Lens quickstart

The lens library is a clever library which brings some useful parts of object oriented syntax to Haskell. Namely the ability to easily access and manipulate nested data structures and. Marvin depends on and uses, both internally and in its interfaces, a library called microlens. microlens is very similar to lens. It offers a smaller set of features (which suffice for marvin) but also has far fewer dependencies. It is however fully compatible with lens, meaning a Lens value from microlens can be used as a Lens value lens as both are simply type aliases. So if you want to use features from lens you don’t need to also use microlens to be compatible with marvin.

The Lens' type can be used to manipulate a field in a data structure. For example a lens foo :: Lens' Bar Int pertains to a field of type Int in a data structure called Bar.


The operator ^. is used to access the contents of a field. x ^. foo accesses the foo field in the x value.

These lenses are composable. If we have a lens bar :: Lens' Baz Bar and a value y :: Baz we can access the nested foo value with y ^. bar . foo.


The same lenses can also be used to modify the contents of the referenced field. foo .~ value creates a function which sets the foo field to value. Often this is combined with the revers application operator & to write code such as x & foo .~ value which sets foo in x to value. Using & we can also chain modifications like so x & foo .~ value & anotherField .~ anotherValue. This does not modify the original x but instead returns a new value of type Bar which is identical to x except for the contents of the foo field.

Another operator for modification is %~ where foo %~ f modifies the content of the foo field with the function f.

Lenses in modification operations are also composable. For instance to set the nested foo field in y we can say y & baz . foo .~ value.