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Today's Topics:
1. Re: Improve my lambda expressions (PATRICK BROWNE)
2. a way to check whether a file is actually being written on
(Silent Leaf)
3. Re: a way to check whether a file is actually being written
on (Silent Leaf)
4. Re: a way to check whether a file is actually being written
on (Jona Ekenberg)
----------------------------------------------------------------------
Message: 1
Date: Tue, 27 Jun 2017 17:35:11 +0100
From: PATRICK BROWNE <patrick.bro...@dit.ie>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Improve my lambda expressions
Message-ID:
<cagflrke3vxonquvtgd8v9q4o6pg_orvledmfkb7jf5zjxzu...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Thanks for all your help.
I was unaware that there was a relation between let/where and lambdas.
Here is my effort to use a single lamda
md3 n = (\n -> (dist (Point (4.0 + 0.5 * n) (4.0 - 0.5 * n)) (Point (n *
1.0) ( n * (-1.0))))) n
I imagine that this function could be written without lambdas, let, or
where.
Is it generally true the all/most functions could be written without
lambdas, let, or where?
Thanks,
Pat
On 26 June 2017 at 12:04, Frerich Raabe <ra...@froglogic.com> wrote:
> On 2017-06-26 11:38, PATRICK BROWNE wrote:
>
>> The code below provides a distance function that works for points and
>> moving point.
>> I am happy with the result, but I have a problem with the lambda
>> expressions.
>>
>
> [..]
>
> -- Sttic points
>> p1, p2 :: Point Float
>> p1 = Point 0.0 0.0
>> p2 = Point 4.0 4.0
>> d = dist p1 p2
>>
>> -- Moving points
>> mp1, mp2 :: Point Float
>> mp1x = (\t -> 4.0 + 0.5 * t)
>> mp1y = (\t -> 4.0 - 0.5 * t)
>> mp1 = Point (mp1x 2) (mp1y 2)
>> mp2x = (\t -> 0.0 + 1.0 * t)
>> mp2y = (\t -> 0.0 - 1.0 * t)
>> mp2 = Point (mp2x 2) (mp2y 2)
>> md = dist mp1 mp2
>>
>
> Maybe you could reduce the number of lambda expressions by extracting
> common logic. It seems to me that 'mp1' and 'mp2' are moved versions of the
> same point (2,2) except that they apply different functions to the
> coordinates. These functions follow a pattern (a factor is applied to the
> component and then an 'offset' is added).
>
> For instance, it might be worthwhile to define
>
> movePoint :: Float -> Float -> Point -> Point
> movePoint offset factor (Point x y) = Point (offset + factor * x)
> (offset - factor * y)
>
> ...such that you could then define
>
> md = let p = Point 2 2 in dist (movePoint 4 0.5 p) (movePoint 0 1 p)
>
> --
> Frerich Raabe - ra...@froglogic.com
> www.froglogic.com - Multi-Platform GUI Testing
>
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Message: 2
Date: Tue, 27 Jun 2017 21:42:03 +0200
From: Silent Leaf <silent.le...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] a way to check whether a file is actually
being written on
Message-ID:
<cagfccjphrco8loear88jot+ghcadasvsqwfx5dccpsgy2j6...@mail.gmail.com>
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Hi,
i created a small clone of dd in haskell. I made it so it only copies block
by block and only if there's any difference between each pair of blocks
from each file. the idea is to use this dd clone as backup system,
especially since my partitions are nearly full, so no real loss in copying
the whole things.
I'm wondering if there's any way to check if my program never ever writes
onto the target unless actually needed. obviously by reading the code i'd
say it does what i want, but we do make test cases rather than rely on what
we think the code does.
i can't run it with a target file that would be made read-only in the
filesys (and hope for an error for trying to write on it) since obviously i
need to open it in read-write right from the beginning, in case of actual
need of writing (as apparently i can't have two handles on the same file...
although maybe there's a way to change the mode of opening on the run? did
not find it in System.IO nor in Hoogle or Hayoo)
so if anyone has an idea, in or outside of haskell, that would be great!
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Message: 3
Date: Tue, 27 Jun 2017 22:14:24 +0200
From: Silent Leaf <silent.le...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] a way to check whether a file is
actually being written on
Message-ID:
<CAGFccjP6j=9G0fb86xF=2ygtqvekuvjwbf0xemsygfrqsow...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
by the way: dd has this option that says "in case of read error, do not
stop, continue". it's recomended to use it altho i never actually gotten
why.
my point is: how to handle any kind of read/write exception with my clone?
i used withBinaryFile which promised to close my handles no matter what,
which is great, but i don't feel much secured since i don't know how my
program would handle a bumpy ride or even if it would tell me anything... i
don't even know what would happen if it were cut in the middle, etc.
on this general topic, by the way, are there good resources for
system-programming with haskell? is it even manageable to do so rather than
use c-like (go, c++, etc) languages?
2017-06-27 21:42 GMT+02:00 Silent Leaf <silent.le...@gmail.com>:
> Hi,
>
> i created a small clone of dd in haskell. I made it so it only copies
> block by block and only if there's any difference between each pair of
> blocks from each file. the idea is to use this dd clone as backup system,
> especially since my partitions are nearly full, so no real loss in copying
> the whole things.
>
> I'm wondering if there's any way to check if my program never ever writes
> onto the target unless actually needed. obviously by reading the code i'd
> say it does what i want, but we do make test cases rather than rely on what
> we think the code does.
>
> i can't run it with a target file that would be made read-only in the
> filesys (and hope for an error for trying to write on it) since obviously i
> need to open it in read-write right from the beginning, in case of actual
> need of writing (as apparently i can't have two handles on the same file...
> although maybe there's a way to change the mode of opening on the run? did
> not find it in System.IO nor in Hoogle or Hayoo)
>
> so if anyone has an idea, in or outside of haskell, that would be great!
>
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Message: 4
Date: Tue, 27 Jun 2017 22:42:53 +0200
From: Jona Ekenberg <saik...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] a way to check whether a file is
actually being written on
Message-ID:
<calveeuckvechzm_pg_6wbwu6thxsvkhg62zh4rz7-pdxt9g...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Den 27 juni 2017 9:47 em skrev "Silent Leaf" <silent.le...@gmail.com>:
Hi,
i created a small clone of dd in haskell. I made it so it only copies block
by block and only if there's any difference between each pair of blocks
from each file. the idea is to use this dd clone as backup system,
especially since my partitions are nearly full, so no real loss in copying
the whole things.
I'm wondering if there's any way to check if my program never ever writes
onto the target unless actually needed. obviously by reading the code i'd
say it does what i want, but we do make test cases rather than rely on what
we think the code does.
i can't run it with a target file that would be made read-only in the
filesys (and hope for an error for trying to write on it) since obviously i
need to open it in read-write right from the beginning, in case of actual
need of writing (as apparently i can't have two handles on the same file...
although maybe there's a way to change the mode of opening on the run? did
not find it in System.IO nor in Hoogle or Hayoo)
so if anyone has an idea, in or outside of haskell, that would be great!
______________________________
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