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Today's Topics:
1. Re: R/W from/to partitions (on linux) (eyeinsky .)
----------------------------------------------------------------------
Message: 1
Date: Mon, 26 Jun 2017 19:12:15 +0200
From: "eyeinsky ." <eyeins...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] R/W from/to partitions (on linux)
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<CADkv3aH9-vPKUqVK4uwkEAHB_4TENtKQvpu=p4bjwk2j_xl...@mail.gmail.com>
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AFAIK, in the list comparison case you mention, it will just build and
destroy the lists until it reaches the end or finds a non-equal pair and
returns False at that point. So no keeping the list in memory, since
computing equality for lists doesn't require it.
On Jun 26, 2017 2:39 PM, "Silent Leaf" <silent.le...@gmail.com> wrote:
> btw i just tried my comparison of lists, and it seems there's an
> intermediate type holding the concept of boundaries and so on, aka it does
> not need to compute them beyond comparing their boundaries. clever haskell!
> a bit too much, you never know how smart it exactly is...
>
> 2017-06-26 13:33 GMT+02:00 Silent Leaf <silent.le...@gmail.com>:
>
>> i'm trying to get for which purpose BS.strict and BS.lazy are adapted. am
>> i right to think that:
>>
>> lazy bytestrings are not optimal when needing to read huge files of
>> several gigabytes, bc they are only lazy to the extent that the very long
>> bytestring is not read to the end. aka it's like haskell's natural
>> laziness, if i compute [0..2^20] == [0..2^20] it won't just take a lot of
>> time but will also (try to) keep in memory both huge lists, aka the
>> operation is not intrinsically on minimal resources, comparing each item of
>> each list and discarding the previous ones at the same time; more generally
>> the lists or bytestrings, be they lazy, are still only deallocated when the
>> whole variable (the whole list/array) is dereferenced.
>>
>> thus if i need say to read very huge files and compare them together
>> block by block, it's better for me to use strict bytestrings for each pair
>> of chunks and manually move the handle from one block to the other, looping
>> over the whole file. correct?
>>
>> 2017-06-26 12:53 GMT+02:00 Silent Leaf <silent.le...@gmail.com>:
>>
>>> yes, conduits really seem optimal! i'll probably use that in my final
>>> version, thanks!
>>> however i'll first try without using pre-made tools, to get the handle
>>> (ha) of manually using (binary) files with haskell, as after all my program
>>> isn that complicated.
>>>
>>> 2017-06-26 10:42 GMT+02:00 Stefan Risberg <steffeno...@gmail.com>:
>>>
>>>> I would use some streaming library instead of lazy bytestring to keep
>>>> memory at reasonably low levels. It will also help with reading chunks, and
>>>> then composing actions on it.
>>>>
>>>> For library need you got: conduits, iostreams and pipes
>>>>
>>>>
>>>>
>>>>
>>>> On 26 Jun. 2017 10:37, "Silent Leaf" <silent.le...@gmail.com> wrote:
>>>>
>>>> i'm reading on the doc of BS.Lazy.hGetContents:
>>>> "Once EOF is encountered, the Handle is closed."
>>>>
>>>> what does that imply if i'm using it inside of withFile? no risk of
>>>> getting prematurely out of the function right? that doesn seem possible in
>>>> a pure function but i'm asking either way.
>>>> if say i do something that reads the whole file, say calculating its
>>>> length, does it mean since EOF will be reached i'll have to open the file
>>>> again? i think i'm a bit lost...
>>>>
>>>> i'm trying to find how to read big chunks of two files, do stuff with
>>>> each pair of chunk, and so on till the EOF, which may or may not happen at
>>>> the same time for both... i don't really know how lazy bytestrings handle,
>>>> for example, taking too much from a file. one way would be to calculate the
>>>> length, ofc, but for files (partition) of several dozens of gigabytes, it's
>>>> a bit delicate... the ideal would be to get the length from the system
>>>> itself rather than calculate the whole string ...
>>>>
>>>> 2017-06-26 7:57 GMT+02:00 Silent Leaf <silent.le...@gmail.com>:
>>>>
>>>>> Darn quick answer! Thanks Sylvain, that may be all i need to start!
>>>>>
>>>>> 2017-06-26 7:51 GMT+02:00 Sylvain Henry <sylv...@haskus.fr>:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> It is not Haskell specific. You just have to read from the partition
>>>>>> device special file (e.g., something like /dev/sdb2) as you would do
>>>>>> with a
>>>>>> normal file. You must have the permission to do so (e.g., be root). Be
>>>>>> careful as you can destroy your system if you write something incorrect
>>>>>> in
>>>>>> your partitions.
>>>>>>
>>>>>> Repositioning handles: https://www.stackage.org/haddo
>>>>>> ck/lts-8.20/base-4.9.1.0/System-IO.html#g:13
>>>>>>
>>>>>> Read/write: https://www.stackage.org/haddo
>>>>>> ck/lts-8.20/base-4.9.1.0/System-IO.html#v:hPutBuf
>>>>>>
>>>>>> Sylvain
>>>>>>
>>>>>> On 26/06/2017 07:35, Silent Leaf wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> I'd like to be able to read and write from/to partitions directly.
>>>>>> I've had trouble with the documentation (honestly i can't find anything,
>>>>>> and any mention of partitions leads to mathematical partitioning of lists
>>>>>> or whatever).
>>>>>>
>>>>>> I obviously would need to be able to write or read from a specific
>>>>>> position in the partition. Mind you that would be good too for files
>>>>>> (that
>>>>>> is, being able to read/write from a specific position in it) since i plan
>>>>>> on making disk images.
>>>>>>
>>>>>> Thanks in advance!
>>>>>>
>>>>>>
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>>>>>
>>>>
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>>>
>>
>
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