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Today's Topics:
1. Re: Understanding some notation (Jack Henahan)
2. Re: Understanding some notation (Benjamin Edwards)
----------------------------------------------------------------------
Message: 1
Date: Sun, 26 Jun 2011 18:48:00 -0400
From: Jack Henahan <[email protected]>
Subject: Re: [Haskell-beginners] Understanding some notation
To: Mike Meyer <[email protected]>
Cc: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"
I know the type signature of (++), I'm just not seeing `n` as a list, I
suppose. I'm reading it as being the Int that is passed to splitAt, though
perhaps my thought process is just wonky.
On Jun 26, 2011, at 6:41 PM, Mike Meyer wrote:
> On Sun, 26 Jun 2011 18:14:44 -0400
> Jack Henahan <[email protected]> wrote:
>
>> I have this code snippet:
>>
>> import Data.List
>>
>> aaa x (y:ys) = case splitAt x (y:ys) of
>> (n, x:xs) -> x:n ++ xs
>> (n, xs) -> n ++ xs
>>
>> I understand what it's meant to do (that is, split a list at index `x` and
>> make a new list with that element at the head, or just return the list when
>> given a singleton), but my brain is failing me when trying to read the
>> notation `n ++ xs`.
>>
>> Is there some obvious explanation that I'm just forgetting?
>
> Yes. But more importantly, you're forgetting that you can ask the REPL:
>
> Prelude> :t (++)
> (++) :: [a] -> [a] -> [a]
>
> So it takes two lists and produces a new one. Trying it:
>
> Prelude> [1, 2, 3] ++ [4, 5, 6]
> [1,2,3,4,5,6]
>
> So ++ concatenates it's operands.
>
> <mike
> --
> Mike Meyer <[email protected]> http://www.mired.org/
> Independent Software developer/SCM consultant, email for more information.
>
> O< ascii ribbon campaign - stop html mail - www.asciiribbon.org
====
"Computer Science is no more about computers than astronomy is about
telescopes."
-- Edsger Dijkstra
====
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Message: 2
Date: Sun, 26 Jun 2011 23:55:53 +0100
From: Benjamin Edwards <[email protected]>
Subject: Re: [Haskell-beginners] Understanding some notation
To: Jack Henahan <[email protected]>
Cc: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
On 26 June 2011 23:48, Jack Henahan <[email protected]> wrote:
> I know the type signature of (++), I'm just not seeing `n` as a list, I
> suppose. I'm reading it as being the Int that is passed to splitAt, though
> perhaps my thought process is just wonky.
>
Look at the type of splitAt, the answers are all in the types :)
splitAt :: Int -> [a] -> ([a],[a])
So when you do a case match on the result of
splitAt x (y:ys)
you must be matching on a tuple of lists, because that's what the return
type of the list is
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