locations: lists employees by their location and position
const locations = {
berlin: {
employees: {
staff: {
list: [
{
name: "Wiley Moen",
phone: "688-031-5608",
id: "cdfa-f2ae"
},
{
name: "Sydni Keebler",
phone: "129-526-0289",
id: "e0ec-e480"
}
]
},
managers: {
list: [
{
name: "Cecilia Wisoky",
phone: "148-188-6725",
id: "9ebf-5a73"
}
]
},
students: {
list: [
{
name: "Kirsten Denesik",
phone: "938-634-9476",
id: "c816-2234"
}
]
}
}
},
paris: {
employees: {
staff: {
list: [
{
name: "Lucius Herman",
phone: "264-660-0107",
id: "c2fc-55da"
}
]
},
managers: {
list: [
{
name: "Miss Rickie Smith",
phone: "734-742-5829",
id: "2095-69a7"
}
]
}
}
}
};
3 different types of `compose`:
// recursive version
const compose = (...fns) => x =>
fns.length
? compose(...fns.slice(0, -1))(
fns[fns.length - 1](x)
)
: x;
// iterative version
const composeItr = (...fns) => x => {
const functions = Array.from(
fns
).reverse();
/* `reverse` mutates the array,
so we make a shallow copy of the functions array */
let result = x;
for (const f of functions) {
result = f(result);
}
return result;
};
// with Array.prototype.reduce
const composeReduce = (...fns) => x =>
fns.reduceRight(
(result, f) => f(result),
x
);
// use it!
console.log(
compose(
x => `Hello ${x}`,
x => `${x}!`
)("World")
); // -> "Hello World!"
Composed getters
const studentsAtLocation = compose(
(students = {}) => students.list || [],
(employees = {}) => employees.students,
(location = {}) => location.employees
);
const locationWithName = locationName => (
locations = {}
) => locations[locationName];
const getBerlinStudents = compose(
studentsAtLocation,
locationWithName("berlin")
);
const getParisStudents = compose(
studentsAtLocation,
locationWithName("paris")
);
console.log(
getBerlinStudents(locations)
); // [ { name: 'Kirsten Denesik', ... ]
console.log(
getParisStudents(locations)
); // []
Function to create a composable getter and setter pair:
Explanation:
1. After calling `createComposableGetterSetter` with a getter and a setter function as arguments, we get back the actutal `composableGetterSetter`.
2. Our `composableGetterSetter` will get a `toGetterAndSetter` function, that takes some data as input and returns an object with a `get` and a `set` method. We return a function, that expects the target data as its only argument.
3. We construct a GetterSetter object by calling **(1)** with the target data from **(2)** and passing the return value to the `toGetterAndSetter` function.
4. We use the GetterSetter objects `set()` method with the return value of calling the setter **(4)** with the value of the constructed GetterSetter object (we call `getterSetter.get()` to simply retrieve this value) and the targetData (we expect, that the setter will return a new version of `targetData` with its focused value set to the return value from `getterSetter.get()`).
5. We return the value (which is again a GetterSetter object) that is returned from `getterSetter.set(...)` in **(5)**.
const createComposableGetterSetter = (
getter, // (1)
// -- getter(targetData: TargetData): Value
setter // (4)
// -- setter(value: Value, targetData: TargetData) => TargetData
) => toGetterAndSetter => targetData => {
// (2)
const getterSetter = toGetterAndSetter(
getter(targetData)
); // (3)
/**
* toGetterAndSetter is called with
* "data" as argument
* and returns a GetterSetter object:
* @typedef {
* {
* get: function(): *,
* set: function(newData: *): GetterSetter
* }
* } GetterSetter
*
*/
return getterSetter.set(
setter(
getterSetter.get(),
targetData
)
); // (5)
};
Used for setting values with a composable getter and setter pair:
const toSetAccessors = data => ({
get: () => data,
set: newData => toSetAccessors(newData)
});
Used for getting values with a composable getter and setter pair:
const toGetAccessors = data => ({
get: () => data,
set() {
return this;
}
});
Creating Lenses
First draft:
let parisLens = createComposableGetterSetter(
(obj = {}) => obj.paris,
(value, obj) => ({
...obj,
paris: value
})
);
let employeesLens = createComposableGetterSetter(
(obj = {}) => obj.employees,
(value, obj) => ({
...obj,
employees: value
})
);
let studentsLens = createComposableGetterSetter(
(obj = {}) => obj.students,
(value, obj) => ({
...obj,
students: value
})
);
After refactoring:
const lensProp = propName =>
createComposableGetterSetter(
(obj = {}) => obj[propName],
(value, obj) => ({
...obj,
[propName]: value
})
);
// we can now create lenses for props like this:
parisLens = lensProp("paris");
employeesLens = lensProp(
"employees"
);
studentsLens = lensProp(
"students"
);
const listLens = lensProp("list"); // needed to get the list of students
Composing (and using) lenses:
const parisStudentListLens = compose(
parisLens,
employeesLens,
studentsLens,
listLens
);
const parisStudentList = parisStudentListLens(
toGetAccessors
)(locations).get();
console.log(parisStudentList);
// -> undefined, since there is no list of students for paris defined.
