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How can I define an HKT for instance (over object) methods in Scala?

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3

How do you define an HKT in Scala to provide methods, such as map, as methods on instances instead of functions on objects?

I know you can

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


object Q extends M[Q] 
 def map[A, B](f: Q[A])(fn: A => B) = new Q(fn(f.e))


class Q[A](val e: A)

Q.map(new Q(1))(_.toDouble)

However, I want to use map on instances instead. I haven't seen any literature that does this, so I've written the following.

trait M[A, F[A]] 
 def map[B](f: A => B): F[B]


class Q[A](val e: A) extends M[A, Q] 
 def map[B](f: A => B) = new Q(f(e))


new Q(1).map(_.toDouble)

Is it idiomatic Scala? Does it do what map should do? Are there limitations compared with the object version above? (An aside, with extends M[A, Q[A]] it won't compile - why?)

I have seen

trait M[A] 
 def map[B](f: A => B): M[B]

but then Q's map could return a class R[A] extends M[A] which isn't desired.

scala monads traits higher-kinded-types

share|improve this question

edited Mar 28 at 22:12

Joel Berkeley

asked Mar 28 at 21:47

Joel BerkeleyJoel Berkeley

1,66288 silver badges2424 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  So, you want a typeclass. The idea is that there is only one instance of the typeclass for your type, because here the relation ship is Q _has_ an instance of M instead of Q _is_ an instance of M. Usually, in Scala, those instances are made implicit, so you can use them without having to pass them manually. Additionally, one usually defines an implicit Ops or Syntax class (like Joe K did) to provide the functions of the typeclass with a regular method notation, just to make their use more natural.
  
  – Luis Miguel Mejía Suárez
  Mar 28 at 22:54
  

  
  
  
  

add a comment
 | 

3

How do you define an HKT in Scala to provide methods, such as map, as methods on instances instead of functions on objects?

I know you can

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


object Q extends M[Q] 
 def map[A, B](f: Q[A])(fn: A => B) = new Q(fn(f.e))


class Q[A](val e: A)

Q.map(new Q(1))(_.toDouble)

However, I want to use map on instances instead. I haven't seen any literature that does this, so I've written the following.

trait M[A, F[A]] 
 def map[B](f: A => B): F[B]


class Q[A](val e: A) extends M[A, Q] 
 def map[B](f: A => B) = new Q(f(e))


new Q(1).map(_.toDouble)

Is it idiomatic Scala? Does it do what map should do? Are there limitations compared with the object version above? (An aside, with extends M[A, Q[A]] it won't compile - why?)

I have seen

trait M[A] 
 def map[B](f: A => B): M[B]

but then Q's map could return a class R[A] extends M[A] which isn't desired.

scala monads traits higher-kinded-types

share|improve this question

edited Mar 28 at 22:12

Joel Berkeley

asked Mar 28 at 21:47

Joel BerkeleyJoel Berkeley

1,66288 silver badges2424 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  So, you want a typeclass. The idea is that there is only one instance of the typeclass for your type, because here the relation ship is Q _has_ an instance of M instead of Q _is_ an instance of M. Usually, in Scala, those instances are made implicit, so you can use them without having to pass them manually. Additionally, one usually defines an implicit Ops or Syntax class (like Joe K did) to provide the functions of the typeclass with a regular method notation, just to make their use more natural.
  
  – Luis Miguel Mejía Suárez
  Mar 28 at 22:54
  

  
  
  
  

add a comment
 | 

How do you define an HKT in Scala to provide methods, such as map, as methods on instances instead of functions on objects?

I know you can

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


object Q extends M[Q] 
 def map[A, B](f: Q[A])(fn: A => B) = new Q(fn(f.e))


class Q[A](val e: A)

Q.map(new Q(1))(_.toDouble)

However, I want to use map on instances instead. I haven't seen any literature that does this, so I've written the following.

trait M[A, F[A]] 
 def map[B](f: A => B): F[B]


class Q[A](val e: A) extends M[A, Q] 
 def map[B](f: A => B) = new Q(f(e))


new Q(1).map(_.toDouble)

Is it idiomatic Scala? Does it do what map should do? Are there limitations compared with the object version above? (An aside, with extends M[A, Q[A]] it won't compile - why?)

