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Today's Topics:
1. Re: How to manage typeclass hierarchies and instances?
(Stuart Hungerford)
2. Re: How to manage typeclass hierarchies and instances?
(Michael Orlitzky)
3. Re: How to manage typeclass hierarchies and instances?
(Brandon Allbery)
4. Re: How to manage typeclass hierarchies and instances?
(PATRICK BROWNE)
5. Re: How to manage typeclass hierarchies and instances?
(PATRICK BROWNE)
----------------------------------------------------------------------
Message: 1
Date: Sun, 22 Feb 2015 08:15:53 +1100
From: Stuart Hungerford <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] How to manage typeclass hierarchies
and instances?
Message-ID:
<CAG+kMrF3PEp0xbYs7pCNDTfaAi-y=hbr6teuhec+-qpdgk4...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
>> -- in Semigroup.hs
>>
>> class Semigroup a where
>> (|.|) :: a -> a -> a
>>
>> instance Semigroup Integer where
>> (|.|) = (+)
>>
>>
>> -- In Monoid.hs
>>
>> class (Semigroup a) => Monoid a where
>> identity :: a
>>
>> instance Monoid Integer where
>> identity = 0
>>
>>
>> -- In Group.hs
>>
>> class (Monoid a) => Group a where
>> inverse :: a -> a
>>
>> instance Group Integer where
>> (|.|) = (+)
>> identity = 0
>> inverse = (-)
>>
>>
>> In Group.hs I'm trying to create an (additive) group instance for
>> Integer values but GHC complains that (|.|) and identity are not
>> "visible" typeclass methods.
>
> You only get one instance per type, so the Semigroup/Monoid instances
> for Integer are "set in stone." When you "import Semigroup" and "import
> Monoid", those instances come into scope. So in Group.hs, '|.|' and
> 'identity' are already defined for Integer. All you need is,
>
> instance Group Integer where
> inverse = negate
Thanks for that insight. Ignoring the instance issue for a minute,
suppose I wanted to define a typeclass Additive where I'd like to set
default implementations for the methods:
class (Num a, Group a) => Additive a where
(|.|) = (+)
-- etc...
In this situation GHC says (|.|) is not a visible method. If I add a
type declaration for (|.|) am I effectively "shadowing" the method
from Semigroup with a new method rather than supplying a default
implementation?
Thanks,
Stu
------------------------------
Message: 2
Date: Sat, 21 Feb 2015 18:44:21 -0500
From: Michael Orlitzky <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] How to manage typeclass hierarchies
and instances?
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
On 02/21/2015 04:15 PM, Stuart Hungerford wrote:
>
> Thanks for that insight. Ignoring the instance issue for a minute,
> suppose I wanted to define a typeclass Additive where I'd like to set
> default implementations for the methods:
>
> class (Num a, Group a) => Additive a where
> (|.|) = (+)
>
> -- etc...
>
> In this situation GHC says (|.|) is not a visible method. If I add a
> type declaration for (|.|) am I effectively "shadowing" the method
> from Semigroup with a new method rather than supplying a default
> implementation?
>
I don't think you can do that, but I wouldn't swear to it. One problem
would be that you could declare a Semigroup instance of Integer
independently of the Additive instance, and GHC would have to choose
which operation to use. In this case Additive is more specific than
Semigroup, but my instinct would be to prefer the Semigroup "override."
I think some of these things are addresses in,
https://ghc.haskell.org/trac/ghc/wiki/IntrinsicSuperclasses
but I've never made it through the whole document =)
------------------------------
Message: 3
Date: Sat, 21 Feb 2015 19:04:02 -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] How to manage typeclass hierarchies
and instances?
Message-ID:
<CAKFCL4XaAmSXMqHWbh5U2dqPUNmLOdGRr8tK9k=wia5W=-_...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Sat, Feb 21, 2015 at 6:44 PM, Michael Orlitzky <[email protected]>
wrote:
> I don't think you can do that
You can't, at present. (Note: Typeclasses are not OOP classes; `class (Num
a, Group a) => Additive a` does not declare superclasses, but
prerequisites.)
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 4
Date: Sun, 22 Feb 2015 07:53:12 +0000
From: PATRICK BROWNE <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] How to manage typeclass hierarchies
and instances?
Message-ID:
<cagflrkemvr85bjjquvdsdbfwnshsoa64hnetwypgm0ikghz...@mail.gmail.com>
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On 22 February 2015 at 00:04, Brandon Allbery <[email protected]> wrote:
> You can't, at present. (Note: Typeclasses are not OOP classes; `class (Num
> a, Group a) => Additive a` does not declare superclasses, but
> prerequisites.)
>
In logic we would say (Num a AND Group a) is a necessary condition for (
Additive a) => (Num a AND Group a) to hold.
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Message: 5
Date: Sun, 22 Feb 2015 08:07:44 +0000
From: PATRICK BROWNE <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] How to manage typeclass hierarchies
and instances?
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OPPS! My previous email should have be stated as a question rather than a
statement!
class (Num a, Group a) => Additive a
Or maybe given ( Additive a) => (Num a AND Group a).
Then (Num a AND Group a) is a necessary condition for (Additive a) to
hold.
On 22 February 2015 at 07:53, PATRICK BROWNE <[email protected]> wrote:
>
> On 22 February 2015 at 00:04, Brandon Allbery <[email protected]> wrote:
>
>> You can't, at present. (Note: Typeclasses are not OOP classes; `class
>> (Num a, Group a) => Additive a` does not declare superclasses, but
>> prerequisites.)
>>
>
> In logic we would say (Num a AND Group a) is a necessary condition for (
> Additive a) => (Num a AND Group a) to hold.
>
>
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