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Today's Topics:
1. Re: get date (Luca Ciciriello)
2. Re: Signatures (Ertugrul Soeylemez)
3. Category Theory (Matt Andrew)
4. Re: get date (Brandon S Allbery KF8NH)
----------------------------------------------------------------------
Message: 1
Date: Thu, 24 Jun 2010 11:20:55 +0200
From: Luca Ciciriello <luca_cicirie...@hotmail.com>
Subject: Re: [Haskell-beginners] get date
To: Ozgur Akgun <ozgurak...@gmail.com>
Cc: beginners@haskell.org
Message-ID: <blu0-smtp79ff0fc10391336c123a179a...@phx.gbl>
Content-Type: text/plain; charset="us-ascii"
Ok this solve my problem.
I've just missed an IO before String in the type signature.
Thanks.
Luca.
On Jun 24, 2010, at 11:16 AM, Ozgur Akgun wrote:
> Then you might want to use something like:
>
> currentTimeStr :: IO String
> currentTimeStr = do ct <- currentTime
> return ("File creation date: " ++ show ct ++ " by
> MyCompany")
>
> Or more concisely,
>
> currentTimeStr :: IO String
> currentTimeStr = liftM (\ ct -> "File creation date: " ++ show ct ++ " by
> MyCompany" ) currentTime
>
> My point is, the type of currentTimeStr is IO String, not just String.
> Because you are working on an IO value.
>
> Best,
>
> PS: If you are not confident with the piece of code I've suggested, please
> have a look at the io/monads/do-notation sections of the Real World Haskell.
>
>
> On 24 June 2010 11:17, Luca Ciciriello <luca_cicirie...@hotmail.com> wrote:
> Starting from the beginning, my original problem is to create the string:
>
> "File creation date: " ++ currentSystemDate ++ " by MyCompany"
>
> to obtain the string
>
> "File creation date: 2010-06-24 by MyCompany"
>
> Probably this is really trivial, but I'm a real beginner in Haskell and this
> seems to me a big problem.
>
> Luca.
>
> On Jun 24, 2010, at 9:59 AM, Ozgur Akgun wrote:
>
>> On 24 June 2010 10:52, Luca Ciciriello <luca_cicirie...@hotmail.com> wrote:
>> Just a question.
>> How can I obtain a String from
>>
>> currentTime :: IO Day
>> currentTime = utctDay `fmap` getCurrentTime
>>
>> Here currentTime returns to me 2010-06-24, but I want "2010-06-24".
>> In another worlds I need a function
>>
>>
>> I bet Day has a Show instance (that's why you get 2010-06-24 in ghci I
>> suppose)
>> Just use that.
>>
>>
>> currentTimeStr :: IO Day -> String
>>
>>
>> Getting out of IO? I would think again. Following might be what you really
>> want:
>>
>> currentTimeStr :: IO Day -> IO String
>>
>>
>> Any Idea?
>>
>> Luca.
>>
>> Best,
>> Ozgur
>
>
>
>
> --
> Ozgur Akgun
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Message: 2
Date: Thu, 24 Jun 2010 12:04:45 +0200
From: Ertugrul Soeylemez <e...@ertes.de>
Subject: [Haskell-beginners] Re: Signatures
To: beginners@haskell.org
Message-ID: <20100624120445.72454...@tritium.streitmacht.eu>
Content-Type: text/plain; charset=US-ASCII
Daniel Fischer <daniel.is.fisc...@web.de> wrote:
> On Wednesday 23 June 2010 19:01:21, Colin Paul Adams wrote:
> > >>>>> "Patrick" == Patrick LeBoutillier <patrick.leboutill...@gmail.com>
> writes:
> > >>> So
> > >>>
> > >>> nightmare :: Offspring ?
> >
> > If you make
> >
> > Patrick> sex :: Partner -> IO [STD]
> >
> > Patrick> then perhaps
> >
> > Patrick> nightmare :: [STD]
> >
> > Patrick> becomes more universal...