I have seen

trait M[A] 
 def map[B](f: A => B): M[B]

but then Q's map could return a class R[A] extends M[A] which isn't desired.

scala monads traits higher-kinded-types

share|improve this question

edited Mar 28 at 22:12

Joel Berkeley

asked Mar 28 at 21:47

Joel BerkeleyJoel Berkeley

1,66288 silver badges2424 bronze badges

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


object Q extends M[Q] 
 def map[A, B](f: Q[A])(fn: A => B) = new Q(fn(f.e))


class Q[A](val e: A)

Q.map(new Q(1))(_.toDouble)

trait M[A, F[A]] 
 def map[B](f: A => B): F[B]


class Q[A](val e: A) extends M[A, Q] 
 def map[B](f: A => B) = new Q(f(e))


new Q(1).map(_.toDouble)

trait M[A] 
 def map[B](f: A => B): M[B]

share|improve this question

edited Mar 28 at 22:12

Joel Berkeley

asked Mar 28 at 21:47

Joel BerkeleyJoel Berkeley

1,66288 silver badges2424 bronze badges

share|improve this question

edited Mar 28 at 22:12

Joel Berkeley

asked Mar 28 at 21:47

Joel BerkeleyJoel Berkeley

1,66288 silver badges2424 bronze badges

asked Mar 28 at 21:47

Joel BerkeleyJoel Berkeley

1,66288 silver badges2424 bronze badges

asked Mar 28 at 21:47

Joel BerkeleyJoel Berkeley

1,66288 silver badges2424 bronze badges

1 Answer
1

active

oldest

votes

4

There's a library specifically for this: https://github.com/mpilquist/simulacrum

You can also do it yourself. The typical pattern is to define a corresponding implicit MOps class or whatever:

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


implicit class MOps[F[_] : M, A](x: F[A]) 
 def map[B](f: A => B): F[B] = implicitly[M[F]].map(x)(f)

Then example usage:

case class Box[A](a: A)

implicit val BoxM = new M[Box] 
 def map[A, B](f: Box[A])(fn: A => B): Box[B] = Box(fn(f.a))


Box(2).map(_.toString)

But it's a lot of boilerplate, so simulacrum does it all for you (and more).

share|improve this answer

edited Mar 28 at 23:09

answered Mar 28 at 22:22

Joe KJoe K

15.8k11 gold badge2626 silver badges4747 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  so you define the map as per the object version then use implicits to give that functionality to the instance?
  
  – Joel Berkeley
  Mar 28 at 22:33
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  what implications are there for this approach compared to mine?
  
  – Joel Berkeley
  Mar 28 at 22:52
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Your approach allows for some weird things like class Foo[A] extends M[A, Bar]. This is a common problem, I think this is the best write-up I've seen about it: tpolecat.github.io/2015/04/29/f-bounds.html
  
  – Joe K
  Mar 28 at 22:56
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, some methods of some typeclasses don't have instances associated with them, e.g. Monoid's empty method. So these typeclasses have to use this style of representation, and once you start having a few of this form, you often want inheritance (Monoid[A] extends Semigroup[A]) and pretty soon it's easier to do everything this way
  
  – Joe K
  Mar 28 at 22:58
  

  
  
  
  

add a comment
 | 

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4

There's a library specifically for this: https://github.com/mpilquist/simulacrum

You can also do it yourself. The typical pattern is to define a corresponding implicit MOps class or whatever:

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


implicit class MOps[F[_] : M, A](x: F[A]) 
 def map[B](f: A => B): F[B] = implicitly[M[F]].map(x)(f)

Then example usage:

case class Box[A](a: A)

implicit val BoxM = new M[Box] 
 def map[A, B](f: Box[A])(fn: A => B): Box[B] = Box(fn(f.a))


Box(2).map(_.toString)

But it's a lot of boilerplate, so simulacrum does it all for you (and more).

share|improve this answer

edited Mar 28 at 23:09

answered Mar 28 at 22:22

Joe KJoe K

15.8k11 gold badge2626 silver badges4747 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  so you define the map as per the object version then use implicits to give that functionality to the instance?
  
