Do people use visitor pattern in Haskell...

Nicholls, Mark

19 Jun 2015 19 Jun '15

3:48 p.m.

My initial guess would be no...its built into the language But.... I find myself tempted to use it (in order to match on a type that's the instance of a typeclass)...which makes me think I've missed something. So...

...

data Foo = Foo data Bar = Bar

Ooo...this looks like an OO design pattern...surely wrong?

...

class Wibble a where visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...

instance Wibble Foo where visit f _ x = f x instance Wibble Bar where visit _ f x = f x

I want a list of Wibbles.... hmmm... (Wibble a) => [a] is clearly wrong... I want [(Wibble a) =>a] so I package it up?

...

data WibblePackage where WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now...so pointless function across Wibbles in a list.

...

fizzBuzz :: [WibblePackage] -> Integer fizzBuzz [] = 0 fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...

help :: Integer help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works! OO nerds will get uptight about "closing" the Wibble typeclass....but I've managed to create a list of different types and have a mechanism for recovering the type. How would a Haskell nerd do this? (I have looked, but they look more complicated....maybe to my OO eye). 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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Anupam Jain

19 Jun 19 Jun

4:03 p.m.

I discovered the same general pattern foroop On Friday, June 19, 2015, Nicholls, Mark wrote:

...

My initial guess would be no…its built into the language

But….

I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something.

So…

...
data Foo = Foo

...
data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...
class Wibble a where

...
visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...
instance Wibble Foo where

...
visit f _ x = f x

...
instance Wibble Bar where

...
visit _ f x = f x

I want a list of Wibbles....

hmmm...

(Wibble a) => [a] is clearly wrong...

I want [(Wibble a) =>a]

so I package it up?

...
data WibblePackage where

...
WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...
fizzBuzz :: [WibblePackage] -> Integer

...
fizzBuzz [] = 0

...
fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...
help :: Integer

...
help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works!

OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type.

How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye).

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.

-- Sent from my hyper-communicator

Anupam Jain

4:05 p.m.

Doh fat fingers! I discovered the same general pattern for oops programming in Haskell. An example - https://github.com/ajnsit/oop-inheritence-in-haskell/blob/master/Main.hs On Friday, June 19, 2015, Anupam Jain wrote:

...

I discovered the same general pattern foroop

On Friday, June 19, 2015, Nicholls, Mark javascript:_e(%7B%7D,'cvml','nicholls.mark@vimn.com');> wrote:

...
My initial guess would be no…its built into the language

But….

I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something.

So…

...
data Foo = Foo

...
data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...
class Wibble a where

...
visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...
instance Wibble Foo where

...
visit f _ x = f x

...
instance Wibble Bar where

...
visit _ f x = f x

I want a list of Wibbles....

hmmm...

(Wibble a) => [a] is clearly wrong...

I want [(Wibble a) =>a]

so I package it up?

...
data WibblePackage where

...
WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...
fizzBuzz :: [WibblePackage] -> Integer

...
fizzBuzz [] = 0

...
fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...
help :: Integer

...
help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works!

OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type.

How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye).

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.

-- Sent from my hyper-communicator

-- Sent from my hyper-communicator

Nicholls, Mark

4:09 p.m.

The design pattern is very well known in the OO community…most of the use cases evaporate in Haskell….in some sense its part of the language… Haskell is my 9th or 10th language, I can barely write “hello world” without the manual, so I’m still at times I’m still trying to discover the basic idioms… How to put heterogenuos things in a list…and yet not loose all type information (I know about HList…but I think that’s for special occasions) From: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Anupam Jain Sent: 19 June 2015 5:04 PM To: haskell-cafe Cafe Subject: Re: [Haskell-cafe] Do people use visitor pattern in Haskell... I discovered the same general pattern foroop On Friday, June 19, 2015, Nicholls, Mark mailto:nicholls.mark@vimn.com> wrote: My initial guess would be no…its built into the language But…. I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something. So…

...

data Foo = Foo data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...

class Wibble a where visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...

instance Wibble Foo where visit f _ x = f x instance Wibble Bar where visit _ f x = f x

I want a list of Wibbles.... hmmm... (Wibble a) => [a] is clearly wrong... I want [(Wibble a) =>a] so I package it up?

