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Today's Topics:
1. Traverse tree with computing current level using Foldable
instance. (Dmitriy Matrosov)
2. haskell for system administration (Ben)
3. Re: haskell for system administration (Mike Meyer)
4. Re: [x] and (x:_) for lists -- did you ever think that odd?
(AntC)
5. Re: [x] and (x:_) for lists -- did you ever think that odd?
(AntC)
6. Re: [x] and (x:_) for lists -- did you ever think that odd?
(Ertugrul S?ylemez)
7. Re: [x] and (x:_) for lists -- did you ever think that odd?
(Ertugrul S?ylemez)
8. Re: Traverse tree with computing current level using
Foldable instance. (Brent Yorgey)
----------------------------------------------------------------------
Message: 1
Date: Mon, 21 May 2012 14:28:03 +0400
From: Dmitriy Matrosov <[email protected]>
Subject: [Haskell-beginners] Traverse tree with computing current
level using Foldable instance.
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hi.
I can't figure out how should i properly solve the following problem.
There is a tree defined like
data Tape a = Tape a [Tape a]
and i want to traverse it in some specific order, computing at the same
time current level (depth). I.e. it should like fold, and folding
function should have access to current level in the tree. Here is my
implementation:
import Data.Monoid
import Control.Monad.State
type TapeState a = State Int a
foldMapS2 :: (Monoid m) => (a -> TapeState m) -> TapeState
(Tape a) -> TapeState m
foldMapS2 f tt = do
t@(Tape name ts) <- tt
foldr (go f) (f name) ts
where
go :: (Monoid m) => (a -> TapeState m) -> Tape a
-> TapeState m -> TapeState m
go f t mz = do
cs <- get
x <- foldMapS2 f (State (\s -> (t, s + 1)))
put cs
z <- mz
put cs
return (x `mappend` z)
and here is example usage
testTape = Tape "A" [ Tape "B" [ Tape "C" []
, Tape "F" [Tape "G"
[Tape "H" []]]
, Tape "E" []
]
, Tape "D" [ Tape "I" []]
]
*Main> runState (foldMapS2 (\name -> get >>= \cs -> if cs == 0 then
return [name] else return mempty) (return (testTape))) 0
(["A"],0)
*Main> runState (foldMapS2 (\name -> get >>= \cs -> if cs == 1 then
return [name] else return mempty) (return (testTape))) 0
(["B","D"],0)
*Main> runState (foldMapS2 (\name -> get >>= \cs -> if cs == 2 then
return [name] else return mempty) (return (testTape))) 0
(["C","F","E","I"],0)
*Main> runState (foldMapS2 (\name -> get >>= \cs -> if cs == 3 then
return [name] else return mempty) (return (testTape))) 0
(["G"],0)
*Main> runState (foldMapS2 (\name -> get >>= \cs -> if cs == 4 then
return [name] else return mempty) (return (testTape))) 0
(["H"],0)
*Main> runState (foldMapS2 (\name -> get >>= \cs -> if cs == 5 then
return [name] else return mempty) (return (testTape))) 0
([],0)
As you can see, this just selects all elements at particular tree level.
So, my foldMapS2 looks similar to foldMap from Foldable, but i can't
figure out, how should i define instances of Foldable (and Monoid?) to
achieve the same functionality?
------------------------------
Message: 2
Date: Mon, 21 May 2012 19:43:47 +0530
From: Ben <[email protected]>
Subject: [Haskell-beginners] haskell for system administration
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hi,
I am new to HASKELL.Currently I am using shell scripting for
Administration tasks and all.Kindly suggest me , does HASKELL usefull
for Linux system administration and reporting purpose.?
Do we use HASKELL for network programming and all?
Regards,
Ben
------------------------------
Message: 3
Date: Mon, 21 May 2012 10:28:45 -0400
From: Mike Meyer <[email protected]>
Subject: Re: [Haskell-beginners] haskell for system administration
To: Ben <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII
On Mon, 21 May 2012 19:43:47 +0530
Ben <[email protected]> wrote:
> I am new to HASKELL.Currently I am using shell scripting for
> Administration tasks and all.Kindly suggest me , does HASKELL usefull
> for Linux system administration and reporting purpose.?
Yes, it is. I wrote "eddie" to let me leverage Haskell list processing
from the command line. The standard Unix tools generally cover this
kind of thing fairly well, so it's only marginally useful. Windows
users seem to find more use for it. http://code.google.com/eddie.
> Do we use HASKELL for network programming and all?
Yup. You can find the code I wrote to extract statistical information
on NCAA FBS seasons from the web at
http://blog.mired.org/2012/01/analysis-of-fbs-controversies-part-i.html.
While doing the fetch is a bit more complicated than using fetch (or
curl, or whatever), the ease of analysis in Haskell and having the
data already loaded when you want to start analysis makes up for that.
