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Today's Topics:
1. Re: Beginners Digest, Vol 108, Issue 12 (Ning Yin)
2. Re: a way to check whether a file is actually being written
on (Silent Leaf)
3. Re: Improve my lambda expressions (Frerich Raabe)
----------------------------------------------------------------------
Message: 1
Date: Wed, 28 Jun 2017 13:39:28 +0800
From: Ning Yin <[email protected]>
To: "[email protected]" <[email protected]>
Subject: Re: [Haskell-beginners] Beginners Digest, Vol 108, Issue 12
Message-ID: <[email protected]>
Content-Type: text/plain; charset=gb2312
发自我的 iPad
> 在 2017年6月24日，20:00，[email protected] 写道：
>
> Send Beginners mailing list submissions to
> [email protected]
>
> To subscribe or unsubscribe via the World Wide Web, visit
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> or, via email, send a message with subject or body 'help' to
> [email protected]
>
> You can reach the person managing the list at
> [email protected]
>
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Beginners digest..."
>
>
> Today's Topics:
>
> 1. Re: help designing types for a gsl fit (David McBride)
> 2. What to use when you need random values? (Silent Leaf)
> 3. Re: What to use when you need random values? (David McBride)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 23 Jun 2017 09:58:35 -0400
> From: David McBride <[email protected]>
> To: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell <[email protected]>
> Subject: Re: [Haskell-beginners] help designing types for a gsl fit
> Message-ID:
> <can+tr42lkxcggzkvfzcsmxuk79t39f36_ena4cwdmp1px_g...@mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> If you look at the type of labelNew
>
> GlibString string => Maybe string -> IO Label
>
> If you look at the instances for GlibString, they could be Text or
> [Char]. You have to decide which. The fact that you are using
> Nothing does not tell you the entire final type. It could be Maybe
> [Char], or Maybe Text. Even though the choice seems arbitrary in this
> instance, you have to decide which it is. So try this.
>
> lprog <- G.labelNew (Nothing :: Maybe [Char])
>
> On Fri, Jun 23, 2017 at 3:22 AM, Agustin Larreinegabe
> <[email protected]> wrote:
>> Hello,
>> I'm trying to install an application Termite - Debug but I get this error in
>> line 929:14
>>
>> when it try to do this
>>
>>
>>
>> lprog <- G.labelNew Nothing
>>
>>
>> The error says:
>>
>> Could not deduce (glib-0.13.4.1:System.Glib.UTFString.GlibString string0)
>> arising from a use of ‘G.labelNew’ from the context (D.Rel c v a s) bound by
>> the type signature for sourceWindowCreate :: D.Rel c v a s => RSourceView c
>> a u -> IO G.Widget at Debug/SourceView.hs:913:23-73 instance
>> glib-0.13.4.1:System.Glib.UTFString.GlibString [Char] -- Defined in
>> ‘glib-0.13.4.1:System.Glib.UTFString’ In a stmt of a 'do' block: lprog <-
>> G.labelNew Nothing In the expression: do { vbox <- G.vBoxNew False 0;
>> G.widgetShow vbox; spec <- getIORef svInputSpec ref; code <- codeWinNew
>> spec; .... } In an equation for ‘sourceWindowCreate’: sourceWindowCreate ref
>> = do { vbox <- G.vBoxNew False 0; G.widgetShow vbox; spec <- getIORef
>> svInputSpec ref; cabal: Error: some packages failed to install:
>>
>> I really don't know how to proceed, I'm new with Haskell
>>
>>
>> Thanks in advance.
>>
>>
>> -----------------
>> Agustin Larreinegabe
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
> ------------------------------
>
> Message: 2
> Date: Fri, 23 Jun 2017 16:24:07 +0200
> From: Silent Leaf <[email protected]>
> To: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell <[email protected]>
> Subject: [Haskell-beginners] What to use when you need random values?
> Message-ID:
> <cagfccjp6ai7x_n9+agqgggjgamwtqou-+ynsubdhaen4sp+...@mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Hi,
> I've had trouble finding the best way(s) to use random values in haskell,
> as it seems like there are several modules that either do the same thing or
> reuse one another i'm not sure.
>
> There is System.Random
> - is it better to use the streams random(R)s or a more imperative randomRIO?
