Styjun

Why is “deal 6 damage” a legit phrase?

Error with uppercase in titlesec's label field

Normally Closed Optoisolators

"Will flex for food". What does this phrase mean?

How do I safety check that there is no light in Darkroom / Darkbag?

Why are sugars in whole fruits not digested the same way sugars in juice are?

Why should I use a big powerstone instead of smaller ones?

Adjective for when skills are not improving and I'm depressed about it

A game of red and black

Is it really a problem to declare that a visitor to the UK is my "girlfriend", in terms of her successfully getting a Standard Visitor visa?

Went to a big 4 but got fired for underperformance in a year recently - Now every one thinks I'm pro - How to balance expectations?

When did J.K. Rowling decide to make Ron and Hermione a couple?

How did Biff return to 2015 from 1955 without a lightning strike?

Word for giving preference to the oldest child

How do I respond appropriately to an overseas company that obtained a visa for me without hiring me?

Can birds evolve without trees?

Should I put my name first or last in the team members list?

Using Python in a Bash Script

How to define a functional that produces at runtime a function by evaluating selected parts in its definition?

The grades of the students in a class

Why do we need a voltage divider when we get the same voltage at the output as the input?

Should 2FA be enabled on service accounts?

PI 4 screen rotation from the terminal

Basic transistor circuit

Is it possible to make a Functor instance for a type parameter other than the last? [duplicate]

Can I write a higher order type for a -> b -> *?Monad instance for pairsAre there “type-level combinators”? Will they exist in some future?Haskell defining Functor instance for an alternative Either data typeLarge-scale design in Haskell?Functor / Applicative instances for State in HaskellWhat functionality do you get for free with Functors or other type-classes?Deriving functor instance, not on last type argumentCan this multi-parameter type class be simplified?Understanding operations on composed functor typesHow to create Functor definition for Either typeIs this property of a functor stronger than a monad?How do I make the partially-applied function type into a functor?What does instance mean in Haskell?

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;

1

This question already has an answer here:

• 
  Can I write a higher order type for a -> b -> *?
  
  1 answer
  
  


• 
  Monad instance for pairs
  
  2 answers
  
  

Sometimes there are data types that contain multiple type parameters (eg Either a b and (a,b)). We can create a Functor instance for the b parameter which turns out to be pretty straight forward. But what about the a parameter? I know that one could wrap everything in a newtype with the parameters in reverse order, but is there a way to create the instance of a second order type class (is that the correct way to describe Functor/ Applicative / Monad?) directly on a parameter other than the last? I don't think so but I could not actually find that written down anywhere.

Context:

Another question on Stack Overflow was from a student studying Haskell who was trying to create a Functor instance for their own Either-like Type presumably as a class assignment. Unfortunately, they had defined their data type as data Alt a b = Success a | Failure b and could not get the functor instance to work because they wanted to fmap over the a parameter. I was about to write that Functor instances only work for the last parameter when I realized that I did not actually know that for sure. Thus my question.

haskell typeclass

share|improve this question

edited Mar 27 at 2:22

duplode

26.7k44 gold badges5454 silver badges104104 bronze badges

asked Mar 26 at 23:34

John F. MillerJohn F. Miller

17.3k88 gold badges5757 silver badges109109 bronze badges

marked as duplicate by jberryman, Daniel Wagner haskell
Users with the  haskell badge can single-handedly close haskell questions as duplicates and reopen them as needed.

StackExchange.ready(function()
if (StackExchange.options.isMobile) return;

$('.dupe-hammer-message-hover:not(.hover-bound)').each(function()
var $hover = $(this).addClass('hover-bound'),
$msg = $hover.siblings('.dupe-hammer-message');

$hover.hover(
function()
$hover.showInfoMessage('',
messageElement: $msg.clone().show(),
transient: false,
position: my: 'bottom left', at: 'top center', offsetTop: -7 ,
dismissable: false,
relativeToBody: true
);
,
function()
StackExchange.helpers.removeMessages();

);
);
);
Mar 27 at 0:37

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  well there are always Bifunctors - you could define a new typeclass which just included first from Bifunctor (presumably under some other name). I think the only reason the last parameter is "privileged" in the way you describe is that type application (like function application) is left-associative - so Either a b is the same as (Either a) b, and can't be rewritten as something applied to a (without some newtype shenanigans as you observe).
  
  – Robin Zigmond
  Mar 26 at 23:40
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also related: Are there “type-level combinators”? Will they exist in some future?
  
  – duplode
  Mar 26 at 23:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I don't think this is actually an exact duplicate of either of the linked questions.
  
  – Benjamin Hodgson♦
  Mar 27 at 15:21
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @John F. Miller: I asked the original questions. Benjamin Hodgson, Chepner and Daniel Wagner provided lots of useful information. I learned a lot from their comments in combination with getting my head around the Typeclassopedia. I am still a long way from the level of understanding these guys have of Haskell, but it nonetheless it fuels my enthousiasm to dive in deeper and deeper ;-)
  
  – Madderote
  Mar 27 at 19:14
  

  
  
  
  

add a comment | 

1

This question already has an answer here:

• 
  Can I write a higher order type for a -> b -> *?
  
