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Today's Topics:
1. represent data sturcture using function (Raeck chiu)
2. Re: about Data.BinaryTree (Daniel Fischer)
3. Re: Re: Restricting integers in a type? (Colin Paul Adams)
4. 'Iterating a data type' ([email protected])
5. Re: Restricting integers in a type? (Ertugrul Soeylemez)
6. Re: Re: Restricting integers in a type? (Colin Paul Adams)
7. Re: 'Iterating a data type' (Paul Visschers)
8. Enum and Bounded in generic type ([email protected])
9. Re: Enum and Bounded in generic type (Philippa Cowderoy)
10. Re: Enum and Bounded in generic type (Paul Visschers)
----------------------------------------------------------------------
Message: 1
Date: Mon, 29 Dec 2008 05:04:28 +0000
From: Raeck chiu <[email protected]>
Subject: [Haskell-beginners] represent data sturcture using function
To: Beginners Haskell <[email protected]>, Cafe Haskell
<[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="windows-1252"
Merry Christmas!
Hope everybody is enjoying the Christmas!
I am doing some readings and I found something seems to be interesting.
People sometime will try to represent a quantity-regard-only data structure
(such a order-regadless List) using functions instead of ta concrete data
structure (like the haskell build-in []), any particular reason for this?
For example,
> data Sex = Male | Female
> classA :: Sex -> Int
> classA Male = 100
> classA Female = 200
when I should prefer the above solution instead of using a concrete data
structure
such as [] to represent the classA members?
It seems to be very difficult to change the number of Male or Female if a
concrete data
structure is not used. Is it possible change the number of Male in classA when
represent
classA using function?
Thank you very much!
Best wishes,
Raeck
_________________________________________________________________
Its the same Hotmail®. If by same you mean up to 70% faster.
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Message: 2
Date: Mon, 29 Dec 2008 07:06:26 +0100
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] about Data.BinaryTree
To: "Liu Jian" <[email protected]>, [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Am Montag, 29. Dezember 2008 04:22 schrieb Liu Jian:
> Hi All,
>
> Is there any implementation of binary tree in haskell library? for
> example, "insert lookup empty adjust isEmpty delete" including in
> it.
>
> cheers,
Well, depending on your use-case, it might be worth looking at Data.Set and
Data.Map, they're implemented via binary trees.
There's an AVL-tree imlementation in
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/collections
And generally a lot of data structures on Hackage, suffix-trees,
finger-trees...
There should be something that suits your needs.
Cheers,
Daniel
------------------------------
Message: 3
Date: Mon, 29 Dec 2008 06:37:12 +0000
From: Colin Paul Adams <[email protected]>
Subject: Re: [Haskell-beginners] Re: Restricting integers in a type?
To: Ertugrul Soeylemez <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
>>>>> "Ertugrul" == Ertugrul Soeylemez <[email protected]> writes:
>> That would seem to allow x = -3 and y = 13, for instance.
Ertugrul> Not if you disallow an x = -3 and y = 13 to happen in
Ertugrul> the first place. Haskell's module and type system
Ertugrul> allows you to do that.
Can you explain how to do that please?
--
Colin Adams
Preston Lancashire
------------------------------
Message: 4
Date: Tue, 30 Dec 2008 01:24:00 -0000
From: <[email protected]>
Subject: [Haskell-beginners] 'Iterating a data type'
To: "Beginners Haskell" <[email protected]>, "Cafe Haskell"
<[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original
Are there anyway to express the "iterating" of a user-defined data type in
Haskell?
For example, in
> data Shape = Square | Circle | Triangle
how can I 'iterate' them and apply them all to the same function without
indicating them explicitly?
such as [func Square, func Circle, func Triangle].
Can I use a form similar to the following case in a list instead:
> Numbers = [1,2,3,4,5]
> [func x | x <- Numbers ]
Actually what I want is to obtain all the possible values of a data type
(Shape).
Thank you very much!
