Visor

LeviySoft.Visor is an optics library for .NET

Rationale

C# 9 introduced record types, which are a first-class implementation of immutable data types. While records are very useful for domain modeling, it can be pretty akward to modify deeply nested structures:

record InnerItem(int Id, string Name);
record ItemWrapper(InnerItem Item, ...);
record Warehouse(ItemWrapper Main, ...);

var warehouse = new WareHouse(..); //We want to modify Id of InnerItem here

var warehouse2 = warehouse with {
    Main = warehouse.Main with {
        Item = warehouse.Main.Item with {
            Id = 42 //Here is our new Id
        }
    }
}

This problem becomes more annoying with every new level of nesting, and here optics come on the scene. The overall concept of optics is to abstract accessing (and modifying) immutable data from the data itself. Let's generate Lens optic for the case above and see, how can we re-write our modification:

[Optics] partial record InnerItem(int Id, string Name);
[Optics(withNested: true)] partial record ItemWrapper(InnerItem Item, ...);
[Optics(withNested: true)] partial record Warehouse(ItemWrapper Main, ...);

var warehouse = new WareHouse(..);

var warehouse2 = Warehouse.FocusMain.FocusItem.IdLens.Set(42)(warehouse);

withNested controls "chained" Lens generation.

Lens is not the only optic on the table, with Prism for example you can abstract over subtyping, with Traverse over enumerable structures etc.

How-to

• install LeviySoft.Visor and LeviySoft.Visor.Gen
• add [Optics] attribute on your record and make your record partial

[Optics]
partial record Sample(int Id, string Description);

• now you can use Sample static class containing optics:

var id = Sample.IdLens.Get(...);

Sample.DescriptionLens.Set("replaced")(...);

Dealing with immutable collections & nullable properties

Visor comes with a useful set of optics for manipulating values at nullable anr/or collection properties. Although System.Collections.Immutable is supported I strongly recommend using language-ext's immutable collections because standart ones breaks value equality of records.

Here is a brief example of using build-in optics:

[Optics] internal partial record InnerItem(int Id, string Name);
[Optics] internal partial record Aux(string Comment, IImmutableList<string> Tags);
[Optics(withNested: true)] internal partial record ItemWrapper(InnerItem Item, Aux? Aux);
[Optics(withNested: true)] internal partial record Warehouse(ItemWrapper Main);

var warehouse = new Warehouse(new ItemWrapper(new InnerItem(1, "Default"), new Aux("test", ImmutableList.Create("tag1", "tag2"))));
var telescope = Warehouse.FocusMain.AuxLens.NotNull().Compose(Aux.TagsLens).Compose(Property.IImmutableList.First<string>());
var warehouse2 = telescope.Set("updated_tag")(warehouse); //Now warehouse2.Main.Aux?.Tags is ("updated_tag", "tag2")

Coming soon

• Prism
• Traverse
• Fold
• Iso

Other notes

LeviySoft.Visor is an independently maintained fork of Tinkoff.Visor and is not related to Tinkoff in any kind.