A Focus is a way to work with particular parts of a large chunk of data. On the
most basic level, it lets you get
and set
fields of a record in a simple and
composable way. This means you could avoid writing special record update syntax
and use something that composes much more elegantly.
It gives you the ability to write stuff like freeze
in the following snippet:
mario =
{ super = False
, fire = False
, physics = { position = { x=3, y=4 }
, velocity = { x=1, y=1 }
}
}
freeze object =
set (physics => velocity) { x=0, y=0 } object
The actual goal of this library is to help architect large projects with many
independent widgets. If we have three widgets with some shared state, we do not
necassarily want to give each widget full access to that shared state. Instead
we can have them return a function that acts just on the state they care about,
and we can use a Focus
to apply that change.
It is not yet clear if this will actually be any nicer than just using typical record update syntax in a well-architected application, because it is likely no one is actually dealing with particularly deep records. More experimentation is needed to figure this out.
It is possible that the concept of a Focus
is harmful to code quality in that
it can help you to be lax with abstraction boundaries. By making it easy to
look deep inside of data structures, it encourages you to stop thinking about
how to make these substructures modular, perhaps leading to an architecture that
is not as nice and has extra conceptual complexity.
The deeper problem may be that lenses are best when they are bidirectional,
whereas a Focus
is only in one direction. The issue is then that making proper
lenses is not necessarily possible without changing the language itself.
This API is inspired by the concept of Bidirectional Lenses as described by Nate Foster and seen in a modified form in Haskell as "lenses" and in ClojureScript as "cursors". My personal opinions and understanding come primarily from this talk by Simon Peyton Jones and Nate Foster's PhD dissertation on bidirectional lenses. I chose the name "Focus" because it is sort of like a lens that only lets you see in one direction.
Implementations of this idea in JS and Clojure rely heavily on the fact that the languages are dynamically typed and you can do runtime introspection. Haskell relies heavily on Template Haskell (a sort of macro system) to generate all of the necessary code. In Elm, type-directed macros may make things more convenient, but it is unclear exactly how this might work or if it is worthwhile.