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Today's Topics:
1. Using Maybe monad with Zipper (Hugo Ferreira)
2. Re: Combinations of choosing n items from a list (Lorenzo Bolla)
3. (no subject) (Alia)
----------------------------------------------------------------------
Message: 1
Date: Mon, 14 Nov 2011 11:02:49 +0000
From: Hugo Ferreira <[email protected]>
Subject: [Haskell-beginners] Using Maybe monad with Zipper
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-15; format=flowed
Hello,
I picked up code that originally had the following function:
leftCursor :: Z.Zipper a -> Maybe a
leftCursor z = if Z.beginp z then
Nothing
else
Z.safeCursor $ Z.left z
However now I need a functions that returns all three elements
to the left of the zipper. I then considered using composition
and came up with the following:
left :: Maybe (Z.Zipper a) -> Maybe (Z.Zipper a)
left (Just z) = if Z.beginp z
then
Nothing
else
Just $ Z.left z
left Nothing = Nothing
cursor :: Maybe (Z.Zipper a) -> Maybe a
cursor (Just z) = Z.safeCursor z
cursor Nothing = Nothing
which I then used as follows:
instPrev1Of3TagRule :: POSTags -> Maybe TransformationRule
instPrev1Of3TagRule z = do
(_, _, prev1) <- leftCursor z
(_, _, prev2) <- (cursor . left . return) z
(_, _, prev3) <- (cursor . left . left . return) z
(_, correct, incorrect) <- Z.safeCursor z
return $ Prev1Of3TagRule (Replacement incorrect correct) prev1
prev2 prev3
But I was not happy with this for two reasons:
1. I was repeating work needlessly. I should pick up the last
zipper and just move the cursor again once for the next
character.
2. I was not taking advantage of the Maybe Monad
So I finally changed the above to:
left :: Maybe (Z.Zipper a) -> Maybe (Z.Zipper a, a)
left (Just z) = if Z.beginp z
then
Nothing
else
Just $ (Z.left z, Z.cursor z)
left Nothing = Nothing
instPrev1Of3TagRule :: POSTags -> Maybe TransformationRule
instPrev1Of3TagRule z = do
(z1,(_, _, prev1)) <- (left . return) z
(z2, (_, _, prev2)) <- (left . return) z1
(_z3, (_, _, prev3)) <- (left . return) z2
(_, correct, incorrect) <- Z.safeCursor z
return $ Prev1Of3TagRule (Replacement incorrect correct) prev1
prev2 prev3
So my question is, is this correct? Can the above code be made
simpler/cleaner? Somehow the double use of Maybe seems wrong.
TIA,
Hugo F.
------------------------------
Message: 2
Date: Mon, 14 Nov 2011 11:02:56 +0000
From: Lorenzo Bolla <[email protected]>
Subject: Re: [Haskell-beginners] Combinations of choosing n items from
a list
To: [email protected]
Cc: [email protected]
Message-ID:
<CADjgTRy=xy2x2gawzmt5yw_fwjepxa0ynpbl22a-iuvpzqx...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
What about something like:
combinations n = filter (\s -> length s == n) . subsequences
hth,
L.
On Sun, Nov 13, 2011 at 10:42 PM, Peter Hall <[email protected]>wrote:
> Hi all,
> This function took me a long time to write, getting my head around the
> double recursion. It returns a list of all possible sub-sets of a
> given length from a list.
>
> I have a couple of questions.
>
> 1. Can it be improved for efficiency or style?
> 2. Is there a library that already has this and other related
> functions? I assumed there would be but I couldn't find it on hoogle.
>
>
> combinations :: Int -> [a] -> [[a]]
> combinations _ [] = []
> combinations 0 _ = []
> combinations 1 x = map (:[]) x
> combinations n (x:xs) = (map (x:) $ combinations (n-1) xs) ++ combinations
> n xs
>
> e.g.
> > combinations 3 [1..5]
>
> [[1,2,3],[1,2,4],[1,2,5],[1,3,4],[1,3,5],[1,4,5],[2,3,4],[2,3,5],[2,4,5],[3,4,5]]
>
> Thanks,
> Peter
>
> _______________________________________________
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Message: 3
Date: Mon, 14 Nov 2011 05:06:21 -0800 (PST)
From: Alia <[email protected]>
Subject: [Haskell-beginners] (no subject)
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