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Today's Topics:
1. Re: Add parenthesis to Sin/Cos expressions
(Sumit Sahrawat, Maths & Computing, IIT (BHU))
2. mapKeysMaybe (Dennis Raddle)
3. Re: mapKeysMaybe (Lyndon Maydwell)
4. Re: mapKeysMaybe (Lyndon Maydwell)
5. Re: mapKeysMaybe (Lyndon Maydwell)
----------------------------------------------------------------------
Message: 1
Date: Thu, 5 Nov 2015 22:46:43 +0530
From: "Sumit Sahrawat, Maths & Computing, IIT (BHU)"
<sumit.sahrawat.ap...@iitbhu.ac.in>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Add parenthesis to Sin/Cos
expressions
Message-ID:
<cajbew8mvjapegzfzhu5rhzo5-ngrz69lusstof6bj6tzfqf...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
The spirit of declarative programming lies in the fact that you try to just
tell the compiler what something means. Instead of thinking about keeping
track of the numbers of parentheses, and then applying them afterwards, I
would suggest you create a function to do the same job:
bracketed :: String -> String
bracketed s = "(" ++ s ++ ")"
This would allow you to remove the numeric argument, and also write cleaner
code.
The second thing that you can improve is the pattern matching. You can use
isPrefixOf
<http://hackage.haskell.org/package/base-4.8.1.0/docs/Data-List.html#v:isPrefixOf>
to
check functions, or use a take inside a case statement to get cleaner
handling of different cases.
case take 3 str of
"sin" -> ...
"cos" -> ...
_ -> ...
This would also let you debug your code more easily, just as you require.
On 4 November 2015 at 14:51, Imants Cekusins <ima...@gmail.com> wrote:
> > a function that is supposed to add parenthesis to sin/cos expressions.
> a) Sin 5 + 3 -> Sin(5) + 3
> b) Sin Cos (5+3) -> Sin (Cos(5+3))
> c) Sin Cos 5 * 3 -> Sin(Cos(5)) * 3
>
> are you looking for textual representation or actual Haskell code which
> runs?
> if actual code, these examples may be re-written like this:
> a) sin 5 + 3
> b) sin $ cos $ 5 + 3
> c) sin (cos 5) * 3
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
--
Regards
Sumit Sahrawat
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Message: 2
Date: Thu, 5 Nov 2015 23:29:36 -0800
From: Dennis Raddle <dennis.rad...@gmail.com>
To: Haskell Beginners <beginners@haskell.org>
Subject: [Haskell-beginners] mapKeysMaybe
Message-ID:
<cakxlvopg4uelsvrvq7z7j_i7wrlezmimtt+sx3abtwoej0h...@mail.gmail.com>
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Can someone give me a clever way to implement mapKeysMaybe, defined as
follows?
Ord k => (k -> Maybe k) -> Map k a -> Map k a
where the transformed keys that evaluate to Just are the only ones kept?
I am thinking this will probably help me understand a new type, like
traversable or something.
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Message: 3
Date: Fri, 6 Nov 2015 18:43:47 +1100
From: Lyndon Maydwell <maydw...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] mapKeysMaybe
Message-ID:
<CAM5QZtxg089PfHuD9kxd4AhekRpJCNK69dd-p=H_q29j0Zi=1...@mail.gmail.com>
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Well, this might not be the most efficient, but:
mapKeysMaybe f = Data.Map.mapMaybeWithKey (const . f)
What do you reckon?
On Fri, Nov 6, 2015 at 6:29 PM, Dennis Raddle <dennis.rad...@gmail.com>
wrote:
> Can someone give me a clever way to implement mapKeysMaybe, defined as
> follows?
>
> Ord k => (k -> Maybe k) -> Map k a -> Map k a
>
> where the transformed keys that evaluate to Just are the only ones kept?
>
> I am thinking this will probably help me understand a new type, like
> traversable or something.
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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Message: 4
Date: Fri, 6 Nov 2015 18:46:40 +1100
From: Lyndon Maydwell <maydw...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] mapKeysMaybe
Message-ID:
<cam5qztzwspo+buoobx8c0wufmo+ptuv_pgy7mf4bodd9f_a...@mail.gmail.com>
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Oops disregard that. The mapMaybeWithKey doesn't quite do what I expected.
On Fri, Nov 6, 2015 at 6:43 PM, Lyndon Maydwell <maydw...@gmail.com> wrote:
> Well, this might not be the most efficient, but:
>
> mapKeysMaybe f = Data.Map.mapMaybeWithKey (const . f)
>
>
> What do you reckon?
>
> On Fri, Nov 6, 2015 at 6:29 PM, Dennis Raddle <dennis.rad...@gmail.com>
> wrote:
>
>> Can someone give me a clever way to implement mapKeysMaybe, defined as
>> follows?
>>
>> Ord k => (k -> Maybe k) -> Map k a -> Map k a
>>
>> where the transformed keys that evaluate to Just are the only ones kept?
>>
>> I am thinking this will probably help me understand a new type, like
>> traversable or something.
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
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Message: 5
Date: Fri, 6 Nov 2015 19:19:03 +1100
From: Lyndon Maydwell <maydw...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] mapKeysMaybe
Message-ID:
<CAM5QZtzcqr2rG1m5ni-hTzMSpgW-5G6+=dal9ycvvd4uzp5...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Here's a more correct solution, unfortunately it's not as nice.
{-# LANGUAGE TupleSections #-}
import Data.Map hiding (mapMaybe)
import Data.Maybe
mkm :: Ord k => (t -> Maybe k) -> Map t a -> Map k a
mkm f = fromList . mapMaybe (\(k,v) -> fmap (,v) (f k)) . toList
On Fri, Nov 6, 2015 at 6:46 PM, Lyndon Maydwell <maydw...@gmail.com> wrote:
> Oops disregard that. The mapMaybeWithKey doesn't quite do what I expected.
>
> On Fri, Nov 6, 2015 at 6:43 PM, Lyndon Maydwell <maydw...@gmail.com>
> wrote:
>
>> Well, this might not be the most efficient, but:
>>
>> mapKeysMaybe f = Data.Map.mapMaybeWithKey (const . f)
>>
>>
>> What do you reckon?
>>
>> On Fri, Nov 6, 2015 at 6:29 PM, Dennis Raddle <dennis.rad...@gmail.com>
>> wrote:
>>
>>> Can someone give me a clever way to implement mapKeysMaybe, defined as
>>> follows?
>>>
>>> Ord k => (k -> Maybe k) -> Map k a -> Map k a
>>>
>>> where the transformed keys that evaluate to Just are the only ones kept?
>>>
>>> I am thinking this will probably help me understand a new type, like
>>> traversable or something.
>>>
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners@haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>
>>>
>>
>
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