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Today's Topics:
1. Re: Better Code (Graham Gill)
2. Re: Better Code (Jeffrey Brown)
3. safe versions of pred and succ? (Graham Gill)
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Message: 1
Date: Fri, 13 Jan 2017 19:09:49 -0500
From: Graham Gill <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Better Code
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
Sorry, I mean, "groups together consecutive and equal terms, that occur
sequentially". Examples speak louder than general descriptions!
On 13-Jan-2017 6:43 PM, Graham Gill wrote:
> Here's one that does what you want, doesn't require the list to be
> sorted, and groups together consecutive and equal terms:
------------------------------
Message: 2
Date: Fri, 13 Jan 2017 17:14:54 -0800
From: Jeffrey Brown <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Better Code
Message-ID:
<caec4ma29sueehutlu1m4n5zyhcmvzh20kjdbo94ednbghv-...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
You could zip the original list to its own tail.
On Fri, Jan 13, 2017 at 4:09 PM, Graham Gill <[email protected]> wrote:
> Sorry, I mean, "groups together consecutive and equal terms, that occur
> sequentially". Examples speak louder than general descriptions!
>
> On 13-Jan-2017 6:43 PM, Graham Gill wrote:
>
>> Here's one that does what you want, doesn't require the list to be
>> sorted, and groups together consecutive and equal terms:
>>
>
>
> _______________________________________________
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>
--
Jeff Brown | Jeffrey Benjamin Brown
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Message: 3
Date: Sat, 14 Jan 2017 02:35:17 -0500
From: Graham Gill <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] safe versions of pred and succ?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8; format=flowed
Suppose I have a function
f :: (Enum a) => a -> ...
f x ... = ...pred x ...
If a is also an instance of Bounded, then I get a runtime error if pred
minBound is evaluated during evaluation of f.
If I try to protect the use of pred x with a check using minBound, then
I have to add a Bounded constraint on type a, which would mean, for
example, that I can no longer use f with type Integer.
Do I need two different versions of f, one for Bounded a and one for
non-Bounded a? Is there a more elegant way to take care of this problem?
I don't know much about all of the type magic available in GHC.
For example, from another list message,
groupConsecutive :: (Enum a,Eq a) => [a] -> [[a]]
groupConsecutive = foldr go []
where go x ls@(hd@(y:_):yss)
| x == y || x == pred y = (x:hd):yss
| otherwise = [x]:ls
go x [] = [[x]]
> groupConsecutive [1,2,3,7,8,10,11,12]
[[1,2,3],[7,8],[10,11,12]]
> groupConsecutive ([1,0,1,2]::[Word])
*** Exception: Enum.pred{Word}: tried to take `pred' of minBound
In the first go case, if type a is also Bounded, y == minBound and x /=
y, then we know already that x /= pred y, we want the guard to fail and
that we pass to the otherwise guard to start a new sublist. But how to
implement it without making the function unavailable for un-Bounded types?
Graham
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