part 3.2, 'Adjunctions' #56
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| ................ | ||
| ```Haskell | ||
| class (Functor f, Representable u) => | ||
| Adjunction f u | f -> u, u -> f where |
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Omitted functional dependencies in scala version
| return :: d -> m d | ||
| ``` | ||
| ```scala | ||
| def `return`[D]: D => M[D] |
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an M[_] type parameter is missing here
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Here, m is the (endo-) functor corresponding to R∘L, and w is the (endo-) functor corresponding to L∘R. As we’ll see later, they are part of the definition of a monad and a comonad, respectively.
It seemed to me like kind of a concrete example, however simplified:
trait Extract[M[_]] { // ingnore
def return[D](d: D): M[D]
} //ignore| extract :: w c -> c | ||
| ``` | ||
| ```scala | ||
| def extract[C]: W[C] => C |
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a W[_] type parameter is missing here
src/main/tut/3.2-adjunctions.md
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| implicit val F: Functor[F], | ||
| val U: Representable[U] | ||
| ){ | ||
| def unit[A](a: => A): U[F[A]] |
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why did you decide to make a byname here?
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I don't remember why I've added it, and don't see a reason to keep it.
src/main/tut/3.2-adjunctions.md
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| /* changed the order of parameters | ||
| to help scala compiler infer types */ | ||
| def leftAdjunct[A, B] | ||
| (a: => A)(f: F[A] => B): U[B] |
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Changing parameter order makes sense from a scala perspective, but it might make seeing the relationship harder if one doesn't also read the haskell code
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That's why I've added a comment.
The reasons to change the order in scala version:
- We are still going to do it in scala for sometime
- This example:
rightAdjunct(a)(identity)
src/main/tut/3.2-adjunctions.md
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| rightAdjunct f = counit . fmap f | ||
| ``` | ||
| ```scala | ||
| def unit[A](a: => A): U[F[A]] = |
src/main/tut/3.2-adjunctions.md
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| rightAdjunct(a)(identity) | ||
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| def leftAdjunct[A, B] | ||
| (a: => A)(f: F[A] => B): U[B] = |
Zelenya
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merged since I think all feedback addressed. |
Fixes #26