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Today's Topics:
1. Re: [Haskell-cafe] Defining a containing function on
polymorphic list (Andrew Wagner)
2. RE: [Haskell-cafe] Defining a containing function on
polymorphic list (Raeck Zhao)
3. Re: [Haskell-cafe] Defining a containing function on
polymorphic list (Andrew Wagner)
4. about the pattern matching (Raeck Zhao)
----------------------------------------------------------------------
Message: 1
Date: Mon, 22 Dec 2008 09:18:37 -0500
From: "Andrew Wagner" <[email protected]>
Subject: [Haskell-beginners] Re: [Haskell-cafe] Defining a containing
function on polymorphic list
To: "Denis Bueno" <[email protected]>
Cc: Raeck Zhao <[email protected]>, [email protected],
[email protected]
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Yes, of course, sorry for the typo.
On Mon, Dec 22, 2008 at 9:17 AM, Denis Bueno <[email protected]> wrote:
> 2008/12/22 Andrew Wagner <[email protected]>:
> > The problem here is even slightly deeper than you might realize. For
> > example, what if you have a list of functions. How do you compare two
> > functions to each other to see if they're equal? There is no good way
> really
> > to do it! So, not only is == not completely polymorphic, but it CAN'T be.
> >
> > There is a nice solution for this, however, and it's very simple:
> >
> > contain :: Eq a -> [a] -> Bool
>
> Please note that the syntax here should be:
>
> contain :: Eq a => a -> [a] -> Bool
>
> Denis
>
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Message: 2
Date: Mon, 22 Dec 2008 14:35:01 +0000
From: Raeck Zhao <[email protected]>
Subject: [Haskell-beginners] RE: [Haskell-cafe] Defining a containing
function on polymorphic list
To: <[email protected]>
Cc: Beginners Haskell <[email protected]>, Cafe Haskell
<[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Thank you very much for your reply! It is really helpful!
But I just found another 'problem', I just realize that the list does not
support the user-defined data type?
the list is also depending on the Eq function?
For example,
data Shape = Square | Triangle | Circle
when I type either
[Square, Triangle, Circle]
or
Square == Square
there are errors!
So there is no way to construct a truly polymorphic List? any way to extend the
list to support some user-defined data type?
Or... I define the Shape in a wrong way actually?
Thanks
Raeck
Date: Mon, 22 Dec 2008 09:02:53 -0500
From: [email protected]
To: [email protected]
Subject: Re: [Haskell-cafe] Defining a containing function on polymorphic list
CC: [email protected]; [email protected]
The problem here is even slightly deeper than you might realize. For example,
what if you have a list of functions. How do you compare two functions to each
other to see if they're equal? There is no good way really to do it! So, not
only is == not completely polymorphic, but it CAN'T be.
There is a nice solution for this, however, and it's very simple:
contain :: Eq a -> [a] -> Bool
contain x [] = False
contain x (y:ys) = if x == y then True else contain x ys
The "Eq a" in the type signature says that 'a' must be a member of the 'Eq'
typeclass. That says, in turn, that 'a' must have == defined for it.
Fortunately, most types have, or can easily derive that definition. Here is the
definition of the typeclass:
class Eq a where
(==) :: a -> a -> Bool
(/=) :: a -> a -> Bool
That is, for 'a' to be a member of 'Eq', it must have a == operator which can
take 2 values of that type and return a Boolean, saying whether or not they're
equal, and it must also have a definition for the /= operator, which is "not
equal". These two are also defined in terms of each other, so if you define ==,
you get /= for free, and vice versa.
That's probably more information than you needed to know, but I hope it helps.
2008/12/22 Raeck Zhao <[email protected]>
I am trying to define a containing function to see if a value is one
of the elements within a list which is polymorphic, but failed with the
following codes:
> contain :: a -> [a] -> Bool
> contain x [] = False
> contain x (y:ys) = if x == y then True else contain x ys
it seems that the problem is the 'operator' == does not support a polymorphic
check?
Any way can solve the problem? or any alternative solution to achieve the
purpose?
Thanks!
Raeck
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Message: 3
Date: Mon, 22 Dec 2008 09:44:41 -0500
From: "Andrew Wagner" <[email protected]>
Subject: [Haskell-beginners] Re: [Haskell-cafe] Defining a containing
function on polymorphic list
To: "Raeck Zhao" <[email protected]>
Cc: Beginners Haskell <[email protected]>, Cafe Haskell
<[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
There are two ways to fix this. Let me see if I can get my syntax right this
time :)
1.) Let GHC work out the Eq instance:
data Shape = Square | Triangle | Circle deriving Eq
2.) Tell GHC how to do it explicitly:
data Shape = Square | Triangle | Circle
instance Eq Shape where
Square == Square = True
Triangle == Triangle = True
Circle == Circle = True
_ == _ = False
Note that the last line here means that any other comparisons are false.
