@freckle/maybe
v2.2.0
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Operations for maybe types
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@freckle/maybe
Provides a collection of helper functions for operations on maybe types.
Motivation
This package seeks to recreate idioms common in strongly typed functional programming languages, with heavy influence from the Data.Maybe Haskell package.
Usage
Operations are used to refine an input from being possibly null
or undefined
:
/* @flow */
import {fromJust, fromMaybe, maybe} from '@freckle/maybe'
// Return input from a user, if any:
declare function getUserInput(): ?string
const mUserInput = getUserInput()
// Go from ?string -> string:
const input = fromMaybe(() => 'No input', mUserInput)
// Run a function on ?string with a default:
const capitalized = maybe(() => 'No input to capitalize', capitalize, mUserInput)
// Or produce an error on null | undefined to make future execution more predictable:
const userInput = fromJust(mUserInput, 'No input was given!')
Other operations carry the possibly null
value after applying a function:
import {mmap, mthen} from '@freckle/maybe'
// Return input from a user, if any:
declare function getUserInput(): ?string
// Function that does not handle a null | undefined value:
declare function transform(input: string): string
const mUserInput = getUserInput()
const mTransformed = mmap(transform, mUserInput)
// => null | undefined | transform(mUserInput)
// Alternate form that is more helpful for control flow:
mthen(mTransformed, (transformedUserInput: string) => {
// Process value
})
For dealing with Arrays that may contain null
or undefined
elements:
import {catMaybes, mapMaybes} from '@freckle/maybe'
const arr = [
null,
'foo',
undefined,
'bar'
]
const out = catMaybes(arr)
console.log(out) // => ['foo', 'bar']
const padString = (input: string) => ` ${input}`
const mapped = mapMaybes(arr, padString)
console.log(mapped) // => [' foo', ' bar']
Two operations are tailored to use with React:
/* @flow */
import {mEffect, asHTMLAttributeValue} from '@freckle/maybe'
type Props = {
myInput: ?string
}
const MyComponent = (props: Props): React.Node => {
// This prop may be string | undefined | null
const possibleInput = props.myInput
React.useEffect(() => {
mEffect(possibleInput, input =>
// Call a side effect that does not handle a null value:
sideEffect(input)
)
}, [possibleInput])
// Rendering an element with an attribute from a maybe value:
const attrObj = {'my-attribute': asHTMLAttributeValue(possibleInput)}
// If input is not a string, <div> is rendered;
// otherwise: <div my-attribute="..." />
return <div {...attrObj} />
}