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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Introducing Assemblist: Fluent Method-Chain Builders for Rust

Hi Rustaceans,

I wanted to share assemblist, a library I created to simplify immutable builder patterns in Rust. I know the ecosystem already offers many builder-related crates, but I wanted to explore a different approach — one that makes defining method chains feel more natural and intuitive.

Unlike traditional builders tied to struct annotations, Assemblist lets you declare method chains as you use them, making APIs clearer and more flexible.

Example Usage

Here's a simple example demonstrating the idea:

assemblist!{
    fn define_movie<'a>(name: &'a str)
        .released_in(release_year: usize)
        .directed_by(director_name: &'a str) -> Movie
    {
        Movie {
            name: name.to_string(),
            release_year,
            director_name: director_name.to_string(),
        }
    }
}

Once declared, method chains can be called fluently:

let movie = define_movie("The Lobster")
    .released_in(2015)
    .directed_by("Yorgos Lanthimos");

Supporting Alternative Paths

Assemblist also introduces alternative blocks, allowing users to define multiple possible continuations for a chain using a .{ … } scope. This mechanism is recursive, enabling expressive branching.

assemblist!{
    fn new_http_request_to(url: Uri)
        .from<'a>(user_agent: &'a str)
        .with_authorization(authorization: HttpAuthorization).{

        fn as_get() -> GetHttpRequest {
            GetHttpRequest {
                url,
                user_agent: user_agent.to_string(),
                authorization,
            }
        }

        fn as_post().{
            fn with_text(body: String) -> PostHttpRequest {
                PostHttpRequest {
                    url,
                    user_agent: user_agent.to_string(),
                    authorization,
                    body: HttpBody::Text(body),
                }
            }

            fn with_json(json: JsonValue) -> PostHttpRequest {
                PostHttpRequest {
                    url,
                    user_agent: user_agent.to_string(),
                    authorization,
                    body: HttpBody::Json(json),
                }
            }
        }
    }
}

Looking for Feedback

Assemblist is available on crate.io, and I’d love to hear your thoughts.

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