> - is it better to use mkStdGen or newStdGen or getStdGen?
> There is Test.QuickCheck and its type(class?) Gen
> There is a module in Control.Monad (i think) which exports the type Rnd
>
> What about performances, and all those options? What do you like to use
> with random numbers?
>
> I know that's a lot of questions. feel free to only answer to a few of
> them. :)
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>
> Message: 3
> Date: Fri, 23 Jun 2017 11:10:05 -0400
> From: David McBride <[email protected]>
> To: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell <[email protected]>
> Subject: Re: [Haskell-beginners] What to use when you need random
> values?
> Message-ID:
> <CAN+Tr43UcOVbES4Sw9BdhuYuu-iYekc2nsCxT0tx5nKp2uTM=g...@mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> When you are unsure about the differences between functions, it can be
> good to read the haddocks for the library.
>
> http://hackage.haskell.org/package/random-1.1/docs/System-Random.html
>
> The standard haskell random library supports the idea of splitting a
> seed randomly. You take one seed and split it, and now you have two
> seeds, which will each generate different randoms independently.
> getStdGen gets the current global seed. newStdGen splits new a seed
> off of the current global seed. mkStdGen allows you to create a seed
> from a value so that you can get the same set of randoms repeatedly.
>
> I would say if you are in IO, just use randomRIO. If you are in
> monadic code that not IO at its base, you should use MonadRandom
> library on hackage. Quickcheck randomness is only really used in
> quickcheck, although it is probably based off the standard libraries.
>
> Just keep in mind that randomness is a concept that is a little hard
> to wrap your head around in haskell until you've been using it a
> little while.
>
>> On Fri, Jun 23, 2017 at 10:24 AM, Silent Leaf <[email protected]> wrote:
>> Hi,
>> I've had trouble finding the best way(s) to use random values in haskell, as
>> it seems like there are several modules that either do the same thing or
>> reuse one another i'm not sure.
>>
>> There is System.Random
>> - is it better to use the streams random(R)s or a more imperative randomRIO?
>> - is it better to use mkStdGen or newStdGen or getStdGen?
>> There is Test.QuickCheck and its type(class?) Gen
>> There is a module in Control.Monad (i think) which exports the type Rnd
>>
>> What about performances, and all those options? What do you like to use with
>> random numbers?
>>
>> I know that's a lot of questions. feel free to only answer to a few of them.
>> :)
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
> ------------------------------
>
> Subject: Digest Footer
>
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> ------------------------------
>
> End of Beginners Digest, Vol 108, Issue 12
> ******************************************
------------------------------
Message: 2
Date: Wed, 28 Jun 2017 08:07:19 +0200
From: Silent Leaf <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] a way to check whether a file is
actually being written on
Message-ID:
<CAGFccjNhycqAS358oWL=ypfltzqyxsukdp-kcmmcsdyzxap...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
strace sounds really great, i'll test it, thanks :)
2017-06-27 22:42 GMT+02:00 Jona Ekenberg <[email protected]>:
>
>
> Den 27 juni 2017 9:47 em skrev "Silent Leaf" <[email protected]>:
>
> 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!
> ______________________________
>
> Maybe you can use strace? https://youtu.be/4pEHfGKB-OE
>
>
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------------------------------
Message: 3
Date: Wed, 28 Jun 2017 08:26:20 +0200
From: Frerich Raabe <[email protected]>
To: [email protected], The Haskell-Beginners Mailing List -
Discussion of primarily beginner-level topics related to Haskell
<[email protected]>
Subject: Re: [Haskell-beginners] Improve my lambda expressions
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII; format=flowed
On 2017-06-27 18:35, PATRICK BROWNE wrote:
> 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.
Indeed, it could. Note that your definition has the form
md3 n = (\n -> (dist )) n
I.e. the expression 'md3 n' is equivalent to the expression '(\n -> (dist
..)) n', which means 'apply the lambda expression to n'. You don't need the
lambda expression if you apply it to a given argument directly though, i.e.
the above definition is equivalent to
md3 n = dist ..
> Is it generally true the all/most functions could be written without
> lambdas, let, or where?
I believe it is true since you could define any function as a global
definition (i.e. not a nested scope as in let..in or where).
--
Frerich Raabe - [email protected]
www.froglogic.com - Multi-Platform GUI Testing
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