[Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
Hello,
I largely don't know what I'm doing or even trying to do, it is a voyage into
the unknownbutif I go...
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
class Foo x y | x - y, y - x
instance Foo Integer Integer
That seems to workand my head seems to say...your created some sort of
binary relation between 2 types...and made Integer,Integer a member of it...
Something like that anyway
Then I go
data Bar
instance Foo Bar x
Error!but I'm think I understand thisI can't claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my functional
dependencies, because Bar,Integer is now a member of Foo..
Bad programmer...
So how I naively go
class NotAnInteger a
instance (NotAnInteger x) = Foo Bar x
I haven't declared integer to be NotAnIntegerso (in a closed
world)this would seem to exclude the contradictionbut...
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12
So
i)I clearly don't understand something about the type
system.
ii) I don't know how to restrict type variables in instance
declarationsi.e. how do I use the notion of Foo across different
combinations of types, without them colliding.
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Viacom, Viacom International Media Networks and VIMN and Comedy Central are all
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Re: [Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
The constraint on an instance never influences which instance is
selected. So as far as instance selection goes, 'instance Foo x' and
'instance C x = Foo x' are the same. The constraint is only checked
after the instance is selected.
Erik
On Fri, Jul 5, 2013 at 2:43 PM, Nicholls, Mark nicholls.m...@vimn.com wrote:
Hello,
I largely don’t know what I’m doing or even trying to do, it is a voyage
into the unknown….but….if I go…
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
class Foo x y | x - y, y - x
instance Foo Integer Integer
That seems to work….and my head seems to say…your created some sort of
binary relation between 2 types…and made Integer,Integer a member of it…
Something like that anyway….
Then I go….
data Bar
instance Foo Bar x
Error!but I’m think I understand this….I can’t claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my
functional dependencies, because Bar,Integer is now a member of Foo..
Bad programmer…….
So how I naively go….
class NotAnInteger a
instance (NotAnInteger x) = Foo Bar x
I haven’t declared integer to be “NotAnInteger”….so (in a closed
world)….this would seem to exclude the contradiction….but…
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12
So
i)I clearly don’t understand something about the type
system.
ii) I don’t know how to restrict type variables in
instance declarations….i.e. how do I use the notion of “Foo” across
different combinations of types, without them colliding.
CONFIDENTIALITY NOTICE
This e-mail (and any attached files) is confidential and protected by
copyright (and other intellectual property rights). If you are not the
intended recipient please e-mail the sender and then delete the email and
any attached files immediately. Any further use or dissemination is
prohibited.
While MTV Networks Europe has taken steps to ensure that this email and any
attachments are virus free, it is your responsibility to ensure that this
message and any attachments are virus free and do not affect your systems /
data.
Communicating by email is not 100% secure and carries risks such as delay,
data corruption, non-delivery, wrongful interception and unauthorised
amendment. If you communicate with us by e-mail, you acknowledge and assume
these risks, and you agree to take appropriate measures to minimise these
risks when e-mailing us.
MTV Networks International, MTV Networks UK Ireland, Greenhouse,
Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions
International, Be Viacom, Viacom International Media Networks and VIMN and
Comedy Central are all trading names of MTV Networks Europe. MTV Networks
Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks
Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent,
London, NW1 8TT.
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Re: [Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
You're running into the open worldassumption--anybody could come along
and make Integer part of your NotAnInteger class, and there's nothing you
can do to stop them. This is a design tradeoff for typeclasses: typeclass
instances are always global and are exported to all other modules you use.
This means you cannot ensure a type is *not* part of a typeclass. (Or, at
the very least, you can't convince GHC of this fact.)
For more information about this, take a look at the following StackOverflow
question:
http://stackoverflow.com/questions/8728596/explicitly-import-instances
On Jul 5, 2013 8:47 AM, Nicholls, Mark nicholls.m...@vimn.com wrote:
Hello,
** **
I largely don’t know what I’m doing or even trying to do, it is a voyage
into the unknown….but….if I go…
** **
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
** **
class Foo x y | x - y, y - x
instance Foo Integer Integer
** **
That seems to work….and my head seems to say…your created some sort of
binary relation between 2 types…and made Integer,Integer a member of it…
** **
Something like that anyway….
** **
Then I go….
** **
data Bar
** **
instance Foo Bar x
** **
Error!but I’m think I understand this….I can’t claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my
functional dependencies, because Bar,Integer is now a member of Foo..***
*
** **
Bad programmer…….
** **
** **
So how I naively go….
** **
** **
class NotAnInteger a
** **
instance (NotAnInteger x) = Foo Bar x
** **
I haven’t declared integer to be “NotAnInteger”….so (in a closed
world)….this would seem to exclude the contradiction….but…
** **
** **
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12**
**
** **
So
**i)**I clearly don’t understand something about the
type system.