<mike
--
Mike Meyer <[email protected]> http://www.mired.org/
Independent Software developer/SCM consultant, email for more information.
O< ascii ribbon campaign - stop html mail - www.asciiribbon.org
------------------------------
Message: 4
Date: Tue, 22 May 2012 00:36:47 +0000 (UTC)
From: AntC <[email protected]>
Subject: Re: [Haskell-beginners] [x] and (x:_) for lists -- did you
ever think that odd?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Brandon Allbery <allbery.b <at> gmail.com> writes:
> You missed experienced Lispers, who are likewise entirely comfortable with
cons-like notation.
Good point Brandon, I should have asked what background and expectations
people were coming from. From ALGOL, square brackets mean arrays, which are
close-ish to lists(?)
(For Lispers: does it seem odd using colon? Does it seem odd that dot is
function composition, not cons pairing?)
I was more getting at the oddness (or perhaps not) of having two different
notations.
>
> I would want to very carefully check the implications of your suggestion:
No, I wasn't anywhere near making a "suggestion" or "proposal". Just asking
for people's impressions.
> it might not integrate well with the rest of pattern matching syntax, ...
FWIW, the double-dot notation is not currently valid in a pattern.
I wanted to show something with square brackets. And to my mind, the double-
dot could be taken as an unknown-length tail of the list, like an ellipsis.
But others (it seems) think of it as enumFrom.
AntC
------------------------------
Message: 5
Date: Tue, 22 May 2012 00:49:40 +0000 (UTC)
From: AntC <[email protected]>
Subject: Re: [Haskell-beginners] [x] and (x:_) for lists -- did you
ever think that odd?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
Ertugrul S?ylemez <es <at> ertes.de> writes:
[snip ..]
> We have access to the two list constructors (:) and []
> directly and they are very convenient,
So Ertugrul, as an experienced Haskeller which of these do you put in your
code for a singleton list (whether appearing in a pattern or as an expression):
[x]
(x: [])
Or perhaps you mix them, depending on whether you want to be more suggestive
of an empty list [] or an unknown-length list (x: xs)?
[.. snippety-snip]
AntC
------------------------------
Message: 6
Date: Tue, 22 May 2012 03:02:10 +0200
From: Ertugrul S?ylemez <[email protected]>
Subject: Re: [Haskell-beginners] [x] and (x:_) for lists -- did you
ever think that odd?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
AntC <[email protected]> wrote:
> So Ertugrul, as an experienced Haskeller which of these do you put in
> your code for a singleton list (whether appearing in a pattern or as
> an expression): [x]
> (x: [])
I tend to use bracket notation for fixed-length list patterns.
> Or perhaps you mix them, depending on whether you want to be more
> suggestive of an empty list [] or an unknown-length list (x: xs)?
Yes. It's quite seldom that I use bracket notation, though. That's
simply because it's seldom that I pattern-match against fixed-length
lists.
Greets,
Ertugrul
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Message: 7
Date: Tue, 22 May 2012 03:04:30 +0200
From: Ertugrul S?ylemez <[email protected]>
Subject: Re: [Haskell-beginners] [x] and (x:_) for lists -- did you
ever think that odd?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
Tom Murphy <[email protected]> wrote:
> I think it was just a joke.
While a discussion is going on off the list, I would like to publicly
apologize for my rough tone. I felt offended by the last remark.
Of course I'm not taking back my statement, I just feel like it doesn't
really apply to Anthony's original post.
For questions like this I would recommend writing it in such a way that
the intention is clear. It was somehow blurred by the rhetorical style.
I also hope that people aren't offended by my mail signature, which
indeed is a bit offensive. =)
Greets,
Ertugrul
--
Key-ID: E5DD8D11 "Ertugrul Soeylemez <[email protected]>"
FPrint: BD28 3E3F BE63 BADD 4157 9134 D56A 37FA E5DD 8D11
Keysrv: hkp://subkeys.pgp.net/
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------------------------------
Message: 8
Date: Mon, 21 May 2012 22:18:34 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Traverse tree with computing current
level using Foldable instance.
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
>
> As you can see, this just selects all elements at particular tree level.
>
> So, my foldMapS2 looks similar to foldMap from Foldable, but i can't
> figure out, how should i define instances of Foldable (and Monoid?)
> to achieve the same functionality?
You cannot. Foldable is not general enough; it does not allow you to
define folds which can observe the *structure* of the container being
folded over (such as the level in a tree, the number of children of a
given node, etc.). It corresponds to simply "flattening" the
structure into a list of elements, turning each of them into a value
of some monoid, and then applying mconcat.
However, you should be able to define a general fold for Tape, with
type
foldTape :: (a -> [b] -> b) -> Tape a -> b
and then define foldMapS2 in terms of foldTape.
-Brent
------------------------------
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