const locationsWithStudentListForParis = parisStudentListLens(
_list => toSetAccessors([])
// ignore current list and replace it with an empty array
)(locations).get();
console.log(
locationsWithStudentListForParis
);
// -> { ..., paris: { employees:{ ..., students: { list: [] } } } }
Helpers: `view`, `set`, `over`:
const view = (lens, targetData) =>
lens(toGetAccessors)(
targetData
).get();
const over = (
lens,
overFn /* like the `mapf` callback in `Array.prototype.map(mapf)`.
i.e.: You get a value and return a new value. */,
targetData
) =>
lens(data =>
toSetAccessors(overFn(data))
)(targetData).get();
const set = (lens, value, targetData) =>
over(
lens,
() =>
value /* we use `over` with a `overFn` function,
that just returns the value argument */,
targetData
);
Using `get`, `set` and `over`:
{
const locationsWithStudentListForParis = set(
parisStudentListLens,
[],
locations
);
const locationsWithOneStudentInParis = over(
parisStudentListLens,
(list = []) => [
...list,
{ name: "You", setVia: "Lens" }
],
locations
);
const locationsWithTwoStudentInParis = over(
parisStudentListLens,
(list = []) => [
...list,
{ name: "Me", setVia: "Lens" }
],
locationsWithOneStudentInParis
);
// logging the results:
console.log(
view(parisStudentListLens, locations)
); // -> undefined
console.log(
view(
parisStudentListLens,
locationsWithStudentListForParis
)
); // -> []
console.log(
view(
parisStudentListLens,
locationsWithTwoStudentInParis
)
); // -> [ { name: 'You', setVia: 'Lens' }, { name: 'Me', setVia: 'Lens' } ]
console.log(
view(
parisStudentListLens,
locationsWithOneStudentInParis
)
); // -> [ { name: 'Me', setVia: 'Lens' } ]
console.log(
locationsWithTwoStudentInParis
); // -> ...
}
Improved version of `createComposableGetterSetter`: `createLens`
const createLens = (getter, setter) => toFunctor => targetData =>
toFunctor(getter(targetData)).map(focus => setter(focus, targetData));
// redifine `toGetAccessors` and `toSetAccessors`
// toGetAccessors
const Const = data => ({
get value() {
return data;
},
map() {
return this;
}
});
// toSetAccessors
const Identity = data => ({
get value() {
return data;
},
map(mapf) {
return Identity(mapf(data));
}
});
// redefine view, set, over
const helpers = {
view: (lens, targetData) => lens(Const)(targetData).value,
over: (
lens,
overFn /* like the `mapf` callback in `Array.prototype.map(mapf)`.
i.e.: You get a value and return a new value. */,
targetData
) => lens(data => Identity(overFn(data)))(targetData).value,
set: (lens, value, targetData) =>
helpers.over(
lens,
() =>
value /* we use `over` with a `overFn` function,
that just returns the value argument */,
targetData
)
};
// lens creator helpers:
const lensCreators = {
lensProp: propName =>
createLens(
(obj = {}) => obj[propName],
(value, obj) => ({
...obj,
[propName]: value
})
),
lensIndex: (index = 0) =>
createLens(
(arr = []) => arr[index],
(value, arr = []) => arr.slice(0, index).concat(value, arr.slice(index))
),
lensPath: (path = []) =>
compose(
...path.reduce(
(lenses, nextPathElem) => [
...lenses,
typeof nextPathElem === "number"
? lensCreators.lensIndex(nextPathElem)
: lensCreators.lensProp(nextPathElem)
],
[]
)
)
};
{
const { lensPath, lensProp } = lensCreators;
const { view, over, set } = helpers;
const studentsList = lensPath(["employees", "students", "list"]);
const berlinStudents = compose(
lensProp("berlin"),
studentsList
);
const parisStudents = compose(
lensProp("paris"),
studentsList
);
const locationsWithStudentListForParis = set(parisStudents, [], locations);
const locationsWithOneStudentInParis = over(
parisStudents,
(list = []) => [...list, { name: "You", setVia: "Lens" }],
locations
);
const locationsWithTwoStudentsInBerlin = over(
berlinStudents,
(list = []) => [...list, { name: "Me", setVia: "Lens" }],
locationsWithOneStudentInParis
);
console.log(
"Paris:",
"before:",
view(parisStudents, locations) // undefined
);
console.log(
"after:",
view(parisStudents, locationsWithOneStudentInParis) // [{name: "You", setVia: "Lens"}]
);
console.log(
"Berlin:",
"before:",
view(berlinStudents, locationsWithOneStudentInParis) // [{id: "c816-2234", name: "Kirsten Denesik", …}]
);
console.log(
"after:",
view(
berlinStudents,
locationsWithTwoStudentsInBerlin
) /*
[
{id: "c816-2234", name: "Kirsten Denesik", …},
{name: "Me", setVia: "Lens"}
] */,
locationsWithTwoStudentsInBerlin
);
}
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