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Today's Topics:
1. Rewriting bits of a data structure (Adrian May)
2. Re: Rewriting bits of a data structure (Kim-Ee Yeoh)
3. Re: Rewriting bits of a data structure (Daniel Trstenjak)
4. Re: Rewriting bits of a data structure (Kim-Ee Yeoh)
5. Re: Rewriting bits of a data structure (Adrian May)
----------------------------------------------------------------------
Message: 1
Date: Wed, 22 May 2013 00:35:35 +0800
From: Adrian May <[email protected]>
Subject: [Haskell-beginners] Rewriting bits of a data structure
To: "[email protected]" <[email protected]>
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<CAD-UbzHDvzV_LAGyp+paxf=wj0+-v_fjvydoyg_12ebsdg_...@mail.gmail.com>
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Hi All,
I have a lot of ugly code like this:
data Thing = Thing Int Int Int Int Int
rewriteFourth :: Int -> Thing -> Thing
rewriteFourth x (Thing a b c _ e) = Thing a b c x e
Is there a better way?
It was thinking about state monads that reminded me of this junk that I'd
already written a while ago. Maybe it can be done by applying five StateT
transformers, but I'm not entirely sure how to write that, or if there's a
simpler fix.
TIA,
Adrian.
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Message: 2
Date: Tue, 21 May 2013 23:41:19 +0700
From: Kim-Ee Yeoh <[email protected]>
Subject: Re: [Haskell-beginners] Rewriting bits of a data structure
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
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On Tue, May 21, 2013 at 11:35 PM, Adrian May <[email protected]
> wrote:
> Is there a better way?
>
Yup. One word: lenses. If you're interested in the history of the design
space: semantic editor combinators, fclabels, data.accessors.
-- Kim-Ee
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Message: 3
Date: Tue, 21 May 2013 21:08:47 +0200
From: Daniel Trstenjak <[email protected]>
Subject: Re: [Haskell-beginners] Rewriting bits of a data structure
To: [email protected]
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Hi Adrian,
> data Thing = Thing Int Int Int Int Int
> rewriteFourth :: Int -> Thing -> Thing
> rewriteFourth x (Thing a b c _ e) = Thing a b c x e
>
> Is there a better way?
If you've nested data structures, than lenses are the way to go,
but in your case using record syntax might also help.
data Thing = Thing {
a :: Int,
b :: Int,
c :: Int,
d :: Int,
e :: Int
}
rewriteFourth :: Int -> Thing -> Thing
rewriteFourth x thing = thing {d = x}
Greetings,
Daniel
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Message: 4
Date: Wed, 22 May 2013 04:43:58 +0700
From: Kim-Ee Yeoh <[email protected]>
Subject: Re: [Haskell-beginners] Rewriting bits of a data structure
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
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On Wed, May 22, 2013 at 2:08 AM, Daniel Trstenjak <
[email protected]> wrote:
> If you've nested data structures, than lenses are the way to go,
> but in your case using record syntax might also help.
>
Yes, agreed. Keep to the lightest solutions first.
-- Kim-Ee
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Message: 5
Date: Wed, 22 May 2013 07:16:46 +0800
From: Adrian May <[email protected]>
Subject: Re: [Haskell-beginners] Rewriting bits of a data structure
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
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Aha! I was using record syntax but I didn't know I could use it like that.
Thanks! Now for that lenses thing.
On 22 May 2013 03:11, "Daniel Trstenjak" <[email protected]> wrote:
>
> Hi Adrian,
>
> > data Thing = Thing Int Int Int Int Int
> > rewriteFourth :: Int -> Thing -> Thing
> > rewriteFourth x (Thing a b c _ e) = Thing a b c x e
> >
> > Is there a better way?
>
> If you've nested data structures, than lenses are the way to go,
> but in your case using record syntax might also help.
>
> data Thing = Thing {
> a :: Int,
> b :: Int,
> c :: Int,
> d :: Int,
> e :: Int
> }
>
> rewriteFourth :: Int -> Thing -> Thing
> rewriteFourth x thing = thing {d = x}
>
>
> Greetings,
> Daniel
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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