#fp #lens


A possibly bad lens library for Rust

3 releases

0.1.2 Oct 2, 2019
0.1.1 Oct 1, 2019
0.1.0 Sep 30, 2019

#6 in #lens

MIT license

243 lines


My optics library, Myopic, is a possibly bad implementation of the concept of a functional lense. This is a beautiful concept, and if you want an elegant implementation see lens from the Haskell ecosystem, and one of the many great introductions such as lens over tea.

Myopic is not elegant- it satisfies a need that I have for an implementation of lense that acts on mutable data, which has no or little performance penalty, and which is simple enough to use in other libraries of mine.


This library contains three implementations of the concept of lenses. They are all the simplest kind of lenses which can only get and set data in structures (not the fancy Haskell ones which can do much more).

The lenses are implemented as struct containing either closures (lens), boxed trait objects (lens_box), or function pointers (lens_fn).

Each lense implementation has a Lens type containing functions for getting and setting data, and a ComposedLens type for nested compositions of lenses and ComposedLens. This implementation comes from the lenses library (credit where credit is due).

The reason there are three implementations was to benchmark them, as described below. The takeaway is that the 'lens' module is at worst much faster, and at best much much faster then the others.


The benchmarks can be run with cargo bench

cargo bench

They show that the 'lens' module is by far the fastest. It appears to be able to inline such that I have to use black_box in criterion just to get any benchmarking data out of them.

The other implementations seem to incur some indirection penalty which doesn't get inlined (I assume). This appears to get worst the more they are composed, while a level of composition in 'lens' does not incur a noticable penalty.

The Name

The joke in the name is that this is both my own optics library, and it is likely a bad optics library. Myopic means having bad vision, and it sounds like "my optics". Hopefully this is a little amusing.


There are a number of Rust lense libraries, some of which are quite extensive, and some of which have vastly different designs.

  • lenses is very simple and is the library I started to use, which lead me to eventually create Myopic. This library uses a Lens trait with one implementation for the Lens type, and one for a type which contains composed lenses. This is the design I ended up using in Myopic based on what I found in lenses. I haven't benchmarked this, but lenses may be just as fast as Myopic. I had trouble getting the specific use case covered where I want to be able to modify data that may not have an address, and wanted to mutate existing data. Perhaps lenses can do this, but I don't know how.
  • photonix is pretty interesting- its a completely different implementation using only traits, and not requiring keeping closures around in memory. It seems to allow reaching into nested structures and building up access into those structures (with macros as far as I can tell). Cool.
  • refraction is pretty extensive, breaking the concepts involved into many small pieces and buliding upwards. It contains additional concepts like Prisms and is overall much better engineered then Myopic. It does seem confusing to me, given the number of traits involved, although I think any lens implementation in Rust will reuqire this.
  • shoggoth is also cool, and seems extensive. At its own admission (given the name) it is quite complex. I wanted something simple that I understood, so I didn't go down this route to madness.
  • fp-core has only a lens trait and I don't believe it supports my use-case of mutating data in-place.
  • rustz has a better implementation then Myopic, which takes a slightly different approach to setting. I believe it precludes me from updating data in-place, but if that doesn't turn out to matter then rustz may be the best option in the sense that it is similar and the implementation is not as opaque as some.


Myopic is licensed under either MIT or APACHE2, whichever you prefer.

No runtime deps