Inject for either #1600
Inject for either #1600
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andyscott
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I'll rebase/finish this after settling #1725 |
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Codecov Report
@@ Coverage Diff @@
## master #1600 +/- ##
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+ Coverage 93.99% 94.02% +0.03%
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Files 253 256 +3
Lines 4180 4202 +22
Branches 155 90 -65
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+ Hits 3929 3951 +22
Misses 251 251
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| def prj: B => Option[A] | ||
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| def apply(a: A): B = inj(a) |
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shall we make these two final as well?
andyscott
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| /** | ||
| * Inject is a type class to inject a value of type `A` into a | ||
| * coproduct of types `B` when `A :≺: B`. The coproduct `B` is | ||
| * modelled with `Either`. |
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It's true that it is the original incentive for introducing this class, but now that we made a type class, this is just one specific usage/instances of Inject and InjectK right?
We could have an Inject between A and List[A] or Int and Double, right?
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Yes, that's correct. I suppose it could be reworded to indicate that that's just the default behavior?
Inject is a type class to inject a value of type `A` into another type `B`.
The default instances provided by cats injects `A` into coproduct of types
`B` when `A :≺: B`. The coproduct `B` is modelled with `Either`.
Also in this explanation, :≺: is used in a conceptual sense. :≺: exists as a type alias for InjectK which could be confusing. Thoughts?
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My understanding of the Inject type class is that it represents an injective function between type A and B. If we treat types as sets, and a function between the two types are just pairs of elements in the two sets A and B. Each pair has an "input" element in set A and an "output" element of B. For an injective function, for every element in A there is exactly one pair. And for every element in B there is either one pair or no pair at all.
A total pure scala function is an injective function, but it only specifies pairs for all elements in A, i.e. given any A you can get a B. It doesn't specify pairs for elements in B, this Inject type class does that through proj.
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Agreed with @kailuowang; I don't think this is particularly informative as to what Inject does, especially because we're likely going to provide these other instances in the future from cats. I'd pick something like:
Inject is a type class providing an injection f from type A into another type B.
An injection is a function which does not destroy any information, and thus for every b: B there is at most one a: A such that f(a) = b.
Because of this all injections admit partial inverses, functions B => Option[A], which pair a value b: B with the single value a: A that f sends to b if it exists.
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@edmundnoble I like your explanation. I tweaked it a tad-- how about:
/**
* Inject is a type class providing an injection from type `A` into
* another type `B`. An injection is a function `inj` which does not
* destroy any information: for every `b: B` there is at most one
* `a: A` such that `inj(a) = b`.
*
* Because of this all injections admit partial inverses `prj` which
* pair a value `b: B` back with a single value `a: A`.
*
* @since 1.0
* @note Prior to cats 1.0, Inject handled injection for type
* constructors. For injection of type constructors, use [[InjectK]].
*
* @see [[InjectK]] for injection for [[cats.data.EitherK]]
*/
abstract class Inject[A, B] // ...
I'll give the same treatment to InjectK once proper doc is decided.
andyscott
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merge with 2 sign-offs |
Re-introduces
Inject, but forEither. I also went ahead and tried to document bothInjectandInjectK.Todo:
Associated issue: #1599