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## Alleycats | ||
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### Overview | ||
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Alleycats is a module of the [Cats](https://github.com/typelevel/cats) | ||
project that exists to support types which are not entirely savory or | ||
*law-abiding* but which may prove useful or necessary in some | ||
situations. | ||
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In some cases, a type class instance can almost (but not quite) obey | ||
the required laws (e.g. a `Monad` instance for `scala.util.Try`). In | ||
other cases, type classes which lack laws or constraints may still be | ||
useful in some cases (e.g. `Empty[_]`, a type class which provides | ||
some notion of "emptiness"). | ||
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Rather than argue about whether to permit these types in cats proper, we | ||
provide a (slightly disreputable) home for them here. | ||
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### Type classes | ||
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Alleycats introduces several new type classes. Here is an overview of | ||
the instances introduced. | ||
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#### Empty[A], Zero[A], and One[A] | ||
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A commonly-requested type class is one that encodes the idea of a set | ||
having an identity element. Normally this would be done by defining a | ||
`Monoid[A]` instance, but in some cases the idea of emptiness is | ||
independent of a particular associative operation. | ||
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In this case, `Empty[A]` may be used in place of `Monoid[A]`. It | ||
provides access to the *identity* element (via the `.empty` method), | ||
and can also provide `.isEmpty` and `.nonEmpty` if an `Eq[A]` is | ||
available. | ||
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The two other type classes, `Zero[A]` and `One[A]`, are similar, | ||
except they correspond to `AdditiveMonoid[A]` and | ||
`MultiplicativeMonoid[A]` (found in the `algebra.ring` package). Their | ||
methods are called `zero` and `one`, respectively. | ||
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While none of these type classes have their own laws, they are | ||
required not to violate the monoid laws when applicable. This means | ||
that if `Empty[A]` and `Semigroup[A]` are both available, that | ||
`Empty[A].empty` **must** function as an identity element for | ||
`Semigroup[A].combine`. In fact, together these instances can be | ||
viewed as a `Monoid[A]` (and there is an implicit method to that | ||
effect). | ||
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The same rules apply to `Zero[A]` and `One[A]` and their respective | ||
associative operations. | ||
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#### Pure[F[\_]] and Extract[F[\_]] | ||
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The `Pure[F]` type class represents the `pure` method of | ||
`Applicative[F]` separated from its `map` and `ap` methods. Like the | ||
previous type classes, if `Pure[F]` and `Apply[F]` are both available | ||
they are required to be consistent (and should provide a valid | ||
`Applicative[F]` instance). | ||
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Similarly, `Extract[F]` represents the `extract` method of | ||
`Comonad[F]` without `coflatMap` and `map` methods. When `Extract[F]` | ||
and `CoflatMap[F]` are available, they should provide a valid | ||
`Comonad[F]` instance. | ||
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#### EmptyK[F[\_]] | ||
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Finally, `EmptyK[F]` generalizes the `empty[A]` method from | ||
`MonoidK[F]`. The pattern here is the same as before -- | ||
`SemigroupK[F]` and `EmptyK[F]` should provide a valid `MonoidK[F]` | ||
instance. | ||
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### Instances | ||
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Alleycats also provides some "disreputable" type class instances. | ||
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#### Set[\_] instances | ||
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Scala's `Set[_]` takes advantage of the universal availability of | ||
`.hashCode` and `.equals`. This makes it difficult to use | ||
[parametricity](https://failex.blogspot.jp/2013/06/fake-theorems-for-free.html) | ||
to reason about sets, and casts some doubt on their use with functors | ||
and monads. | ||
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Alleycats provides `Monad[Set]` and `Traverse[Set]`. You can import | ||
these instances via `import alleycats.std.set._`. | ||
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#### Try[\_] instances | ||
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Scala's `Try[_]` is intended to replace the need for `try { ... } | ||
catch { ... }` syntax in Scala programs, to ease error-handling, and | ||
to transport exceptions as data. Due to the way `Try` transparently | ||
catches exceptions in `.map` and `.flatMap`, some people are skeptical | ||
that `Try` fulfills the necessary functor/monad laws. | ||
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Alleycats provides a `Monad[Try]`. You can import this instance via | ||
`import alleycats.std.try._`. | ||
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#### Iterable[\_] instances | ||
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Scala's `collection.Iterable[_]` offers no guarantees that it's immutable, | ||
since it abstracts over the `mutable` and `immutable` variants. However it's | ||
the type used to represent a `Map`s `values`, and its often desirable to treat the | ||
values of a map as a `Foldable` collection. Alleycats provides a `Foldable[Iterable]`, eg: | ||
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``` | ||
import cats.implicits._ | ||
import alleycats.std.iterable._ | ||
//Result "AppleOrange" | ||
Map(1 -> "Apple", 2 -> "Orange").values.combineAll | ||
``` | ||
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### Contributing | ||
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This module's goal is to be very liberal about accepting type class | ||
instances, but to only provide instances which are absent from | ||
cats proper. Law-abiding instances will end up in cats, and everything else will | ||
end up here. |