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add ascending to IterableExtension.sortedBy #731

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add ascending to IterableExtension.sortedBy #731

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12 changes: 8 additions & 4 deletions pkgs/collection/lib/src/iterable_extensions.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,6 @@ import 'dart:math' show Random;

import 'algorithms.dart';
import 'functions.dart' as functions;
import 'utils.dart';

/// Extensions that apply to all iterables.
///
Expand Down Expand Up @@ -71,10 +70,15 @@ extension IterableExtension<T> on Iterable<T> {
/// Creates a sorted list of the elements of the iterable.
///
/// The elements are ordered by the natural ordering of the
/// property [keyOf] of the element.
List<T> sortedBy<K extends Comparable<K>>(K Function(T element) keyOf) {
/// [keyOf] property when [ascending] is `true` or reverse ordering
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The sorted function should get the same behavior. Really, every sort function should behave the same way if at all possible.
(It's probably impossible because there is some sort([int Function(T, T) compare]) somewhere which prevents a named parameter.)

/// when [ascending] is `false`.
List<T> sortedBy<K extends Comparable<K>>(
K Function(T element) keyOf, {
bool ascending = true,
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I think I would prefer to name it descending, so the default is false.
Just because it's usually best to have "nothing" mean false.
Then it feels like you are opting in to the non-default behavior. Using ascending: false feels ... conflicted. You are opting out of the default behavior, but that doesn't directly say what it opts in to.

}) {
int compare(K a, K b) => ascending ? a.compareTo(b) : b.compareTo(a);
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At the risk of premature optimization, I think we should keep this as a static tear-off instead of a new lambda on every use. If we decide to go forward with this change I think something like

final compare = ascending ? compareComparable : compareComparableDescending;

int compareComparableDescending<T extends Comparable<T>>(T a, T b) => b.compareTo(a);

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If it's typed, it's a new closure on every use anyway, because generic instantiation also causes a new closure.
That said: I don't want to do the branch on every comparison, so something like what @natebosch says is better.

var elements = [...this];
mergeSortBy<T, K>(elements, keyOf, compareComparable);
mergeSortBy<T, K>(elements, keyOf, compare);
return elements;
}

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10 changes: 10 additions & 0 deletions pkgs/collection/test/extensions_test.dart
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Expand Up @@ -64,6 +64,16 @@ void main() {
test('multiple', () {
expect(iterable(<int>[3, 20, 100]).sortedBy(toString), [100, 20, 3]);
});
test('multiple ascending', () {
expect(
iterable(<int>[3, 20, 100]).sortedBy(toString, ascending: true),
[100, 20, 3]);
});
test('multiple descending', () {
expect(
iterable(<int>[3, 20, 100]).sortedBy(toString, ascending: false),
[3, 20, 100]);
});
});
group('.sortedByCompare', () {
test('empty', () {
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