...

data WibblePackage where WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...

fizzBuzz :: [WibblePackage] -> Integer fizzBuzz [] = 0 fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...

help :: Integer help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works! OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type. How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye). 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. -- Sent from my hyper-communicator 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.

Patrick Chilton

4:10 p.m.

This is a frequent antipattern: https://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-... If all you can do with WibblePackage is recover either a Foo or a Bar, why not just use Either Foo Bar? On Fri, Jun 19, 2015 at 5:09 PM, Nicholls, Mark wrote:

...

The design pattern is very well known in the OO community…most of the use cases evaporate in Haskell….in some sense its part of the language…

Haskell is my 9th or 10th language, I can barely write “hello world” without the manual, so I’m still at times I’m still trying to discover the basic idioms…

How to put heterogenuos things in a list…and yet not loose all type information (I know about HList…but I think that’s for special occasions)

*From:* Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] *On Behalf Of *Anupam Jain *Sent:* 19 June 2015 5:04 PM *To:* haskell-cafe Cafe *Subject:* Re: [Haskell-cafe] Do people use visitor pattern in Haskell...

I discovered the same general pattern foroop

On Friday, June 19, 2015, Nicholls, Mark wrote:

My initial guess would be no…its built into the language

But….

I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something.

So…

...
data Foo = Foo

...
data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...
class Wibble a where

...
visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...
instance Wibble Foo where

...
visit f _ x = f x

...
instance Wibble Bar where

...
visit _ f x = f x

I want a list of Wibbles....

hmmm...

(Wibble a) => [a] is clearly wrong...

I want [(Wibble a) =>a]

so I package it up?

...
data WibblePackage where

...
WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...
fizzBuzz :: [WibblePackage] -> Integer

...
fizzBuzz [] = 0

...
fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...
help :: Integer

...
help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works!

OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type.

How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye).

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.

-- Sent from my hyper-communicator

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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Alexander Solla

4:46 p.m.

...

If all you can do with WibblePackage is recover either a Foo or a Bar, why not just use Either Foo Bar?

Typically, I just go straight for "data types a la carte" when I need any kind of type-level disjunction. Define the type synonym: type (:+:) a b = Either a b and define a type class class (:<:) a b where inj :: a -> b with some appropriate instances (as specified in the paper)[1], and you can chain disjunctions almost automatically: data Commercial = Commercial data Retail = Retail data Investment = Investment type Bank = Retail :+: Commercial :+: Investment bac :: Bank bac = inj $ Retail [1]: http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesALaCarte.pdf On Fri, Jun 19, 2015 at 9:10 AM, Patrick Chilton wrote:

...

This is a frequent antipattern: https://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-...

If all you can do with WibblePackage is recover either a Foo or a Bar, why not just use Either Foo Bar?

On Fri, Jun 19, 2015 at 5:09 PM, Nicholls, Mark wrote:

...
The design pattern is very well known in the OO community…most of the use cases evaporate in Haskell….in some sense its part of the language…

Haskell is my 9th or 10th language, I can barely write “hello world” without the manual, so I’m still at times I’m still trying to discover the basic idioms…

How to put heterogenuos things in a list…and yet not loose all type information (I know about HList…but I think that’s for special occasions)

*From:* Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] *On Behalf Of *Anupam Jain *Sent:* 19 June 2015 5:04 PM *To:* haskell-cafe Cafe *Subject:* Re: [Haskell-cafe] Do people use visitor pattern in Haskell...

I discovered the same general pattern foroop

On Friday, June 19, 2015, Nicholls, Mark wrote:

My initial guess would be no…its built into the language

But….

I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something.

