partial.lenses

This library provides a collection of Ramda compatible partial lenses. While an ordinary lens can be used to view and update an existing part of a data structure, a partial lens can view optional data, insert new data, update existing data and delete existing data.

Index:

• Examples
• Reference
• Background

Examples

TBD

Reference

Usage

The lenses and operations on lenses are accessed via the default import:

import L from "partial.lenses"

Operations on lenses

For convenience, you can access basic operations on lenses via the default import L:

• L.compose(l1, ..., ln) is the same as R.compose(l1, ..., lN) (see compose).
• L.lens(get, set) is the same as R.lens(get, set) (see lens).
• L.over(l, x2x, s) is the same as R.over(l, x2x, s) (see over).
• L.set(l, x, s) is the same as R.set(l, x, s) (see set).
• L.view(l, s) is the same as R.view(l, s) (see view).

For convenience, there is also a shorthand for delete:

• L.delete(l, s) is the same as R.set(l, undefined, s).

Shorthand composition and lifting

The default import, L, is also a shorthand function for lens composition (see compose) and lifting. The semantics can be described as

L(l1, ..., lN) === R.compose(lift(l1), ..., lift(lN))

where

const lift = l => {
  switch (typeof l) {
  case "string": return L.prop(l)
  case "number": return L.index(l)
  default:       return l
  }
}

Note that L.compose does not perform lifting.

Lenses

L.prop(string)

L.prop(string) is much like R.lensProp(string) (see lensProp), but composes as a partial lens:

• When viewing an undefined property or an undefined object, the result is undefined.
• When setting property to undefined, the property is removed from the result. If the result would be an empty object, the whole result will be undefined.

Examples:

> L.set(L("x", "y"), undefined, {x: {y: 1}})
undefined
> L.set(L("x", "y"), 2, {x: {y: 1}})
{ x: { y: 2 } }
> L.set(L("x", "y"), undefined, {x: {y: 1}, z: 3})
{ z: 3 }
> L.set(L("x", "y"), 2, {x: {y: 1}, z: 3})
{ x: { y: 2 }, z: 3 }
> L.view(L("x", "y"), undefined)
undefined

L.index(integer)

L.index(integer) is like R.lensIndex(integer) (see lensIndex), but composes as a partial lens:

• When viewing an undefined array index or an undefined array, the result is undefined.
• When setting an array index to undefined, the element is removed from the resulting array, shifting all higher indices down by one. If the result would be an array without indices (ignoring length), the whole result will be undefined.

L.find(predicate)

L.find(predicate) operates on arrays like L.index, but the index to be viewed is determined by finding the first element from the input array that matches the given unary predicate. When no matching element is found the effect is same as with R.index with the index set to the length of the array.

L.normalize(transform)

L.normalize(transform) maps the value with same given transform when viewed and set and implicitly maps undefined to undefined. More specifically, L.normalize(transform) is equivalent to R.lens(toPartial(transform), toPartial(transform)) where

const toPartial = transform => x => undefined === x ? x : transform(x)

The use case for normalize is to make it easy to determine whether, after a change, the data has actually changed. By keeping the data normalized, a simple R.equals comparison will do.

L.replace(inn, out)

L.replace(inn, out), when viewed, replaces the value inn with out and vice versa when set. Values are compared using R.equals (see equals).

Examples:

> L.view(L(L.replace(undefined, {type: "title", text: ""}),
           "text"),
         undefined)
""
> L.set(L(L.replace(undefined, {type: "title", text: ""}),
          "text"),
        "",
        {type: "title", text: "not empty"})
undefined

The use case for replace is to handle optional properties and elements.

L.default(out)

L.default(out) is the same as L.replace(undefined, out).

Background

Motivation

Consider the following REPL session using Ramda 0.19.1:

> R.set(R.lensPath(["x", "y"]), 1, {})
{ x: { y: 1 } }
> R.set(R.compose(R.lensProp("x"), R.lensProp("y")), 1, {})
TypeError: Cannot read property 'y' of undefined
> R.view(R.lensPath(["x", "y"]), {})
undefined
> R.view(R.compose(R.lensProp("x"), R.lensProp("y")), {})
TypeError: Cannot read property 'y' of undefined
> R.set(R.lensPath(["x", "y"]), undefined, {x: {y: 1}})
{ x: { y: undefined } }
> R.set(R.compose(R.lensProp("x"), R.lensProp("y")), undefined, {x: {y: 1}})
{ x: { y: undefined } }

One might assume that R.lensPath([p1, ..., pN]) is equivalent to R.compose(R.lensProp(p1), ..., R.lensProp(pN)), but that is not the case.

With partial lenses you can robustly compose a path lens from prop lenses R.compose(L.prop(p1), ..., L.prop(pN)) or just use the shorthand notation L(p1, ..., pN).

Types

To illustrate the idea we could give lenses the naive type definition

type Lens s a = (s -> a, a -> s -> s)

defining a lens as a pair of a getter and a setter. The type of a partial lens would then be

type PartialLens s a = (s -> Maybe a, Maybe a -> s -> s)

which we can simplify to

type PartialLens s a = Lens s (Maybe a)

This means that partial lenses can be composed, viewed, mapped over and set using the same operations as with ordinary lenses. However, primitive partial lenses (e.g. L.prop) are not necessarily the same as primitive ordinary lenses (e.g. Ramda's lensProp).

In Javascript, optional data can be mapped to undefined, which is what partial lenses also do. When the viewed part of a data structure is missing, the result is undefined. When a part of a data structure is set to undefined, the part is deleted. Partial lenses are defined in such a way that operations compose and one can conveniently and robustly operate on deeply nested data structures.