Problem Statement

Suppose you have to sort a list of strings based on a property of each, called "the key", i.e. their lenghts. The most popular solution would require the use of a sorting algorithm with a custom comparator. Now suppose that key computation is an expensive operation, for example, it might involve intensive reading of files, databases, or exchanging information over a network. In such a case, using a sorting algorithm with a custom comparator is not optimal, because the key is computed multiple times for each element in the array, once each time the element needs to be compared to another (and also requires the calculation of the key for that other). One solution of the problem is called the "decorate-sort-undecorate" idiom, pattern or technique. It was originated and named by the Lisp community. Suppose we want to sort the following list of words according to their lengths (the length was chosen for illustrative purposes, it is not precisely an "expensive" operation). {"Rosetta", "Code", "is", "a", "programming", "chrestomathy", "site"} According to the decorate-sort-decorate idiom, we have to "decorate" each element with its key, so each element of the original list becomes a pair. Notice that the key is calculated once (and only once) for each element in the list: {{"Rosetta", 7}, {"Code", 4}, {"is", 2}, {"a", 1}, {"programming", 11}, {"chrestomathy", 12}, {"site", 4}} The list is then sorted according to the second element of each pair (the key), perhaps using a custom comparator: {{"a", 1}, {"is", 2}, {"site", 4}, {"Code", 4}, {"Rosetta", 7}, {"programming", 11}, {"chrestomathy", 12}} And finally, the list must be undecorated, we have to remove the second element of each pair (the key): {"a", "is", "site", "Code", "Rosetta", "programming", "chrestomathy"} So, the decoration acts as a form of memoization. Randal L. Schwartz wrote an implementation of the decorate-sort-undecorate idiom in Perl in 1994, which gained much popularity in the Perl community. It was named "Schwartzian transform". Today the terms decorate-sort-undecorate and the Schwartzian transform are used interchangeably, even outside the Perl community. The Wikipedia page states that a solution can be called a "Schwartzian transform" only if it does not use named temporary lists or arrays. The Lisp solution and even the solution shown by Schwartz actually use intermediate lists, but these lists do not have explicit names, instead they use a functional composition of map-sort-map operations. Write in your programming language, a function, procedure, method, routine, etc. to sort a list of words by length (the key function), using the decorate-sort-undecorate idiom. You can, at your choice, show two solutions, the first using intermediate named lists/arrays, and the second one not using them (a Schwartzian transform)(if applicable). The first solution is sometimes more valuable, because the use of named entities makes the code clearer.

Let's start with the solution:

def 'sort by key' [keyfunc] {
	( each {|v| {k: ($v | do $keyfunc ), v: $v}}
	| sort-by k
	| each {get v} )
}

"Rosetta Code is a programming chrestomathy site" | split words | sort by key {str length}