On Mon, Dec 22, 2008 at 9:35 AM, Raeck Zhao <[email protected]> wrote:
> Thank you very much for your reply! It is really helpful!
>
> But I just found another 'problem', I just realize that the list does not
> support the user-defined data type?
> the list is also depending on the Eq function?
>
> For example,
>
> data Shape = Square | Triangle | Circle
>
> when I type either
>
> [Square, Triangle, Circle]
>
> or
>
> Square == Square
>
> there are errors!
>
> So there is no way to construct a truly polymorphic List? any way to extend
> the list to support some user-defined data type?
>
> Or... I define the Shape in a wrong way actually?
>
> Thanks
>
> Raeck
>
>
> ------------------------------
> Date: Mon, 22 Dec 2008 09:02:53 -0500
> From: [email protected]
> To: [email protected]
> Subject: Re: [Haskell-cafe] Defining a containing function on polymorphic
> list
> CC: [email protected]; [email protected]
>
> The problem here is even slightly deeper than you might realize. For
> example, what if you have a list of functions. How do you compare two
> functions to each other to see if they're equal? There is no good way really
> to do it! So, not only is == not completely polymorphic, but it CAN'T be.
>
> There is a nice solution for this, however, and it's very simple:
>
> contain :: Eq a -> [a] -> Bool
> contain x [] = False
> contain x (y:ys) = if x == y then True else contain x ys
>
> The "Eq a" in the type signature says that 'a' must be a member of the 'Eq'
> typeclass. That says, in turn, that 'a' must have == defined for it.
> Fortunately, most types have, or can easily derive that definition. Here is
> the definition of the typeclass:
>
> class
> Eq<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#t:Eq>a
> where
> (==)<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v:%3D%3D>::
> a -> a ->
> Bool<http://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t:Bool>
> (/=)<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v:/%3D>::
> a -> a ->
> Bool<http://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t:Bool>
> That is, for 'a' to be a member of 'Eq', it must have a == operator which
> can take 2 values of that type and return a Boolean, saying whether or not
> they're equal, and it must also have a definition for the /= operator, which
> is "not equal". These two are also defined in terms of each other, so if you
> define ==, you get /= for free, and vice versa.
>
> That's probably more information than you needed to know, but I hope it
> helps.
>
> 2008/12/22 Raeck Zhao <[email protected]>
>
> I am trying to define a containing function to see if a value is one of
> the elements within a list which is polymorphic, but failed with the
> following codes:
> > contain :: a -> [a] -> Bool
> > contain x [] = False
> > contain x (y:ys) = if x == y then True else contain x ys it seems that
> the problem is the 'operator' == does not support a polymorphic check?
> Any way can solve the problem? or any alternative solution to achieve the
> purpose?
> Thanks!
> Raeck
>
> ------------------------------
> It's the same Hotmail(R). If by "same" you mean up to 70% faster. Get your
> account
> now.<http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_hotmail_acq_broad1_122008>
>
> _______________________________________________
> Haskell-Cafe mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
>
> ------------------------------
> Life on your PC is safer, easier, and more enjoyable with Windows Vista(R).
> See
> how <http://clk.atdmt.com/MRT/go/127032870/direct/01/>
>
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Message: 4
Date: Wed, 24 Dec 2008 01:45:05 +0000
From: Raeck Zhao <[email protected]>
Subject: [Haskell-beginners] about the pattern matching
To: Beginners Haskell <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
hi, good ... morning : )
I am just confused by the following code
> oneOnly :: [Int]
> oneOnly = [1]
> isOneOnly :: [Int] -> Bool
> isOneOnly oneOnly = True
> isOneOnly tester = False
what I want to do is to define a 'type' oneOnly as [1] and use it on
the pattern matching in function isOneOnly. But it does not work as
I expect:
When I type
isOneOnly [1]
it will be True which is the result I expect but for
is OneOnly [1,2]
the result keeps True, it seems the second pattern has been ignored,
I think I try to achieve the purpose in a wrong way, any suggestion?
Thanks and Merry Christmas
Best wishes,
Raeck
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