Thanks, your correction explains what the benefit would be (the awkward
need to cache the value). However this seems simpler to me: `if
(!map.has(key)) { map.set(key, getValue()); } const value = map.get(key);`On Wed, Oct 10, 2018 at 9:07 PM Isiah Meadows <[email protected]> wrote: > I presume you mean this? > > ```js > // Proposed > map.putIfAbsent(key, init) > > // How you do it now > let value > if (map.has(key)) { > value = map.get(key) > } else { > map.set(key, value = init()) > } > ``` > > BTW, I'd like to see this make it myself, just slightly different: > > - How you call it: `map.getOrPut(key, init, thisValue=undefined)` > - How `init` is called: `init.call(thisValue, key, map)` > > This pattern is incredibly common for caching. It'd be nice if I didn't > have to repeat myself so much with it. I would be more willing to stuff it > in a utility function if it weren't for the fact the use cases often entail > performance-sensitive paths, and engines aren't reliable enough in my > experience with inlining closures passed to non-builtins. > > On Wed, Oct 10, 2018 at 22:54 Jordan Harband <[email protected]> wrote: > >> It seems like your proposed `const value = map.putIfAbsent(key, >> getExpensiveValue);` is achievable already with `const value = map.has(key) >> ? map.get(key) : map.set(getExpensiveValue());` - am I understanding your >> suggestion correctly? >> >> On Wed, Oct 10, 2018 at 12:46 AM Man Hoang <[email protected]> wrote: >> >>> Consider the following function >>> >>> ``` js >>> >>> /** >>> >>> * Parses the locale sensitive string [value] into a number. >>> >>> */ >>> >>> export function parseNumber( >>> >>> value: string, >>> >>> locale: string = navigator.language >>> >>> ): number { >>> >>> let decimalSeparator = decimalSeparators.get(locale); >>> >>> if (!decimalSeparator) { >>> >>> decimalSeparator = Intl.NumberFormat(locale).format(1.1)[1]; >>> >>> decimalSeparators.set(locale, decimalSeparator); >>> >>> } >>> >>> >>> >>> let cleanRegExp = regExps.get(decimalSeparator); >>> >>> if (!cleanRegExp) { >>> >>> cleanRegExp = new RegExp(`[^-+0-9${decimalSeparator}]`, 'g'); >>> >>> regExps.set(decimalSeparator, cleanRegExp); >>> >>> } >>> >>> >>> >>> value = value >>> >>> .replace(cleanRegExp, '') >>> >>> .replace(decimalSeparator, '.'); >>> >>> >>> >>> return parseFloat(value); >>> >>> } >>> >>> >>> >>> const decimalSeparators = new Map<string, string>(); >>> >>> const regExps = new Map<string, RegExp>(); >>> >>> ``` >>> >>> >>> >>> This function can be simplified quite a bit as follows >>> >>> ``` js >>> >>> export function parseNumber( >>> >>> value: string, >>> >>> locale: string = navigator.language >>> >>> ): number { >>> >>> const decimalSeparator = decimalSeparators.putIfAbsent( >>> >>> locale, () => Intl.NumberFormat(locale).format(1.1)[1]); >>> >>> >>> >>> const cleanRegExp = regExps.putIfAbsent( >>> >>> decimalSeparator, () => new >>> RegExp(`[^-+0-9${decimalSeparator}]`, 'g')); >>> >>> >>> >>> value = value >>> >>> .replace(cleanRegExp, '') >>> >>> .replace(decimalSeparator, '.'); >>> >>> >>> >>> return parseFloat(value); >>> >>> } >>> >>> ``` >>> >>> if `Map` has the following instance method >>> >>> ``` js >>> >>> export class Map<K, V> { >>> >>> /** >>> >>> * Look up the value of [key], or add a new value if it isn't there. >>> >>> * >>> >>> * Returns the value associated to [key], if there is one. >>> >>> * Otherwise calls [ifAbsent] to get a new value, associates [key] to >>> >>> * that value, and then returns the new value. >>> >>> */ >>> >>> putIfAbsent(key: K, ifAbsent: () => V): V { >>> >>> let v = this.get(key); >>> >>> if (v === undefined) { >>> >>> v = ifAbsent(); >>> >>> this.set(key, v); >>> >>> } >>> >>> return v; >>> >>> } >>> >>> } >>> >>> ``` >>> >>> >>> >>> Java's Map has a `putIfAbsent` method, which accepts a value rather than >>> a function for the second parameter. This is not ideal as computing the >>> value may be expensive. A function would allow the value to be computed >>> lazily. >>> >>> >>> >>> References >>> >>> - [Dart] [Map.putIfAbsent]( >>> https://api.dartlang.org/stable/2.0.0/dart-core/Map/putIfAbsent.html) >>> >>> - [Java] [Map.putIfAbsent]( >>> https://docs.oracle.com/javase/8/docs/api/java/util/Map.html#putIfAbsent-K-V- >>> ) >>> _______________________________________________ >>> es-discuss mailing list >>> [email protected] >>> https://mail.mozilla.org/listinfo/es-discuss >>> >> _______________________________________________ >> es-discuss mailing list >> [email protected] >> https://mail.mozilla.org/listinfo/es-discuss >> >
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