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Today's Topics:
1. Re: Guards problem (Stijn Muylle)
2. Re: Guards problem (Magnus Therning)
3. recursive 'let' ? (John M. Dlugosz)
4. Re: recursive 'let' ? (Karl Voelker)
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Message: 1
Date: Thu, 10 Apr 2014 14:20:17 +0200
From: Stijn Muylle <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Guards problem
Message-ID:
<cagvaf9ztsevrecehbqlhtz7ia60rr4nexlm6slg7z+ofmbn...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Found the problem: I had a bug in my implementation (see Marcus' comment).
The solution provided by Dan (using a 2d array) seems better, but right now
I'm just focussing on making it work ;)
On Thu, Apr 10, 2014 at 9:48 AM, Stijn Muylle <[email protected]> wrote:
>
> All,
>
> I ran into a small problem using guards, and I don't really understand
> what I'm doing wrong.
>
> I have this code:
>
> ----------------------------------
> import Data.List.Split
> type Cell = Bool
> data Board = Board [[Cell]] deriving Show
>
> trueList = True : trueList
>
> createBoard :: Int -> Int -> Board
> createBoard x y = Board (chunksOf x (take (x * y) trueList))
>
> getCellAt :: Int -> Int -> Board -> Cell
> getCellAt x y (Board b)
> | x >= (length b) = False
> | y >= (length (b !! 0)) = False
> | otherwise = (b !! x) !! y
>
> -----------------------------------
>
> When I try to execute this:
>
> getCellAt (-1) 0 $ createBoard 3 3
>
> I get a negative index exception. However, as -1 is smaller than 3 (length
> of the board), I would expect False to be returned.
>
> Am I overlooking something?
>
> Thanks!
> Stijn
>
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Message: 2
Date: Thu, 10 Apr 2014 23:19:16 +0200
From: Magnus Therning <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Guards problem
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"
On Thu, Apr 10, 2014 at 02:20:17PM +0200, Stijn Muylle wrote:
> Found the problem: I had a bug in my implementation (see Marcus' comment).
I think that should be "Magnus' comment" ;)
/M
--
Magnus Therning OpenPGP: 0xAB4DFBA4
email: [email protected] jabber: [email protected]
twitter: magthe http://therning.org/magnus
Heuristic is an algorithm in a clown suit. It?s less predictable, it?s more
fun, and it comes without a 30-day, money-back guarantee.
-- Steve McConnell, Code Complete
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Message: 3
Date: Thu, 10 Apr 2014 19:12:25 -0500
From: "John M. Dlugosz" <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] recursive 'let' ?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8; format=flowed
I understand that the definitions introduced by 'let' can be recursive, even
mutually
recursive among several names. Why would you want to do that? I saw contrived
examples,
and wonder why the authors never show a realistic example.
let b = f a c
c = f a b
in ...
I see that makes sense in light of lazy evaluation: b is really an alias for a
(recursive)
function, not a value that needs to find fixed points.
Is this used for common idioms and problem-solving approaches in Haskell?
--John
------------------------------
Message: 4
Date: Thu, 10 Apr 2014 23:50:19 -0700
From: Karl Voelker <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] recursive 'let' ?
Message-ID:
<[email protected]>
Content-Type: text/plain
On Thu, Apr 10, 2014, at 05:12 PM, John M. Dlugosz wrote:
> I understand that the definitions introduced by 'let' can be recursive,
> even mutually
> recursive among several names. Why would you want to do that? I saw
> contrived examples,
> and wonder why the authors never show a realistic example.
>
> let b = f a c
> c = f a b
> in ...
I am a bit confused by your question.
I assume you know about the usefulness of recursion in general, and that
your question is only about recursion in the context of a let-binding.
But it seems to me that there is nothing particularly special about the
let-binding context, as compared to, for example, the top level of a
module. So what seems different about it to you?
Your code snippet makes me think that perhaps you are specifically
interested in definitions of non-function values. Is that so?
-Karl
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