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Today's Topics:
1. Equality instance for lists (Patrick Browne)
2. Re: Equality instance for lists (Brandon Allbery)
3. Re: Equality instance for lists (Brent Yorgey)
4. This code does not work : conversion error (willie ekaputra)
5. Re: This code does not work : conversion error (David McBride)
6. Re: This code does not work : conversion error (mukesh tiwari)
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Message: 1
Date: Sat, 30 Nov 2013 22:40:59 +0000
From: Patrick Browne <[email protected]>
To: <[email protected]>
Subject: [Haskell-beginners] Equality instance for lists
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Message: 2
Date: Sat, 30 Nov 2013 18:01:14 -0500
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Equality instance for lists
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On Sat, Nov 30, 2013 at 5:40 PM, Patrick Browne <[email protected]>wrote:
> I am trying to write my own simplified class and instances for equality.
> I have trouble with the equality equation for the empty list.
> Even though I can use the [] == [] at the GHCi prompt I cannot use it in
> my equality test.
> How can I make my eq test handle empty lists while staying within the
> context of my current code?.
>
You can't unless you include an Eq constraint on a, since Eq is what
"defines" (==). In your other instance, you use Int and Int has an Eq
instance.
ghci can do it because of extended defaulting: it uses () for a, and there
is an Eq instance for (). But this would not help you in an instance
definition.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 3
Date: Sat, 30 Nov 2013 22:21:27 -0500
From: Brent Yorgey <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Equality instance for lists
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Sat, Nov 30, 2013 at 10:40:59PM +0000, Patrick Browne wrote:
> Hi,
> I am trying to write my own simplified class and instances for equality.
> I have trouble with the equality equation for the empty list.
> Even though I can use the [] == [] at the GHCi prompt I cannot use it in
> my equality test.
> How can I make my eq test handle empty lists while staying within the
> context of my current code?.
> Thanks,
> Pat
>
> class Eq1 a where
> eq :: a -> a -> Bool
>
> instance Eq1 Int where
> eq a b = a == b
>
> instance Eq1 a => Eq1 [a] where
> -- This line compiles but gives an run time error.
> eq [] [] = True
This code does not give a run time error. The error you are getting
is a type inference error, which has nothing to do with this code.
> eq [] []
You simply need to give a type annotation on one of the empty lists.
eq ([] :: [Int]) []
The standard Eq class has some built-in magic support by GHCI
(extended default rules) which means this does not apply in the case
of using == directly.
-Brent
------------------------------
Message: 4
Date: Sun, 1 Dec 2013 08:22:12 +0100
From: willie ekaputra <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] This code does not work : conversion
error
Message-ID:
<CAMCAAFcOAmtvBMcZwdXW98AawV=rcthjns-ra0jdb6-c_4a...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Hi everyone !
I am newbie and I made this code for counting k, so that 2^k divisor of
n.Somehow it doesn't work.
Anyone knows what is wrong?
Regards and thanks.
Wili.
maxexp2:: Int -> Int
maxexp2 n
|n== 0 || 2^k 'mod' n /=0 =0
|otherwise = k
Where
k= e ' div' f
e=round (fromIntegral (log n))
f = round (fromIntegral (log 2))
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Message: 5
Date: Sun, 1 Dec 2013 03:12:08 -0500
From: David McBride <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] This code does not work : conversion
error
Message-ID:
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You just have a few problems. Where should be lowercase. The single
quotes should be backticks "`". You may have some indentation issues. And
lastly, you have some problems with e and f
e = round (log $ fromIntegral n)
f = round (log 2)
Otherwise it seems fine.
On Sun, Dec 1, 2013 at 2:22 AM, willie ekaputra <[email protected]>wrote:
> Hi everyone !
> I am newbie and I made this code for counting k, so that 2^k divisor of
> n.Somehow it doesn't work.
> Anyone knows what is wrong?
>
> Regards and thanks.
> Wili.
>
> maxexp2:: Int -> Int
> maxexp2 n
> |n== 0 || 2^k 'mod' n /=0 =0
> |otherwise = k
> Where
> k= e ' div' f
> e=round (fromIntegral (log n))
> f = round (fromIntegral (log 2))
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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Message: 6
Date: Sun, 1 Dec 2013 13:47:03 +0530
From: mukesh tiwari <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] This code does not work : conversion
error
Message-ID:
<CAFHZvE9YRH3MTe88bYnY4ac00RaHH=bvqr5wc_lljobpnwm...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Hi Willie,
Here is the code modified
maxexp2:: Int -> Int
maxexp2 n
|n== 0 || 2^k `mod` n /=0 =0
|otherwise = k
where
k = e `div` f
e = round ( log ( fromIntegral n ) )
f = round ( (log 2.0 ))
When you computing k then use backticks (`) [1] not the single quote ( ' )
[1] http://book.realworldhaskell.org/read/functional-programming.html( See
Infix function )
On Sun, Dec 1, 2013 at 12:52 PM, willie ekaputra
<[email protected]>wrote:
> Hi everyone !
> I am newbie and I made this code for counting k, so that 2^k divisor of
> n.Somehow it doesn't work.
> Anyone knows what is wrong?
>
> Regards and thanks.
> Wili.
>
> maxexp2:: Int -> Int
> maxexp2 n
> |n== 0 || 2^k 'mod' n /=0 =0
> |otherwise = k
> Where
> k= e ' div' f
> e=round (fromIntegral (log n))
> f = round (fromIntegral (log 2))
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
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