[Haskell-beginners] Beginners Digest, Vol 100, Issue 20

[Mahdi Dibaiee]

 That's not generally true (but I will give my guess about your assumption after this). See the example in the article I linked:

```

import Data.Char(toUpper)

main = do 
       inpStr <- readFile "input.txt"
       writeFile "output.txt" (map toUpper inpStr)
```

Here, `inputStr` has to be evaluated in order to run `map toUpper` on it,
but it's done character by character. Character is read, `toUpper` is run on it, it's written to the file.
(Though it might not be a character unless we use `NoBuffering`, it's going to be lines or blocks)

Now, I guess your assumption _might_ be true in my case, because `show` might not be able to
generate the string character by character (which is very likely), in this case, show will give us the whole string in one
piece, leading to an out-of-memory error as the big string is getting loaded to memory in order to be used by `writeFile`.

This is just a guess, I would appreciate it if someone could actually prove it.

What do you think Imants?

 

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To: beginners <beginners at haskell.org>
Sent: Sat, Oct 29, 2016 3:44 pm
Subject: Beginners Digest, Vol 100, Issue 20

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From
			
Mahdi Dibaiee <mdibaiee at aol.com>
                    
                    
			
To
			
beginners at haskell.org
                    
                
			
Subject
			
[Haskell-beginners] Writing huge result of `show` to file resultsin out of memory
                
                
			
Date
			
Fri, 28 Oct 2016 12:57:22 -0400
		
	


I know that there are other ways of doing it, I just want to understand
where lies the problem here.

I just read the realworldhaskell book by O'Reilly, in one section, Lazy I/O [0], it's explained
that there should be no problem writing big strings, without consuming the whole string
to files, however big they are.

I would appreciate it if someone could point out the problem there.

Thanks

[0]: http://book.realworldhaskell.org/read/io.html#io.lazy


 
	
Attached Message
	
                    
			
From
			
Imants Cekusins <imantc at gmail.com>
                    
                    
			
To
			
The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners at haskell.org>
                    
                
			
Subject
			
Re: [Haskell-beginners] Writing huge result of `show` to fileresults in out of memory
                
                
			
Date
			
Fri, 28 Oct 2016 19:13:23 +0200
		
	


> there should be no problem writing big strings, without consuming the whole string
​
lazy means evaluated when needed. However when a string is evaluated, it is evaluated fully, I guess.



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