What is this?

This is a small library that allows you to define your calls as a testable list of steps that map your abstract data to the specific logic associated with that data.

I.e.: Define a flow for your data in small composable parts and combine those parts to solve the bigger problems.

Why?

So you can easily compose and test them

How?

1. The scenario

Suppose you want to write a function that queries a database based on some input and transform the results in order to send them to another system.

2. The problem

Normally you would create something to mock the server or even test against a local full-blown instance of the database. But why is that? You don't want to test the database, do you? The provider of the database should have already tested that.

What you want to test is:

• whether the correct query is being used and whether the result is transformed the way you expect it.
• how the function behaves if the query returns something unexpected

For that you'd have to write and export a function that takes your arguments and returns a query, and another function that handles the results of the query. Sure, you could do that, but why export functions that have no use otherwise? That are only relevant in the context of this one call?

3. The solution

Enter, the future(-fun): with this library you can define a "Call" that does exactly the same with the added benefit that you can structure it into defined "steps" and test each of those steps individually.

Not only does this make every important step of your "function" testable, you can also create some sort of pipelines and combine several pipelines.

For example, in a typical mongodb setup, you could define a call that returns a Database based on some configuration. And then, based on that call, another one that returns all collections. Then, from that, a specific collection. And all of these calls have the same "root" argument, namely the configuration of the database, a specific collection is, after all, nothing more than the configuration for the database, enriched with its name.

Conclusion

Think of this package as a way to wrap your logic into a list of actions, where each action has a defined (typesafe) input and a defined (typesafe) output. The resulting list gives you access to each action and lets you test this action independently by providing it with fake input. This way you can simulate how the action will behave even though you don't actually have the prerequisites.

API

Call

Call.of

Create a new ICallMonad from scratch a.k.a. lift/of

Call.of: <In, Out>(fn: (arg?: In) => Out, thisArg?: any) => ICallMonad<Out, In>

Input

• fn - any function that takes zero or one arguments of type In and produces some output of type Out
• thisArg? - an optional argument on which fn will be called. Necessary if you want to pass a function from a certain object i.e. Promise.resolve needs Promise as thisArg

Output

An ICallMonad<Out, In> representing the fn as well as the context that enables you to .pipe your fn into other functions

Example

const c = Call.of((x: number) => Promise.resolve(x))

// executing
const number$ = c(1)

// testing
const testResult = testCall(c, 1)
assert(testResult instanceof Promise)
testResult.then(num => assert(num === 1))

useless-testing-example-disclaimer: just for demonstration

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Call.all

aggregate any number of calls into a single call

Call.all: (...calls: ICallMonad[]): ICallMonad<any[], any[]>

Input

• ...calls - the ICallMonads to aggregate

Output

A new ICallMonad that takes an array of all the arguments of the passed calls as its argument and returns all their results

Example

const c = Call.all(Call.of((x: number) => x), Call.of((y: string) => Promise.resolve(y)))

// executing
const [num, str$] = c([1, 'a'])

// testing
const [num, str$] = testCall(c, [1, 'a'])
assert(num === 1)
assert(str$ instanceof Promise)
str$.then(str => assert(str === 'a'))

note: currently typesafe for up to five calls but can be used with any number of calls

useless-testing-example-disclaimer: just for demonstration

---

call.pipe

pipe from one call to another

call.pipe: <I extends ICallMonad, O1, M1 extends Morphism> (this: I, op1: IOperator<OutOf<I>, O1, M1>): IPipedCallMonad<O1, I>

Input

• this - the instance of an ICallMonad that pipe is executed on
• op1 - op5 - the operators used to transform this call

Output

An IPipedCallMonad<O1, I> where I is the instance of the ICallMonad<Out, In> that .pipe was called on. This means that the resulting IPipedCallMonad takes the result of the previous ICallMonad as its argument.

