '09 at 17:10

Arthur Ulfeldt

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This is the best answer. – Felipe Jul 25 '14 at 16:20
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The accepted answer is technically correct, but this answer was most useful to me. I was trying to map a function over a vector and then spit the results to a file, and even after calling doall, the file contained "clojure.lang.LazySeq@address" instead of the contents of the sequence. Calling vec on value map returned got me what I needed to spit out to the file. – Jesse Rosalia Nov 23 '15 at 23:17
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@JesseRosalia It's good to know that the one and only Rich Hickey response in all of SO was was technically correct. ;-) – Phil Cooper Mar 15 '16 at 21:02
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`(vec (my-lazy-seq))` is not so nice in situations like the following: `(vec (json/parse-string "{\"foo\":\"bar\"}")) ;; => [["foo" "bar"]]` Since `cheshire` chooses to produce a lazy-seq from `(json/parse-string)` – codeasone Feb 07 '18 at 16:06
Mitigation for the above was to use eager `(json/parse-string-strict)` – codeasone Feb 07 '18 at 16:38

score 24 · Answer 3 · answered Sep 06 '13 at 09:07

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This Rich guy seems to know his clojure and is absolutely right.
Buth I think this code-snippet, using your example, might be a useful complement to this question :

=> (realized? (take 3 (repeatedly rand))) 
false
=> (realized? (doall (take 3 (repeatedly rand)))) 
true

Indeed type has not changed but realization has

answered Sep 06 '13 at 09:07

Peter

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It's worth noting, though, that you don't need to force the entire sequence for `realized?` to return `true`. E.g. `(let [r (range) r? (realized? r)] (doall (take 1 r)) [r? (realized? r)]) => [false true]` – Alex Coventry Apr 21 '14 at 17:45
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This Rich guy :D haha – Julian Krispel-Samsel Aug 26 '14 at 04:40
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@nimrod :) the pun however was meant to be in "hís clojure". – Peter Nov 04 '14 at 08:31
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For those who don't know, "the Rich guy" invented Clojure. – erturne Nov 16 '14 at 01:42
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@AlexCoventry your example returns `[true true]` – Jan 24 '16 at 11:38
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Huh, the behavior must have changed since I wrote that comment. – Alex Coventry Feb 08 '16 at 22:11

score 8 · Answer 4 · edited Aug 27 '21 at 22:22

8

I stumbled on this this blog post about doall not being recursive. For that I found the first comment in the post did the trick. Something along the lines of:

(use 'clojure.walk)
(postwalk identity nested-lazy-thing)

I found this useful in a unit test where I wanted to force evaluation of some nested applications of map to force an error condition.

edited Aug 27 '21 at 22:22

SiKing

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answered Aug 29 '15 at 20:32

leeor

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score 5 · Answer 5 · edited Aug 16 '11 at 02:14

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(.getClass (into '() (take 3 (repeatedly rand))))

edited Aug 16 '11 at 02:14

agf

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answered Oct 29 '09 at 16:03

stupito

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This is a terrible idea. It reverses the input seq. – amalloy Aug 16 '11 at 17:52
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Of course, in this case reversing the input makes no difference, since they are just 3 random numbers.... :-) – mikera Jan 03 '13 at 00:24

How to convert lazy sequence to non-lazy in Clojure

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