const functor (or parameterized type)

[Vlad Patryshev]

So, there seems to be no way to define a constant functor (or a constant parameterized type), or am I wrong?

class ConstantFunctor[T[_](value: T[???]) extends Functor[T] {

  def map[A, B](f: (A) => B) = value

}

Scalaz has a special class called Const, but I was kind of lost trying to figure out how it could be useful.

Or is it all thanks to Free Theorems? How about phantom types then...

Thanks,
-Vlad

[Vlad Patryshev]

Sorry, I meant something like this:

abstract class ConstantFunctor[T[_], T0 <: T[Any]] extends Functor[T] {

  def f1[A, B](f: (A) => B) = (identity[T0]_).asInstanceOf[T[A] => T[B]]

}

Thanks,
-Vlad

[Tony Morris]

apocalisp.wordpress.com/2010/04/21/a-proper-constant-function-in-scala/

On 04/06/12 08:38, Vlad Patryshev wrote:
> Sorry, I meant something like this:
>

> *abstract class ConstantFunctor[T[_], T0 <: T[Any]] extends Functor[T] {*
> * def f1[A, B](f: (A) => B) = (identity[T0]_).asInstanceOf[T[A] => T[B]]*
> *}*

>
>
> Thanks,
> -Vlad
>
>
> On Sun, Jun 3, 2012 at 2:27 PM, Vlad Patryshev <vpatr...@gmail.com> wrote:
>
>> So, there seems to be no way to define a constant functor (or a constant
>> parameterized type), or am I wrong?
>>
>> class ConstantFunctor[T[_](value: T[???]) extends Functor[T] {
>> def map[A, B](f: (A) => B) = value
>> }
>>
>> Scalaz has a special class called Const, but I was kind of lost trying to
>> figure out how it could be useful.
>>
>> Or is it all thanks to Free Theorems? How about phantom types then...
>>
>> Thanks,
>> -Vlad
>>

--
Tony Morris
http://tmorris.net/