>
> Right, that makes sense, and indeed
>
> implicit def monoidOfTagged[T, A](implicit M0: Monoid[A]): Monoid[A @@ T] =
> Tag.subst[A, Monoid, T](M0)
>
>
> works perfectly.
>
> Now, when I am thinking back, I realise, that at the beginning the goal
> was to apply binary operations on tagged values. For example you can add
> Kilogramms or Cms to each other, but you can not add Kg to Cm, so it should
> not compile. While tagging solves the second problem, the first one still
> remains (I think to #unwrap manually every time you wan to add two numbers
> - is not a real solution), so I thought to solve it with Monoid. But now I
> see, that it does not give you arithmetical operations at all, and
> moreover, this reasoning is only valid when Numbers are tagged.
>
> So actually, it makes sense to have this functionality on a Numeric
> typeclass (which is most likely noy the scalaZ domain):
>
> implicit def numericOfTagged[T, A: Numeric]: Numeric[A @@ T]
>
>
On Sunday, 25 September 2016 18:35:17 UTC+2, Parakhonskiy Gleb wrote:
>
> Hi everyone,
> I suggest to add such thing to Monoid object. Basically it gives you a
> monoid for a tagged type, given a monoid for this type. I found it quite
> useful,
> and if you think that it makes sense, I can make PR.
>
>
>
> implicit def monoidOfTagged[T, A](implicit M0: Monoid[A]): Monoid[A @@ T] =
> new Monoid[@@[A, T]] {
>
> override def zero: @@[A, T] = Tag.of[T](M0.zero)
>
> override def append(f1: @@[A, T], f2: ⇒ @@[A, T]): @@[A, T] =
> Tag.of[T](M0.append(Tag.unwrap(f1), Tag.unwrap(f2)))
> }
>
>
>
>
> And one more suggestion.
> You can always take out function from Monad - if it returns the value
> inside this Monad:
>
> def ???[A, B, F[_]: Monad](f: F[A ⇒ F[B]]): A ⇒ F[B] = a ⇒ {
> Monad[F].bind(f)(ff ⇒ ff(a))
> }
>
>
> It just feels a bit weird to have such high level function laying in the
> Utils. I did not find such thing in scalaZ, so may be it makes sense to add
> it as well.
>
>
> Thanks for a great library!
> Cheers,
> Gleb
>
>
>
>
>
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