implementation

A crate for targeting and accessing actual implementation.

Take an example trait:

rust trait ScrapeTheInternet { fn scrape_the_internet(&self) -> Vec<Website>; }

The trait represents some abstract computation. The trait exports a method signature that can be implemented by types. In this case, we can imagine what a true implementation of the trait will do: Actually scrape the internet.

implementation provides the [Impl] type as an implementation target for traits having the following semantics:

• The trait has only one actual, true implementation.
• Other implementations of the trait may exist, but these are interpreted as fake, mocked in some way.

implementation enables a standardized way of writing these actual implementations in a way that allows the actual Self-receiver type to be unknown.

Usage

To define the actual, generic implementation of ScrapeTheInternet, we can write the following impl:

rust impl<T> ScrapeTheInternet for implementation::Impl<T> { fn scrape_the_internet(&self) -> Vec<Website> { todo!("find all the web pages, etc") } }

This code implements the trait for [Impl], and by doing that we have asserted that it is the actual, true implementation.

The implementation is fully generic, and works for any T.

```rust use implementation::Impl;

struct MyType;

let websites = Impl::new(MyType).scrapetheinternet(); ```

Trait bounds

The advantage of keeping trait implementations generic, is that the self type might live in a downstream crate. Let's say we need to access a configuration parameter from scrape_the_internet. E.g. the maximum number of pages to scrape:

```rust use implementation::Impl;

trait GetMaxNumberOfPages { fn getmaxnumberofpages(&self) -> Option; }

impl ScrapeTheInternet for Impl where Impl: GetMaxNumberOfPages { fn scrapetheinternet(&self) -> Vec { let maxnumberofpages = self.getmaxnumberof_pages(); todo!("find all the web pages, etc") } } ```

Now, for this to work, Impl<T> also needs to implement GetMaxNumberOfPages (for the same T that is going to be used).

GetMaxNumberOfPages would likely be implemented for a specific T rather than a generic one, since that T would typically be some configuration holding that number:

```rust struct Config { maxnumberof_pages: Option }

impl GetMaxNumberOfPages for implementation::Impl { fn getmaxnumberofpages(&self) -> Option { self.maxnumberof_pages } } ```

Explanation

This crate is the solution to a trait coherence problem.

Given the trait above, we would like to provide an actual and a mocked implementation. We might know what its actual implementation looks like as an algorithm, but not what type it should be implemented for. There could be several reasons to have a generic Self:

• The Self type might live in a downstream crate
• It is actually designed to work generically

If we had used a generic Self type (impl<T> DoSomething for T), the trait would be unable to also have distinct fake implementations, because that would break the coherence rules: A generic ("blanket") impl and a specialized impl are not allowed to exist at the same time, because that would lead to ambiguity.

To solve that, a concrete type is needed as implementation target. But that type is allowed to be generic internally. It's just the root level that needs to be a concretely named type.

That type is the [Impl] type.

When we use this implementation, we can create as many fake implementations as we want.

License: MIT