Example

const c = Call.of((x: number) => x * 2)
  .pipe(map(x => x + ''))

// executing
const stringified = c(1)

// testing
assert(testCall(c.previous, 1) === 2)
assert(testCall(c, 1) === '1')

Note

It is important to note that with each .pipe you create a new ICallMonad that has a link to the previous step under previous. That is why testCall(c, 1) does not double the number but only stringifies it

Operators

Operators are at the heart of this library and are basically functions that take any instance of an ICallMonad and transform the instance into another ICallMonad. This can happen based on any morphism you want to implement or statically for things you need really often.

future-fun ships with three basic operators and two utility functions to create them

Note

Operators only transform calls, they do not keep track of the "previous step".

---

map

map from one value to another

function map<From, To> (morphism: UnaryFunction<From, To>): IOperator<From, To, UnaryFunction<From, To>>

Input

• morphism - the function that transforms the output of the previous ICallMonad to a new value

Output

An IOperator that transforms the call it is applied on so that it changes the output type of the resulting call to the result of the morphism

Example

const c = Call.of(parseInt)
const double = map((x: number) => x * 2)

// executing
const parseAndDouble = double(c)

// testing
assert(double.morphism(1) === 2)
assert(testCall(parseAndDouble, '1') === 2)

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flatMap

map from one value to the result of another ICallMonad

function flatMap<From, To> (morphism: UnaryFunction<From, ICallMonad<To, any, From>>): IOperator<From, To, typeof morphism>

Input

• morphism - a function that takes the result of the previous ICallMonad and returns another ICallMonad that takes the same type as its argument

Output

An IOperator that transforms the call it is applied on so that it changes the output type of the resulting call to the result of the ICallMonad returned from the morphism

Example

const identity = Call.of((x: number) => x)
const stringify = Call.of((x: number) => x + '')
const conditional = flatMap((x: number) => x > 9999 ? stringify : identity)

// executing
const stringifyLarge = conditional(identity)

// testing
assert(conditional.morphism(10000) === stringify)
assert(conditional.morphism(1) === identity)

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mapPromise

map the value that a promise will resolve with to another value

function mapPromise<From, To> (morphism: UnaryFunction<From, To | Promise<To>>): IOperator<Promise<From>, Promise<To>, UnaryFunction<From, To | Promise<To>>>

Input

• morphism - a function that receives the value the promise is going to resolve with and maps it to another value

Output

An IOperator that transforms the call it is applied on so that it changes the resolved value of the promise returned from the call to a promise that resolves with another value

Example

const doublePromise = mapPromise((x: number) => x * 2)
const identity$ = Call.of((x: number) => Promise.resolve(x))

// executing
const double$ = doublePromise(identity$)

// testing
assert(doublePromise.morphism(1) === 2)
testCall(double$, Promise.resolve(1)).then(num => assert(num === 2))

flatMapTo

map to the result of a nested ICallMonad

Example

const double = Call.of((x: number) => x).pipe(flatMapTo(Call.of((x: number) => x * 2)))

// executing
assert(double(1) === 2)

Note

This is technically just an alias to map. Works because ICallMonads are functions themselves.

---

aggregate

put multiple operators together into one

function(...operators: IOperator<any, any, any>[]): IOperator<any, any, any>

Input

• operators - any number of IOperators

Output

A new IOperator that simply combines all the morphisms into one and can transform any call by putting it through all the passed operators.

Example

const doubleAndIncrement = aggregate(map((x: number) => x * 2), map((x: number) => x + 1))
const identity = Call.of((x: number) => x)

// executing
const doubleInc = doubleAndIncrement(identity)

// testing
assert(doubleAndIncrement.morphism(1) === 3)

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createOperator

create a new custom operator

function createOperator<In, Out> (morphism: UnaryFunction<In, Out> | NullaryFunction<Out>, transform: (result: In) => Out): IOperator<In, Out, typeof morphism>

Input

• morphism - a UnaryFunction or NullaryFunction that defines any mapping from zero or one arguments to any output
• transform - the logic for transforming the result of the call the operator is applied on.

Output

A new IOperator that transforms a call that returns In into a call that returns Out

Example

// actual implementation of `mapPromise` operator
const mapPromise = createOperator(morphism, result => result.then(morphism))

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Further Examples:

See demo.spec.ts (WIP)