notion is that of a prism, in which the signatures of the functions are instead , . Unlike a lens, a prism may not always give a view; also unlike a lens, given a prism, a view is sufficient to construct a source. If lenses allow "focusing" (viewing, updating) on a part of a product type, prisms allow focusing (possible viewing, building) on a part of a sum type.

Both lenses and prisms, as well as other constructions such as traversals, are more