  1 answer
  
  


• 
  Monad instance for pairs
  
  2 answers
  
  

Sometimes there are data types that contain multiple type parameters (eg Either a b and (a,b)). We can create a Functor instance for the b parameter which turns out to be pretty straight forward. But what about the a parameter? I know that one could wrap everything in a newtype with the parameters in reverse order, but is there a way to create the instance of a second order type class (is that the correct way to describe Functor/ Applicative / Monad?) directly on a parameter other than the last? I don't think so but I could not actually find that written down anywhere.

Context:

Another question on Stack Overflow was from a student studying Haskell who was trying to create a Functor instance for their own Either-like Type presumably as a class assignment. Unfortunately, they had defined their data type as data Alt a b = Success a | Failure b and could not get the functor instance to work because they wanted to fmap over the a parameter. I was about to write that Functor instances only work for the last parameter when I realized that I did not actually know that for sure. Thus my question.

haskell typeclass

share|improve this question

edited Mar 27 at 2:22

duplode

26.7k44 gold badges5454 silver badges104104 bronze badges

asked Mar 26 at 23:34

John F. MillerJohn F. Miller

17.3k88 gold badges5757 silver badges109109 bronze badges

marked as duplicate by jberryman, Daniel Wagner haskell
Users with the  haskell badge can single-handedly close haskell questions as duplicates and reopen them as needed.

StackExchange.ready(function()
if (StackExchange.options.isMobile) return;

$('.dupe-hammer-message-hover:not(.hover-bound)').each(function()
var $hover = $(this).addClass('hover-bound'),
$msg = $hover.siblings('.dupe-hammer-message');

$hover.hover(
function()
$hover.showInfoMessage('',
messageElement: $msg.clone().show(),
transient: false,
position: my: 'bottom left', at: 'top center', offsetTop: -7 ,
dismissable: false,
relativeToBody: true
);
,
function()
StackExchange.helpers.removeMessages();

);
);
);
Mar 27 at 0:37

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  well there are always Bifunctors - you could define a new typeclass which just included first from Bifunctor (presumably under some other name). I think the only reason the last parameter is "privileged" in the way you describe is that type application (like function application) is left-associative - so Either a b is the same as (Either a) b, and can't be rewritten as something applied to a (without some newtype shenanigans as you observe).
  
  – Robin Zigmond
  Mar 26 at 23:40
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also related: Are there “type-level combinators”? Will they exist in some future?
  
  – duplode
  Mar 26 at 23:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I don't think this is actually an exact duplicate of either of the linked questions.
  
  – Benjamin Hodgson♦
  Mar 27 at 15:21
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @John F. Miller: I asked the original questions. Benjamin Hodgson, Chepner and Daniel Wagner provided lots of useful information. I learned a lot from their comments in combination with getting my head around the Typeclassopedia. I am still a long way from the level of understanding these guys have of Haskell, but it nonetheless it fuels my enthousiasm to dive in deeper and deeper ;-)
  
  – Madderote
  Mar 27 at 19:14
  

  
  
  
  

add a comment | 

This question already has an answer here:

• 
  Can I write a higher order type for a -> b -> *?
  
  1 answer
  
  


• 
  Monad instance for pairs
  
  2 answers
  
  

Sometimes there are data types that contain multiple type parameters (eg Either a b and (a,b)). We can create a Functor instance for the b parameter which turns out to be pretty straight forward. But what about the a parameter? I know that one could wrap everything in a newtype with the parameters in reverse order, but is there a way to create the instance of a second order type class (is that the correct way to describe Functor/ Applicative / Monad?) directly on a parameter other than the last? I don't think so but I could not actually find that written down anywhere.

Context:

Another question on Stack Overflow was from a student studying Haskell who was trying to create a Functor instance for their own Either-like Type presumably as a class assignment. Unfortunately, they had defined their data type as data Alt a b = Success a | Failure b and could not get the functor instance to work because they wanted to fmap over the a parameter. I was about to write that Functor instances only work for the last parameter when I realized that I did not actually know that for sure. Thus my question.

haskell typeclass

share|improve this question

edited Mar 27 at 2:22

duplode

26.7k44 gold badges5454 silver badges104104 bronze badges

asked Mar 26 at 23:34

John F. MillerJohn F. Miller

17.3k88 gold badges5757 silver badges109109 bronze badges

share|improve this question

edited Mar 27 at 2:22

duplode

26.7k44 gold badges5454 silver badges104104 bronze badges

asked Mar 26 at 23:34

John F. MillerJohn F. Miller

17.3k88 gold badges5757 silver badges109109 bronze badges

share|improve this question

edited Mar 27 at 2:22

duplode

26.7k44 gold badges5454 silver badges104104 bronze badges

asked Mar 26 at 23:34

John F. MillerJohn F. Miller

17.3k88 gold badges5757 silver badges109109 bronze badges

edited Mar 27 at 2:22

duplode

26.7k44 gold badges5454 silver badges104104 bronze badges

edited Mar 27 at 2:22

duplode

26.7k44 gold badges5454 silver badges104104 bronze badges

asked Mar 26 at 23:34

John F. MillerJohn F. Miller

17.3k88 gold badges5757 silver badges109109 bronze badges

asked Mar 26 at 23:34

John F. MillerJohn F. Miller

17.3k88 gold badges5757 silver badges109109 bronze badges