So…

...
data Foo = Foo

...
data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...
class Wibble a where

...
visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...
instance Wibble Foo where

...
visit f _ x = f x

...
instance Wibble Bar where

...
visit _ f x = f x

I want a list of Wibbles....

hmmm...

(Wibble a) => [a] is clearly wrong...

I want [(Wibble a) =>a]

so I package it up?

...
data WibblePackage where

...
WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...
fizzBuzz :: [WibblePackage] -> Integer

...
fizzBuzz [] = 0

...
fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...
help :: Integer

...
help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works!

OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type.

How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye).

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.

-- Sent from my hyper-communicator

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.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

MigMit

4:08 p.m.

data WibblePackage = WPFoo Foo | WPBar Bar Отправлено с iPhone

...

19 июня 2015 г., в 17:48, Nicholls, Mark написал(а):

My initial guess would be no…its built into the language

But….

I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something.

So…

...
data Foo = Foo data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...
class Wibble a where visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...
instance Wibble Foo where visit f _ x = f x instance Wibble Bar where visit _ f x = f x

I want a list of Wibbles.... hmmm... (Wibble a) => [a] is clearly wrong... I want [(Wibble a) =>a]

so I package it up?

...
data WibblePackage where WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...
fizzBuzz :: [WibblePackage] -> Integer fizzBuzz [] = 0 fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...
help :: Integer help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works!

OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type.

How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye).

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.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

Nicholls, Mark

4:10 p.m.

The shortest answers are the best. Of course. Thanks. From: MigMit [mailto:miguelimo38@yandex.ru] Sent: 19 June 2015 5:09 PM To: Nicholls, Mark Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Do people use visitor pattern in Haskell... data WibblePackage = WPFoo Foo | WPBar Bar Отправлено с iPhone 19 июня 2015 г., в 17:48, Nicholls, Mark mailto:nicholls.mark@vimn.com> написал(а): My initial guess would be no…its built into the language But…. I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something. So…

...

data Foo = Foo data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...

class Wibble a where visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...

instance Wibble Foo where visit f _ x = f x instance Wibble Bar where visit _ f x = f x

I want a list of Wibbles.... hmmm... (Wibble a) => [a] is clearly wrong... I want [(Wibble a) =>a] so I package it up?

...

data WibblePackage where WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...

fizzBuzz :: [WibblePackage] -> Integer fizzBuzz [] = 0 fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...

help :: Integer help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works! OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type. How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye). 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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.orgmailto:Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe 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.

Christopher Allen

4:11 p.m.

This is a pattern people usually refer to as the "existential typeclass antipattern". An article goes into more depth here: https://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-... Sometimes you need a GADT with an existential type and a typeclass, but it doesn't occur as often as the equivalent does in OO. On Fri, Jun 19, 2015 at 10:48 AM, Nicholls, Mark wrote:

...

My initial guess would be no…its built into the language

But….

I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something.

So…

...
data Foo = Foo

...
data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...
class Wibble a where

...
visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...
instance Wibble Foo where

...
visit f _ x = f x

...
instance Wibble Bar where

...
visit _ f x = f x

I want a list of Wibbles....

hmmm...

(Wibble a) => [a] is clearly wrong...

I want [(Wibble a) =>a]

so I package it up?

...
data WibblePackage where

...
WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...
fizzBuzz :: [WibblePackage] -> Integer

...
fizzBuzz [] = 0

...
fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...
help :: Integer

...
help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works!

OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type.

How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye).

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.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- Chris Allen Currently working on http://haskellbook.com

Christopher Allen

4:12 p.m.

Sorry for adding to the dogpile, should've guessed others would point to the same resource :) On Fri, Jun 19, 2015 at 11:11 AM, Christopher Allen wrote:

...

This is a pattern people usually refer to as the "existential typeclass antipattern".

An article goes into more depth here: https://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-...

Sometimes you need a GADT with an existential type and a typeclass, but it doesn't occur as often as the equivalent does in OO.

On Fri, Jun 19, 2015 at 10:48 AM, Nicholls, Mark wrote:

...
My initial guess would be no…its built into the language

But….