  – Joel Berkeley
  Mar 28 at 22:33
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  what implications are there for this approach compared to mine?
  
  – Joel Berkeley
  Mar 28 at 22:52
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Your approach allows for some weird things like class Foo[A] extends M[A, Bar]. This is a common problem, I think this is the best write-up I've seen about it: tpolecat.github.io/2015/04/29/f-bounds.html
  
  – Joe K
  Mar 28 at 22:56
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, some methods of some typeclasses don't have instances associated with them, e.g. Monoid's empty method. So these typeclasses have to use this style of representation, and once you start having a few of this form, you often want inheritance (Monoid[A] extends Semigroup[A]) and pretty soon it's easier to do everything this way
  
  – Joe K
  Mar 28 at 22:58
  

  
  
  
  

add a comment
 | 

4

There's a library specifically for this: https://github.com/mpilquist/simulacrum

You can also do it yourself. The typical pattern is to define a corresponding implicit MOps class or whatever:

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


implicit class MOps[F[_] : M, A](x: F[A]) 
 def map[B](f: A => B): F[B] = implicitly[M[F]].map(x)(f)

Then example usage:

case class Box[A](a: A)

implicit val BoxM = new M[Box] 
 def map[A, B](f: Box[A])(fn: A => B): Box[B] = Box(fn(f.a))


Box(2).map(_.toString)

But it's a lot of boilerplate, so simulacrum does it all for you (and more).

share|improve this answer

edited Mar 28 at 23:09

answered Mar 28 at 22:22

Joe KJoe K

15.8k11 gold badge2626 silver badges4747 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  so you define the map as per the object version then use implicits to give that functionality to the instance?
  
  – Joel Berkeley
  Mar 28 at 22:33
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  what implications are there for this approach compared to mine?
  
  – Joel Berkeley
  Mar 28 at 22:52
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Your approach allows for some weird things like class Foo[A] extends M[A, Bar]. This is a common problem, I think this is the best write-up I've seen about it: tpolecat.github.io/2015/04/29/f-bounds.html
  
  – Joe K
  Mar 28 at 22:56
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Also, some methods of some typeclasses don't have instances associated with them, e.g. Monoid's empty method. So these typeclasses have to use this style of representation, and once you start having a few of this form, you often want inheritance (Monoid[A] extends Semigroup[A]) and pretty soon it's easier to do everything this way
  
  – Joe K
  Mar 28 at 22:58
  

  
  
  
  

add a comment
 | 

There's a library specifically for this: https://github.com/mpilquist/simulacrum

You can also do it yourself. The typical pattern is to define a corresponding implicit MOps class or whatever:

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


implicit class MOps[F[_] : M, A](x: F[A]) 
 def map[B](f: A => B): F[B] = implicitly[M[F]].map(x)(f)

Then example usage:

case class Box[A](a: A)

implicit val BoxM = new M[Box] 
 def map[A, B](f: Box[A])(fn: A => B): Box[B] = Box(fn(f.a))


Box(2).map(_.toString)

But it's a lot of boilerplate, so simulacrum does it all for you (and more).

share|improve this answer

edited Mar 28 at 23:09

answered Mar 28 at 22:22

Joe KJoe K

15.8k11 gold badge2626 silver badges4747 bronze badges

trait M[F[_]] 
 def map[A, B](f: F[A])(fn: A => B): F[B]


implicit class MOps[F[_] : M, A](x: F[A]) 
 def map[B](f: A => B): F[B] = implicitly[M[F]].map(x)(f)

case class Box[A](a: A)

implicit val BoxM = new M[Box] 
 def map[A, B](f: Box[A])(fn: A => B): Box[B] = Box(fn(f.a))


Box(2).map(_.toString)

share|improve this answer

edited Mar 28 at 23:09

answered Mar 28 at 22:22

Joe KJoe K

15.8k11 gold badge2626 silver badges4747 bronze badges

answered Mar 28 at 22:22

Joe KJoe K

15.8k11 gold badge2626 silver badges4747 bronze badges

answered Mar 28 at 22:22

Joe KJoe K

15.8k11 gold badge2626 silver badges4747 bronze badges