> >
> >
> >
> > What's an STD?
>
> Sexually transmitted disease
Anyway, don't perform IO unsafely. You might regret it. Sex is another
form of -- well -- input and output.
Here are the types:
getWrongWife :: IO BadWife
sex :: F___able a => a -> IO (MVar Offspring, Set Disease)
Of course you don't get offspring, if you don't "take" it.
Greets,
Ertugrul
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://blog.ertes.de/
------------------------------
Message: 3
Date: Thu, 24 Jun 2010 20:33:01 +1000
From: Matt Andrew <mjsa...@gmail.com>
Subject: [Haskell-beginners] Category Theory
To: beginners@haskell.org
Message-ID: <m2fx0cyb36.wl%mjsa...@gmail.com>
Content-Type: text/plain; charset=US-ASCII
Hi all,
I have spend some time over the last couple of days trying to get my head
around category theory, in order to understand concepts like monads a little
better, in order to understand Haskell a little better =) I have grasped, at
least to a degree, the basic concepts of 'category,' 'functor' and 'natural
transformation' but I am still fuzzy on a few specifics and had a couple of
questions. I'm doing this for self-study and have no one to ask these questions
of and so thought I'd ask them here.
My first question is this: where are functors understood to 'live,' or exist?
They operate on categories, are they therefore understood to exist outside of
the categories they operate on? (I understand that functors can form a category
themselves, but I'm asking this question in relation to the categories they
operate on. Are they inside or outside of them?) In other words when I define a
functor in Haskell, are the pair of functions 'fmap,' and whatever unary type
constructor is defined along with it, then part of the morphisms of the Haskell
category? Or are they outside of the Haskell category (where the Haskell
category I've seen defined in the articles I've read is one where its objects
are types and its morphisms functions on those types)?
This leads me to my next question. It seems that all functors in Haskell (or at
least all members of the functor typeclass) are covalent endofunctors. Does
this mean that whenever a new functor is defined in Haskell, that the Haskell
category (H) grows to accomodate the effects of that functor? In other words,
for any covalent endofunctor F : H -> H, do H's morphisms then grow to include
F(f) and objects to include F(a), where a is any object in H and f is any
morphism in H?
I have been working off the assumption that the answer to my last question is
'yes.' The reason for this is natural transformations. Where functors seem to
be able to be 'outside' of the categories they operate on (whether they are in
fact 'outside' or not is my first question), natural transformations appear to
have to be 'inside.' My understanding of the theory is that a natural
transformation is a collection of morphisms in the target category of two
parallel functors (where such morphisms operate on the functors and meet some
specific properties). Surely the only way such a collection of morphisms would
make sense would be if the objects they map to/from (i.e. the structures
imposed by the two functors) were part of that category also?
I hope these questions make sense. I really appreciate anyone who takes to time
to read this!
Matt
------------------------------
Message: 4
Date: Thu, 24 Jun 2010 10:08:50 -0400
From: Brandon S Allbery KF8NH <allb...@ece.cmu.edu>
Subject: Re: [Haskell-beginners] get date
To: beginners@haskell.org
Message-ID: <4c2366f2.3060...@ece.cmu.edu>
Content-Type: text/plain; charset=UTF-8
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On 6/24/10 03:52 , Luca Ciciriello wrote:
> Just a question.
> How can I obtain a String from
>
> currentTime :: IO Day
> currentTime = utctDay `fmap` getCurrentTime
>
> Here currentTime returns to me 2010-06-24, but I want "2010-06-24".
> In another worlds I need a function
>
> currentTimeStr :: IO Day -> String
You can't escape from IO, but you can lift a formatter into IO. Formatters
are in Data.Time.Format; look at the documentation for your system's
strftime() function to see how it works.
- --
brandon s. allbery [linux,solaris,freebsd,perl] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon university KF8NH
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