Laziness in filter

2013-03-17 Thread bruce li 
Hello, everyone. I'm writing some code that utilizes the lazy sequence. But
I found something strange. Here is how:

The code is like:

(first (filter some-expensive-io urls))

The code is aimed to find the first result of the operations on the urls
that is not nil. However, it seems that the io operations are executed once
more than needed. As the operations are slow, one more round increases the
overhead dramatically.

Then I tested other pieces of code, such as:

(first (filter #(when ( % 1) (println %) %) (range)))

It prints out:
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31

and returns: 2

So why the lazy sequence is realized more than it is needed? Could I
enforce the laziness and save unnecessary operation?


Thanks,
Bruce Li

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Re: Laziness in filter

2013-03-17 Thread Evan Mezeske 
I'd guess that what you're seeing is related to chunked 
sequences: http://blog.fogus.me/2010/01/22/de-chunkifying-sequences-in-clojure/ 
.

On Sunday, March 17, 2013 1:12:17 AM UTC-7, bruce li wrote:

 Hello, everyone. I'm writing some code that utilizes the lazy sequence. 
 But I found something strange. Here is how:

 The code is like:

 (first (filter some-expensive-io urls))

 The code is aimed to find the first result of the operations on the urls 
 that is not nil. However, it seems that the io operations are executed once 
 more than needed. As the operations are slow, one more round increases the 
 overhead dramatically.

 Then I tested other pieces of code, such as:

 (first (filter #(when ( % 1) (println %) %) (range)))

 It prints out:
 2
 3
 4
 5
 6
 7
 8
 9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31

 and returns: 2

 So why the lazy sequence is realized more than it is needed? Could I 
 enforce the laziness and save unnecessary operation?


 Thanks,
 Bruce Li


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Re: Laziness in filter

2013-03-17 Thread dennis zhuang 
Yep,it's chunked sequence,just like batch processing.
You can use the seq1 function in fogus blog.

2013/3/17 Evan Mezeske emeze...@gmail.com

 I'd guess that what you're seeing is related to chunked sequences:
 http://blog.fogus.me/2010/01/22/de-chunkifying-sequences-in-clojure/ .


 On Sunday, March 17, 2013 1:12:17 AM UTC-7, bruce li wrote:

 Hello, everyone. I'm writing some code that utilizes the lazy sequence.
 But I found something strange. Here is how:

 The code is like:

 (first (filter some-expensive-io urls))

 The code is aimed to find the first result of the operations on the urls
 that is not nil. However, it seems that the io operations are executed once
 more than needed. As the operations are slow, one more round increases the
 overhead dramatically.

 Then I tested other pieces of code, such as:

 (first (filter #(when ( % 1) (println %) %) (range)))

 It prints out:
 2
 3
 4
 5
 6
 7
 8
 9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31

 and returns: 2

 So why the lazy sequence is realized more than it is needed? Could I
 enforce the laziness and save unnecessary operation?


 Thanks,
 Bruce Li

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Re: Laziness in filter

2013-03-17 Thread Marko Topolnik 
This is one of the most frequenly-asked questions and a source of surprise 
to practically every new Clojure user. An update to the official 
documentation on lazy sequences would surely help a lot here.

-marko

On Sunday, March 17, 2013 9:18:05 AM UTC+1, Evan Mezeske wrote:

 I'd guess that what you're seeing is related to chunked sequences: 
 http://blog.fogus.me/2010/01/22/de-chunkifying-sequences-in-clojure/ .

 On Sunday, March 17, 2013 1:12:17 AM UTC-7, bruce li wrote:

 Hello, everyone. I'm writing some code that utilizes the lazy sequence. 
 But I found something strange. Here is how:

 The code is like:

 (first (filter some-expensive-io urls))

 The code is aimed to find the first result of the operations on the urls 
 that is not nil. However, it seems that the io operations are executed once 
 more than needed. As the operations are slow, one more round increases the 
 overhead dramatically.

 Then I tested other pieces of code, such as:

 (first (filter #(when ( % 1) (println %) %) (range)))

 It prints out:
 2
 3
 4
 5
 6
 7
 8
 9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31

 and returns: 2

 So why the lazy sequence is realized more than it is needed? Could I 
 enforce the laziness and save unnecessary operation?


 Thanks,
 Bruce Li



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Re: Laziness in filter

2013-03-17 Thread bruce li 
Ah, it works. It is really chunked sequences. Thanks.

Having been using clojure for half a year, it keeps really bringing me
surprise and fun :)

2013/3/17 Marko Topolnik marko.topol...@gmail.com

 This is one of the most frequenly-asked questions and a source of surprise
 to practically every new Clojure user. An update to the official
 documentation on lazy sequences would surely help a lot here.

 -marko


 On Sunday, March 17, 2013 9:18:05 AM UTC+1, Evan Mezeske wrote:

 I'd guess that what you're seeing is related to chunked sequences:
 http://blog.fogus.**me/2010/01/22/de-chunkifying-**sequences-in-clojure/http://blog.fogus.me/2010/01/22/de-chunkifying-sequences-in-clojure/.

 On Sunday, March 17, 2013 1:12:17 AM UTC-7, bruce li wrote:

 Hello, everyone. I'm writing some code that utilizes the lazy sequence.
 But I found something strange. Here is how:

 The code is like:

 (first (filter some-expensive-io urls))

 The code is aimed to find the first result of the operations on the urls
 that is not nil. However, it seems that the io operations are executed once
 more than needed. As the operations are slow, one more round increases the
 overhead dramatically.

 Then I tested other pieces of code, such as:

 (first (filter #(when ( % 1) (println %) %) (range)))

 It prints out:
 2
 3
 4
 5
 6
 7
 8
 9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31

 and returns: 2

 So why the lazy sequence is realized more than it is needed? Could I
 enforce the laziness and save unnecessary operation?


 Thanks,
 Bruce Li

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