Best wishes,
Raeck
------------------------------
Message: 5
Date: Tue, 30 Dec 2008 10:13:15 +0100
From: Ertugrul Soeylemez <[email protected]>
Subject: [Haskell-beginners] Re: Restricting integers in a type?
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=US-ASCII
Colin Paul Adams <[email protected]> wrote:
> >>>>> "Ertugrul" == Ertugrul Soeylemez <[email protected]> writes:
>
> >> That would seem to allow x = -3 and y = 13, for instance.
>
> Ertugrul> Not if you disallow an x = -3 and y = 13 to happen in
> Ertugrul> the first place. Haskell's module and type system
> Ertugrul> allows you to do that.
>
> Can you explain how to do that please?
Sure:
module Even (Even) where
newtype Even a = Even a
deriving (Eq, Show)
instance Integral a => Num (Even a) where
Even a + Even b = Even (a+b)
-- ...
fromInteger x = if even x then Even (fromInteger x) else undefined
The only way to introduce incorrect Even values would be to access the
constructor directly, but in that example, it's not exported. The
interface to constructing Even values is the fromInteger function.
Greets,
Ertugrul.
--
nightmare = unsafePerformIO (getWrongWife >>= sex)
http://blog.ertes.de/
------------------------------
Message: 6
Date: Tue, 30 Dec 2008 09:15:42 +0000
From: Colin Paul Adams <[email protected]>
Subject: Re: [Haskell-beginners] Re: Restricting integers in a type?
To: Ertugrul Soeylemez <[email protected]>
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
>>>>> "Ertugrul" == Ertugrul Soeylemez <[email protected]> writes:
Ertugrul> Colin Paul Adams <[email protected]> wrote:
>> >>>>> "Ertugrul" == Ertugrul Soeylemez <[email protected]> writes:
>>
>> >> That would seem to allow x = -3 and y = 13, for instance.
>>
Ertugrul> Not if you disallow an x = -3 and y = 13 to happen in
Ertugrul> the first place. Haskell's module and type system
Ertugrul> allows you to do that.
>>
>> Can you explain how to do that please?
Ertugrul> Sure:
Ertugrul> module Even (Even) where
Ertugrul> newtype Even a = Even a deriving (Eq, Show)
Ertugrul> instance Integral a => Num (Even a) where Even a +
Ertugrul> Even b = Even (a+b) -- ... fromInteger x = if even x
Ertugrul> then Even (fromInteger x) else undefined
Ertugrul> The only way to introduce incorrect Even values would be
Ertugrul> to access the constructor directly, but in that example,
Ertugrul> it's not exported. The interface to constructing Even
Ertugrul> values is the fromInteger function.
Thanks for all your help.
--
Colin Adams
Preston Lancashire
------------------------------
Message: 7
Date: Tue, 30 Dec 2008 11:30:17 +0100
From: Paul Visschers <[email protected]>
Subject: Re: [Haskell-beginners] 'Iterating a data type'
To: [email protected]
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
You actually have two different questions. The first about iteration can
be done by the function map in the following way:
Instead of [func Square, func Circle, func Triangle] you use:
map func [Square, Circle, Triangle].
The list comprehensions should also work:
[func x | x <- [Square, Circle, Triangle]]
Now as for obtaining/generating all values of Shape, the easiest way is
to make Shape an instance of Enum, like this:
data Shape = Square | Circle | Triangle deriving Enum
You can then generate a list of all the values by:
enumFrom Square
You use Square here because it is the first constructor of Shape, and
you want to enumerate them all.
I hope this helps,
Paul
[email protected] wrote:
> Are there anyway to express the "iterating" of a user-defined data type
> in Haskell?
>
> For example, in
>
>> data Shape = Square | Circle | Triangle
>
> how can I 'iterate' them and apply them all to the same function without
> indicating them explicitly?
> such as [func Square, func Circle, func Triangle].
