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/*********************************************************************************************************************** | |
* Partition an array into two arrays based on passing each element to a predicate function. | |
* | |
* This function is designed to extend the Array prototype, but only by the explicit action of the consuming library. | |
* E.G. Assuming the function below belongs to a "utils" module, you must write something like the following: | |
* | |
* const utils = require('./utils.js') | |
* Array.prototype.partitionWith = utils.partitionWith | |
* | |
* This function must be defined using a traditional "function" declaration instead of an arrow function. If an arrow | |
* function were used, then the reference to "this.reduce" wouldn't work... | |
* | |
* partitionWith takes a simple predicate function as an argument and returns an array containing two other arrays. | |
* The first array contains all the elements that do not match the predicate, and the second, all the elements that do | |
* match the predicate | |
* | |
* E.G. | |
* | |
* var isEven = x => x % 2 === 0 | |
* var numbers = [1,2,3,4,5,6,7,8,9,10] | |
* var [odd, even] = numbers.partitionWith(isEven) | |
* | |
* odd = [1,3,5,7,9] | |
* even = [2,4,6,8,10] | |
*/ | |
const partitionWith = function(predFn) { | |
const partitionReducer = (acc, el) => | |
(success => success ? [acc[0], acc[1].concat(el)] | |
: [acc[0].concat(el), acc[1]]) | |
(predFn(el)) | |
return this.reduce(partitionReducer, [[],[]]) | |
} |
Thanks Chris, I did think it might be related to academic principles (I know you very well!)
Interesting term, "meaning obscuring". I quite like f
, a
, b
, x
and xs
and think that they help, rather than obscure. Theoretically they should also force (or at least persuade me to think about) splitting out logic into further separate functions if it all gets "a bit too much". But I know some folks see that differently. I subscribe to the Erik Meijer school of "x over xs".
partition
vs partitionWith
- interesting too - Ramda calls it partition. After a beer, I'd argue that the 'with' was superfluous, as (almost) "of course you're going to provide a predicate function" :-)
I have no problem with single cases of x
over xs
, but when you're looking at a reasonably deep stack trace containing lots of functions that all use single letter variable names like names like f
and a
and b
, you (or more specifically, I) quickly loose track of what value is stored where. Quinsiquontly, in my attempt to remove any ambiguity, I tend to use longer, self-descriptive names
The preference of partitionWith
over partition
is motivated by the same reasoning
Alles sollte Idiotensicher sein, weil manchmal, bin Ich das Idiot...
:-P
Goodness me you wouldn't believe how lovely Ramda's implementation of partition
is. Might be worth a beer and a live stream to look at it...
Yes, the Ramda implementation is:
partition = juxt([filter, reject])
Where juxt
first applies the filter
function to produce the first array, then its logical complement reject
to produce the second array
And to answer your questions above...
I try to use IIFEs wherever possible simply because I'm trying to conform to several academic principles:
let
declarations are impossibleIt must be remembered however that it is not always either easy or practical to write functions this way. Sometimes, a function's implementation would become so obscure if written this way, that is ceases to provide any practical value
However, I've found following these principles to be something of a mixed blessing...
f
ora
orb
Therefore, as a compromise, I've developed the style above in which all the internal variables within a function (whose values are bound at invocation time) are given meaningful, self-descriptive names. This gives you a fighting chance to see what's gone wrong when in debug mode
Yes, the use of
concat
also allows forel
itself to be an arrayIt think the name
partitionWith
makes the function's purpose more self-descriptive - you're partitioning an array with (I.E. by the means of) the function supplied as an argument. I also lifted this function name directly out of the Erlanglists
module:-)