Apply functions in method-position.

The Problem

Much functionality in Rust is provided by methods, but occasionally we're forced to use certain standalone functions. Examples are functions like std::str::from_utf8 or std::sync::Arc::new:

let user = fetch_user()
    .await
    .map_err(Error::UserIsLost)?;
let arc = Arc::new(user);
Ok(arc)

Many functions that we write are also effectful, and require us to wrap some final value in an Ok. The combination of these factors means that we often have to bind values to names, even when we don't want to. Naming is hard, and bad names can cause confusion for later readers. In the above, who benefits from seeing a symbol named arc? Names should reflect what something is, not what it's wrapped in.

And what if the only appropriate name would be the one it already had, say in the case of a "response" value, etc.? This encourages "variable shadowing", as in:

let resp = make_request(req).await?;
let resp = foo(resp);
let resp = bar(resp);

This code is fragile to ownership and refactors. We shouldn't have to write code that looks like this.

The Solution

This library provides the trait Apply, which exposes an apply method that is injected into all types via a "blanket implementation". apply allows you to call top-level functions in a chain with the rest of your method calls. This way, the original example becomes:

fetch_user()
    .await
    .map_err(Error::UserIsLost)?
    .apply(Arc::new)
    .apply(Ok)

Ah, beautiful, consistent nesting. And no spurrious names to confuse the peasantry.