> Can I use a form similar to the following case in a list instead:
>
>> Numbers = [1,2,3,4,5]
>
>> [func x | x <- Numbers ]
>
> Actually what I want is to obtain all the possible values of a data type
> (Shape).
>
> Thank you very much!
>
> Best wishes,
>
> Raeck
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 8
Date: Tue, 30 Dec 2008 17:12:29 -0000
From: <[email protected]>
Subject: [Haskell-beginners] Enum and Bounded in generic type
To: <[email protected]>, <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; format=flowed; charset="ISO-8859-1";
reply-type=original
Hi, how can I make the following code work? (show all the possible values of
a type 'a')
> showAll :: (Eq a, Bounded a, Enum a) => a -> [a]
> showAll a = [minBound..maxBound]::[a]
it keeps saying that I could fix the problem by adding (Enum a) and (Bounded
a) the the context, but I failed when I try.
anything goes wrong?
Thanks!
Raeck
------------------------------
Message: 9
Date: Tue, 30 Dec 2008 17:17:37 +0000
From: Philippa Cowderoy <[email protected]>
Subject: Re: [Haskell-beginners] Enum and Bounded in generic type
To: [email protected]
Cc: [email protected]
Message-ID: <1230657457.1125.1048.ca...@flippa-eee>
Content-Type: text/plain
[-cafe left out]
On Tue, 2008-12-30 at 17:12 +0000, [email protected] wrote:
> Hi, how can I make the following code work? (show all the possible values of
> a type 'a')
>
> > showAll :: (Eq a, Bounded a, Enum a) => a -> [a]
> > showAll a = [minBound..maxBound]::[a]
>
> it keeps saying that I could fix the problem by adding (Enum a) and (Bounded
> a) the the context, but I failed when I try.
>
The problem is the inner annotation, ::[a]. It's not the same a as in
(Eq a, Bounded a, Enum a) => a -> [a], and in Haskell98 it can't be. I'm
not surprised that's confused you! If you remove the inner annotation it
should work fine though.
You can go a step further and make showAll a value rather than a
function:
showAll = [minBound..maxBound]
(I've left out the type annotations, but you can put ":t
[minBound..maxBound]" into ghci or hugs if you want one)
--
Philippa Cowderoy <[email protected]>
I left the snappy .sigs on the other computer
------------------------------
Message: 10
Date: Tue, 30 Dec 2008 18:50:22 +0100
From: Paul Visschers <[email protected]>
Subject: Re: [Haskell-beginners] Enum and Bounded in generic type
To: [email protected]
Cc: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1
You seem to be having some trouble with the type system, not just in
this instance. I found when I was learning Haskell that when this
happens, it is useful to not add type annotations, then load it up in
GHCi and see what it comes up with (with the aforementions :t option).
Then you can see why that makes sense and possibly change the function.
Once you're happy with the function and are confident you understand its
type, go ahead and put the type annotation back into your source code.
I also found it useful to play around with various expressions and see
what their types where. Especially with things like partial application,
function composition and monadic functions.
Paul
Philippa Cowderoy wrote:
> [-cafe left out]
>
> On Tue, 2008-12-30 at 17:12 +0000, [email protected] wrote:
>> Hi, how can I make the following code work? (show all the possible values of
>> a type 'a')
>>
>>> showAll :: (Eq a, Bounded a, Enum a) => a -> [a]
>>> showAll a = [minBound..maxBound]::[a]
>> it keeps saying that I could fix the problem by adding (Enum a) and (Bounded
>> a) the the context, but I failed when I try.
>>
>
> The problem is the inner annotation, ::[a]. It's not the same a as in
> (Eq a, Bounded a, Enum a) => a -> [a], and in Haskell98 it can't be. I'm
> not surprised that's confused you! If you remove the inner annotation it
> should work fine though.
>
> You can go a step further and make showAll a value rather than a
> function:
>
> showAll = [minBound..maxBound]
>
> (I've left out the type annotations, but you can put ":t
> [minBound..maxBound]" into ghci or hugs if you want one)
>
------------------------------
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