**ii) **I don’t know how to restrict type variables in
instance declarations….i.e. how do I use the notion of “Foo” across
different combinations of types, without them colliding.
** **
** **
** **
** **
** **
** **
** **
CONFIDENTIALITY NOTICE
This e-mail (and any attached files) is confidential and protected by
copyright (and other intellectual property rights). If you are not the
intended recipient please e-mail the sender and then delete the email and
any attached files immediately. Any further use or dissemination is
prohibited.
While MTV Networks Europe has taken steps to ensure that this email and
any attachments are virus free, it is your responsibility to ensure that
this message and any attachments are virus free and do not affect your
systems / data.
Communicating by email is not 100% secure and carries risks such as delay,
data corruption, non-delivery, wrongful interception and unauthorised
amendment. If you communicate with us by e-mail, you acknowledge and assume
these risks, and you agree to take appropriate measures to minimise these
risks when e-mailing us.
MTV Networks International, MTV Networks UK Ireland, Greenhouse,
Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions
International, Be Viacom, Viacom International Media Networks and VIMN and
Comedy Central are all trading names of MTV Networks Europe. MTV Networks
Europe is a partnership between MTV Networks Europe Inc. and Viacom
Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley
Crescent, London, NW1 8TT.
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Re: [Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
Ah
So it isn't a closed world
So how do I stop my instances clashing?
The x in
instance Foo Bar x
is never intended to be Integer.
Mark Nicholls | Lead broadcast corporate architect, Programmes Development
- Viacom International Media Networks
A: 17-29 Hawley Crescent London NW1 8TT | e:
nicholls.m...@vimn.commailto:m...@vimn.com T: +44 (0)203 580 2223
[Description: cid:image001.png@01CD488D.9204D030]
From: Tikhon Jelvis [mailto:tik...@jelv.is]
Sent: 05 July 2013 2:08 PM
To: Nicholls, Mark
Cc: haskell-cafe
Subject: Re: [Haskell-cafe] newbie question about Functional dependencies
conflict between instance declarations:.
You're running into the open worldassumption--anybody could come along and
make Integer part of your NotAnInteger class, and there's nothing you can do to
stop them. This is a design tradeoff for typeclasses: typeclass instances are
always global and are exported to all other modules you use. This means you
cannot ensure a type is *not* part of a typeclass. (Or, at the very least, you
can't convince GHC of this fact.)
For more information about this, take a look at the following StackOverflow
question: http://stackoverflow.com/questions/8728596/explicitly-import-instances
On Jul 5, 2013 8:47 AM, Nicholls, Mark
nicholls.m...@vimn.commailto:nicholls.m...@vimn.com wrote:
Hello,
I largely don't know what I'm doing or even trying to do, it is a voyage into
the unknownbutif I go...
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
class Foo x y | x - y, y - x
instance Foo Integer Integer
That seems to workand my head seems to say...your created some sort of
binary relation between 2 types...and made Integer,Integer a member of it...
Something like that anyway
Then I go
data Bar
instance Foo Bar x
Error!but I'm think I understand thisI can't claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my functional
dependencies, because Bar,Integer is now a member of Foo..
Bad programmer...
So how I naively go
class NotAnInteger a
instance (NotAnInteger x) = Foo Bar x
I haven't declared integer to be NotAnIntegerso (in a closed
world)this would seem to exclude the contradictionbut...
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12
So
i)I clearly don't understand something about the type
system.
ii) I don't know how to restrict type variables in instance
declarationsi.e. how do I use the notion of Foo across different
combinations of types, without them colliding.
CONFIDENTIALITY NOTICE
This e-mail (and any attached files) is confidential and protected by copyright
(and other intellectual property rights). If you are not the intended recipient
please e-mail the sender and then delete the email and any attached files
immediately. Any further use or dissemination is prohibited.
While MTV Networks Europe has taken steps to ensure that this email and any
attachments are virus free, it is your responsibility to ensure that this
message and any attachments are virus free and do not affect your systems /
data.
Communicating by email is not 100% secure and carries risks such as delay, data
corruption, non-delivery, wrongful interception and unauthorised amendment. If
you communicate with us by e-mail, you acknowledge and assume these risks, and
you agree to take appropriate measures to minimise these risks when e-mailing
us.
MTV Networks International, MTV Networks UK Ireland, Greenhouse, Nickelodeon
Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be
Viacom, Viacom International Media Networks and VIMN and Comedy Central are all
trading names of MTV Networks Europe. MTV Networks Europe is a partnership
between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for
service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.
___
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This e-mail (and any attached files) is confidential and protected by copyright
(and other intellectual property rights). If you are not the intended recipient
please e-mail the sender and then delete the email and any attached files
immediately. Any further use or dissemination is prohibited.
While MTV Networks Europe has taken steps to ensure that this email and any
attachments are virus free, it is your responsibility to ensure that this
message and any attachments are virus free and do not affect your systems /
data.
Communicating by email is not 100
