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Today's Topics:
1. Re: Why does this list comprehension return an empty list?
(Kim-Ee Yeoh)
2. Re: Why does this list comprehension return an empty list?
(Kim-Ee Yeoh)
3. Re: Shorten this code (Nadav Chernin)
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Message: 1
Date: Wed, 24 Jul 2013 07:52:14 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Why does this list comprehension
return an empty list?
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<CAPY+ZdTUQzyLuLxxGE7JKEvP3w7FWxnkLm=8fmsodpg66_o...@mail.gmail.com>
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On Wed, Jul 24, 2013 at 7:47 AM, Kim-Ee Yeoh <[email protected]> wrote:
> Try: [(a,b) | a <- f xs | b <- g xs]
>
Oops, that'll only work if you define g xs = [[]], not []. So yes, you've
basically got a zip.
-- Kim-Ee
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Message: 2
Date: Wed, 24 Jul 2013 08:07:00 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Why does this list comprehension
return an empty list?
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On Tue, Jul 23, 2013 at 3:53 PM, Costello, Roger L. <[email protected]>wrote:
> I have a list comprehension that extracts the singletons from xs using f
> and g, and creates a pair from their output:
>
> [(a,b) | a <- f xs, b <- g xs]
>
> I executed this and the result is the empty list:
>
> []
>
> That is odd. Why is the empty list the result?
>
For the same reason that the cartesian product of any set with the empty
set is the empty set.
Also,
> I have a list of singletons:
> xs = [("a")]
is immediately suspect, since you have xs:: [String], whereas "a list of
singletons" appears to lean quite a lot more toward [Char], which is just
String.
-- Kim-Ee
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Message: 3
Date: Wed, 24 Jul 2013 09:58:52 +0300
From: Nadav Chernin <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Shorten this code
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Hi, the target of this program is to encode string of [A..Z] characters by
method of Polybius http://en.wikipedia.org/wiki/Polybius_square.
Additionally, most short code will receive more points.
Nadav
On Wed, Jul 24, 2013 at 3:36 AM, Kim-Ee Yeoh <[email protected]> wrote:
>
> On Tue, Jul 23, 2013 at 5:41 PM, Nadav Chernin <[email protected]>wrote:
>
>> Please try to shorten it:
>>
>
> What would you achieve by doing so?
>
> -- Kim-Ee
>
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