I find myself tempted to use it (in order to match on a type that’s the instance of a typeclass)…which makes me think I’ve missed something.

So…

...
data Foo = Foo

...
data Bar = Bar

Ooo…this looks like an OO design pattern…surely wrong?

...
class Wibble a where

...
visit :: (Foo -> b) -> (Bar -> b) -> a -> b

...
instance Wibble Foo where

...
visit f _ x = f x

...
instance Wibble Bar where

...
visit _ f x = f x

I want a list of Wibbles....

hmmm...

(Wibble a) => [a] is clearly wrong...

I want [(Wibble a) =>a]

so I package it up?

...
data WibblePackage where

...
WibblePackage :: (Wibble a) => a -> WibblePackage

lets try this now…so pointless function across Wibbles in a list.

...
fizzBuzz :: [WibblePackage] -> Integer

...
fizzBuzz [] = 0

...
fizzBuzz ((WibblePackage x) : xs) = (visit (\_ -> 1) (\_ -> 2) x) + (fizzBuzz xs)

...
help :: Integer

...
help = fizzBuzz [WibblePackage Foo,WibblePackage Bar]

That works!

OO nerds will get uptight about “closing” the Wibble typeclass….but I’ve managed to create a list of different types and have a mechanism for recovering the type.

How would a Haskell nerd do this? (I have looked, but they look more complicated….maybe to my OO eye).

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.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- Chris Allen Currently working on http://haskellbook.com

-- Chris Allen Currently working on http://haskellbook.com

Phil Ruffwind

6:36 p.m.

...

class Wibble a where

visit :: (Foo -> b) -> (Bar -> b) -> a -> b

The type `forall b. (X -> b) -> (Y -> b) -> b` is isomorphic to `Either X Y`. If you rearrange the signature of `visit` a little bit, visit_ :: a -> forall b . (Foo -> b) -> (Bar -> b) -> b you can see that you're really just defining a function `a -> Either Foo Bar`. So the class can be simplified to: class Wibble' a where visit' :: a -> Either Foo Bar It follows that `WibblePackage'` is equivalent to type WibblePackage' = forall a . (WibbleInstance a, a) where type WibbleInstance a = a -> Either Foo Bar Because of the `forall`, there's nothing you can do with the second component of the tuple (`a`) except to apply it to the first (`a -> Either Foo Bar`), so it's just a long-winded way of saying type WibblePackage'' = Either Foo Bar In a non-strict language like Haskell, there's no reason to use such a complicated type signature to delay the application of `a -> Either Foo Bar` to `a`.

Nicholls, Mark

22 Jun 22 Jun

7:28 a.m.

Brilliant, v interesring Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. On 19 Jun 2015, at 19:37, Phil Ruffwind wrote:

...

...
class Wibble a where

visit :: (Foo -> b) -> (Bar -> b) -> a -> b

The type `forall b. (X -> b) -> (Y -> b) -> b` is isomorphic to `Either X Y`. If you rearrange the signature of `visit` a little bit,

visit_ :: a -> forall b . (Foo -> b) -> (Bar -> b) -> b

you can see that you're really just defining a function `a -> Either Foo Bar`. So the class can be simplified to:

class Wibble' a where visit' :: a -> Either Foo Bar

It follows that `WibblePackage'` is equivalent to

type WibblePackage' = forall a . (WibbleInstance a, a)

where

type WibbleInstance a = a -> Either Foo Bar

Because of the `forall`, there's nothing you can do with the second component of the tuple (`a`) except to apply it to the first (`a -> Either Foo Bar`), so it's just a long-winded way of saying

type WibblePackage'' = Either Foo Bar

In a non-strict language like Haskell, there's no reason to use such a complicated type signature to delay the application of `a -> Either Foo Bar` to `a`. 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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Alexander Solla
Anupam Jain
Christopher Allen
MigMit
Nicholls, Mark
Patrick Chilton
Phil Ruffwind

Do people use visitor pattern in Haskell...

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