RR Fetch Syntax

[raulraja]

The following PR provides implicit syntax to lift any A to Fetch[A] as well as syntax for the most useful combinators on a Fetch[A].

@dialelo @rafaparadela Can you please review? Thanks!

Pseudo extract from the included docs contribution:

Syntax

Implicit syntax

Fetch provides implicit syntax to lift any value to the context of a Fetch in addition to the most common used combinators active within Fetch instances.

pure

Plain values can be lifted to the Fetch monad with value.fetch:

val fetchPure: Fetch[Int] = 42.fetch

Executing a pure fetch doesn't query any data source, as expected.

val result: Int = fetchPure.runA[Eval].value

error

Errors can also be lifted to the Fetch monad via exception.fetch. Note that interpreting
an errorful fetch to Eval will throw the exception so we won't do that:

val fetchFail: Fetch[Int] = (new Exception("Something went terribly wrong")).fetch[Int]

join

We can compose two independent fetches with fetch1.join(fetch2).

val fetchJoined: Fetch[(Post, User)] = getPost(1).join(getUser(2))

If the fetches are to the same data source they will be batched; if they aren't, they will be evaluated at the same time.

val result: (Post, User) = fetchJoined.runA[Eval].value

runA

Run directly any fetch to a target any target Monad with a MonadError instance in scope fetch1.runA[Eval].

val post: Eval[Post] = getPost(1).runA[Eval]

runE

Extract a fetch an get it's runtime environment fetch1.runE[Eval].

val env: Eval[FetchEnv] = getPost(1).runE[Eval]

runF

Run a fetch obtaining the environment and final value fetch1.runF[Eval].

val env: Eval[(FetchEnv, Post)] = getPost(1).runF[Eval]

[ghost]

In the case of Eval it won't throw the exception unless accessing the value with .value, maybe we can show how the exception is conveyed on the Xor.Left when ussing the attempt combinator?

[ghost]

Other than the comment LGTM, really awesome work 👍

[raulraja]

Feel free to reword or remove that part for now if you want this to be included in the release.I won't be close to a computer this evening

[ghost]

All right, i'll edit that part and cut another release with the new syntax and docs.

[ghost]