#functional #haskell #macro #generics


A library of traits to provide access to internal fields of generic structs

5 releases

0.0.5 Mar 3, 2023
0.0.4 Mar 3, 2023
0.0.3 Mar 3, 2023
0.0.2 Jan 23, 2023
0.0.1 Jan 10, 2023

#381 in Rust patterns

Download history 24/week @ 2023-01-07 2/week @ 2023-01-14 25/week @ 2023-01-21 5/week @ 2023-01-28 6/week @ 2023-02-04 4/week @ 2023-02-11 4/week @ 2023-02-18 31/week @ 2023-02-25 25/week @ 2023-03-04 7/week @ 2023-03-11

67 downloads per month

Custom license



License: MIT

Every wanted to write incidentlly generic code? and in the middle of it wanted to access a field inside it? We generally end up using a trait and binding the behavior of accessing to certain ambiguous function and then forgetting it. It's alright we all have done it, it's so common in fact that other generic and statically typed language provide us with a way to do that. It's call the lens operator. So, why not employ it for rust too.


There are exactly 3 traits that we need to concern ourself with, [See][See], Load and a special trait [Look][Look]. Derive [See][See] on your structure and bam! now your struct has implemented [See][See] for all it's fields. But it won't take effect on the counter part it will throw some error, it's alright thats where Load trait comes into picture. The sole purpose of this trait is to load all the visitor for any fields that were found during derivation. So, the this reason consider using it after all the #[derive(Look)] in your codebase.

This trait let's us visit a specific field inside a struct. similar to lens in Haskell e.g. Consider a scenerio where, you are passing a generic to a function and incidentally you also want to get a value inside it. Let's assume that there are multiple structs with the same field foo you might wish to for example view it. But, it would be impossible to do so, as the argument that you have passed is generic. Here, See is for the rescue, If you implement See on a struct and use an unit struct to perform indexing, then you can get the field that's inside. But that's exactly similar to creating an interface to get the field, so what's the catch. There's also a #[derive(See)], which does the for you. And to get the unit structs add see_derive::auto_load!() in the root i.e. src/main.rs or src/lib.rs.

Extending See

Though [See][See] provides us with ample ways to access inner field states, yet the syntax isn't friendly and easily understandable for someone new to rust. To counteract this [Look][Look] trait is implemented. Though by looking at the macro expansion it isn't obvious. It's actually auto implemented after certain conditions are met.


Checkout this example, it will make a lot of thing clear

use see::{See, Look, see_derive::{Look, self}};
//        ↑                       ↑
//        |                       +------ The derive
//        +----------------------------- The trait           

struct Point2D {
    x: i32,
    y: i32

struct Point3D {
    x: i32,
    y: i32,
    z: i32

// Once done using the derive or loading modules that have these derive calls do this

// Done! Now let's write a function that modifies x coordinate

fn modify_x<T>(loc: &mut T, change: i32) 
    T: See<see_t::X, Inner = i32> 
    // use see_t for getting the visitors and just specify your field with 2 underscores, 
    // and if need you can specify the type of inner or a morphism that's allowed on the inner type
    *loc.set() = change;

fn modify_y<T>(loc: &mut T, change: i32)
    T: Look<see_t::Y, Inner = i32>
    // `Look` is exactly similar to `See`
    loc[see_t::Y] = change;

fn main3d() {
    let p = Point3D {
        x: 0,
        y: 0,
        z: 0
    // go left
    modify_x(&mut p, -1);
    // go right
    modify_x(&mut p, 1);
fn main2d() {
    let p = Point2D {
        x: 0,
        y: 0
    // go left
    modify_x(&mut p, -1);
    // go right
    modify_x(&mut p, 1);


  • Use the See trait to get raw access to the methods
    1. fn get(&self) -> &Self::Inner here the inner is the type of the data in question
    2. fn set(&mut self) -> &mut Self::Inner
  • Use the Look trait to have similar access, as it extends the See trait. But also allows more verbose way of access control while also allowing a simpler interface when trying to access multiple fields from a generic struct


  • Follow conventional comments.
  • Create issue for any change request
  • Provide proper reference and description for PR