# Lenses can be seen as a bijection with some residual¶

A `Lens a b`

is a bijection between a and (b, r), where r is a - b. This represents a Lens as an isomorphism. Given (b, r) one could construct a.

A `Lens a b`

is a bijection between a and (b, r), where r is a - b. This represents a Lens as an isomorphism. Given (b, r) one could construct a.