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Today's Topics:
1. Re: What is operator :| ? (Francesco Ariis)
2. Re: What is operator :| ? (John M. Dlugosz)
3. Re: Array/Vector like DS in haskell (John M. Dlugosz)
4. Re: What is operator :| ? (Francesco Ariis)
5. random monad (Dennis Raddle)
6. algorithms books or tutorial in haskell (Nishant)
7. Re: random monad (Nikita Danilenko)
----------------------------------------------------------------------
Message: 1
Date: Fri, 28 Mar 2014 07:48:28 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] What is operator :| ?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Fri, Mar 28, 2014 at 12:54:03AM -0500, John M. Dlugosz wrote:
> More generally, is there some effective way to search for
> non-alphabetical Haskell things? Google just ignores the
> "punctuation".
>
You can find :| on Hayoo [1], the other handy place where to
look for APIs stuff being Hoogle [2] (Hoogle is more focused on
'standard' Haskell libraries, Hayoo searches in all Hackage,
both have their usefulness).
>From there, if I need to search, say, a blog post, I will refer
to the name of the typeclass/module/data and feed it to a search
engine (so in this case "data NonEmpty etc. etc.").
Apparently Google ignores punctuation in most cases [3].
Back to the original question
> What is the meaning of :| ?
Clicking on the first occurrence in Hoogle brings me to
Data.List.NonEmpty, where the specific operator is listed as
the lone constructor of |data NonEmpty|.
If the conspicuous name were not enough, checking the code:
data NonEmpty a = a :| [a]
we can see this looks like a list without the empty-list
constructor.
Does that answer your question?
[1] http://holumbus.fh-wedel.de/hayoo/hayoo.html#0:%3A|
[2] http://www.haskell.org/hoogle/
[3] https://support.google.com/websearch/answer/2466433
[4]
http://hackage.haskell.org/package/semigroups-0.12.2/docs/Data-List-NonEmpty.html#v::-124-
------------------------------
Message: 2
Date: Fri, 28 Mar 2014 01:58:11 -0500
From: "John M. Dlugosz" <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] What is operator :| ?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=windows-1252; format=flowed
On 3/28/2014 1:41 AM, Tony Morris wrote:
> It is the constructor for a non-empty list (Data.List.NonEmpty).
>
> http://hackage.haskell.org/package/semigroups-0.12.2/docs/Data-List-NonEmpty.html
>
Interesting? this is the page I had found:
http://www.haskell.org/haskellwiki/Non-empty_list
So why do we need both :| and <| (or cons) ?
------------------------------
Message: 3
Date: Fri, 28 Mar 2014 01:59:26 -0500
From: "John M. Dlugosz" <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Array/Vector like DS in haskell
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
On 3/28/2014 12:30 AM, Nishant wrote:
> Can someone elaborate on how to create a DS which acts like an array or
> vector and has
> same time complexity ?
>
>
> A list though can act like array but getting value at index i is linear time
> but in actual
> array it is constant.
I was just reading http://www.haskell.org/haskellwiki/Arrays
------------------------------
Message: 4
Date: Fri, 28 Mar 2014 08:21:46 +0100
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] What is operator :| ?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
On Fri, Mar 28, 2014 at 01:58:11AM -0500, John M. Dlugosz wrote:
> On 3/28/2014 1:41 AM, Tony Morris wrote:
> >It is the constructor for a non-empty list (Data.List.NonEmpty).
> >
> >http://hackage.haskell.org/package/semigroups-0.12.2/docs/Data-List-NonEmpty.html
> >
>
> Interesting? this is the page I had found:
> http://www.haskell.org/haskellwiki/Non-empty_list
>
>
> So why do we need both :| and <| (or cons) ?
>
Look at the definitions and type signatures:
(<|) :: a -> NonEmpty a -> NonEmpty a
a <| ~(b :| bs) = a :| b : bs
(:|) :: a -> [a] -> NonEmpty a
data NonEmpty a = a :| [a]
(I found the type-sig for (:|) using ghci ":t").
It is now evident that the constructor (:|) takes element/list to
build |NonEmpty a|, while (<|) appends an |a| to a |NonEmpty a|.
------------------------------
Message: 5
Date: Fri, 28 Mar 2014 02:10:51 -0700
From: Dennis Raddle <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] random monad
Message-ID:
<cakxlvopcz9l3zkchsbv0vakbeyceae7+w5ovhqyleldowxk...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I'm writing a program which uses a lot of pseudorandom numbers, and for
that reason it would be nice to put the StdGen in a state monad.
like let's say I want to combine error handling and storing the StdGen
state.
import Control.Monad.Error
import Control.Monad.State
data RandState = RandState StdGen
-- Er is a monad that combines error handling and pseudorandom state
type Er a = ErrorT String (State RandState) a
-- to access random numbers, I could define things like
erRandomR :: Random r => (r,r) -> Er r
erRandomR (lo, hi) = do
RandState g <- get
let (value, g') = randomR (lo, hi) g
put $ RandState g'
return value
erRandoms :: Random r => Er [r]
erRandoms = do
RandState g <- get
let (g1, g2) = split g
let values = randoms g1
put $ RandState g2
return values
erRandomRs :: Random r => (r,r) -> Er [r]
erRandomRs (lo,hi) = do
RandState g <- get
let (g1, g2) = split g
let values = randomRs (lo,hi) g1
put $ RandState g2
return values
-- I could define new ways of using random values, like choosing a random
element of a list
erChooseList :: [a] -> Er a
erChooseList xs = do
let l = length xs
when (l==0) (throwError "in randomChooseList, passed null list")
idx <- erRandomR (0,l-1)
return $ xs !! idx
However, after I got done with that, I realized that I wanted to add
additional state, maybe a ReaderT , stuff like that--different in different
parts of the program. But I always want access to random numbers with the
same functions: erRandomR, erRandoms, etc.
So I thought
class Monad m => RandMonad m where
putGen :: StdGen -> m ()
getGen :: m StdGen
Then I could make Er an instance of RandMonad, like this
type Er a = ErrorT String (State RandState) a
instance RandMonad Er where
putGen g = put (RandState g)
getGen = do RandState g <- get
return g
Clearly I don't know what I'm doing, because when I tried to run this much
I got the error
"Type synonym Er should have 1 argument, but has been given none."
I tried a bunch of variations of this but got nowhere. Can someone explain
how I should conceive of this?
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Message: 6
Date: Fri, 28 Mar 2014 15:15:43 +0530
From: Nishant <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] algorithms books or tutorial in haskell
Message-ID:
<CAEQDmz4nUsWr0Kb_=xKHoQicBiaFkTo_SYOJqacvj3sN=ay...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Hi,
Can someone provide me link to some pdf or online tutorial where I can find
backtracking / dynamic programming kind of algorithms discussed in details.
Regards.
Nishant
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Message: 7
Date: Fri, 28 Mar 2014 10:46:37 +0100
From: Nikita Danilenko <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] random monad
Message-ID: <[email protected]>
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