On 11/11/16 1:37 AM, ace_coder wrote:
However when I declare it's usage within another monad I get a type
error.
trait Foo[ F[ _ ], +A, +B] {
def getFoo(): F[A \/ B]
}
The above code throws a compile-time error regarding type parameters A
and B being
covariant types at invariant positions, but I thought scalaz.\/ is
defined with covariant types.
Yes, I described this problem in "The missing diamond of Scala variance"
<https://failex.blogspot.com/2016/09/the-missing-diamond-of-scala-variance.html#markdown-header-whence-monad-transformer-variance>
(video from 17:02 <https://youtu.be/h4LzUkYQGyE?t=17m2s>, might be a
little clearer explanation for your purposes). Unfortunately, without
the "variance variables" described therein, while you can mark F as
F[+_], as Scalaz used to, this will restrict you to F-monads that are
themselves marked covariant.
--
Stephen Compall
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