From: "shugo (Shugo Maeda)" <redmine@...>
Date: 2012-04-13T11:21:53+09:00
Subject: [ruby-core:44325] [ruby-trunk - Feature #5663] Combined map/select method


Issue #5663 has been updated by shugo (Shugo Maeda).


matz (Yukihiro Matsumoto) wrote:
> I am OK with the original map_select behavior, but I don't like the name #map_select.
> It is combination of -ect family name with map.
> 
> I prefer the name #filter, but it might be confused by simple alias of select.
> Any idea?

I think filter should be a simple alias of select.
# Otherwise, we can't complain about Scala's collect:p
How about filter_map?

I've found that Scheme has filter-map.
From SRFI-1:

  filter-map f clist1 clist2 ... -> list
  Like map, but only true values are saved.
  (filter-map (lambda (x) (and (number? x) (* x x))) '(a 1 b 3 c 7))
      => (1 9 49)
  The dynamic order in which the various applications of f are made is not specified.
  At least one of the list arguments must be finite.

----------------------------------------
Feature #5663: Combined map/select method
https://bugs.ruby-lang.org/issues/5663#change-25872

Author: wycats (Yehuda Katz)
Status: Feedback
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: lib
Target version: 2.0.0


It is pretty common to want to map over an Enumerable, but only include the elements that match a particular filter. A common idiom is:

enum.map { |i| i + 1 if i.even? }.compact

It is of course also possible to do this with two calls:

enum.select { |i| i.even? }.map { |i| i + 1 }

Both cases are clumsy and require two iterations through the loop. I'd like to propose a combined method:

enum.map_select { |i| i + 1 if i.even? }

The only caveat is that it would be impossible to intentionally return nil here; suggestions welcome. The naming is also a strawman; feel free to propose something better.


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