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Add `update-keys` function to transform keys of a map

Description

Problem

Functions related to update that work on all keys or values of a map are frequently needed by Clojure devs but writing versions that are performant and preserve type and metadata are nuanced. (Many utility libraries have something called map-keys for this.)

Background

The maps requiring application of update-keys almost always have keys of homogeneous types with values of varying types.

In practice the need for update-keys arises in server applications that send and receive info packets or deal with entity maps and need to perform transformations on the keys as a whole -- such as turning string keys from json maps into keywords and back again.

Approach

For performance, use reduce-kv and collect into a transient empty map (as a rebuild of the map is required). Preserve metadata on the map.

Answered Questions

  • Is this generic across associative types? What does it mean to update the keys of a vector? The function is written assuming map in and map out – coverage of other associative types in is incidental.

  • Should the type preserve on return? NO

  • Should metadata preserve on return? YES

  • what if (f kx) == (f ky) – UNDEFINED BEHAVIOR

Docstring

"m f => {(f k) v ...} Given a map m and a function f of 1-argument, returns a new map whose keys are the result of applying f to the keys of m, mapped to the corresponding values of m. f must return a unique key for each key of m, else the behavior is undefined"

Patch: clj-1959-v5.patch

Screened: Alex Miller - the only thing that bothers me is whether the error validation check based on counts is slow, particularly if m is not Counted. Also, same question from update-vals ticket on whether any of the answered questions should be reflected in the docstring.

Discussions

GGROUPS: https://groups.google.com/forum/# !topic/clojure-dev/kkPYIl5qj0o

Environment

None

Attachments

5
  • 14 Sep 2021, 02:59 PM
  • 07 Sep 2021, 07:32 PM
  • 11 Jul 2016, 09:27 AM
  • 15 Jun 2016, 05:01 PM
  • 14 Jun 2016, 05:22 PM

Activity

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Alex Miller September 14, 2021 at 3:33 PM

Applied for 1.11.0-alpha2

Rich Hickey September 14, 2021 at 1:08 PM

Please investigate implications of using count

Michael Fogus September 8, 2021 at 12:35 PM

Added a comment about the coverage of associative types. As for the count check, the function was written to be performant for the most common case (map in). While it can operate given a seq of map-entries, that’s not the prevalent case and indeed not mentioned in the docstring. As for the generative tests, I created a set of tests in the spirit of existing update-ish functions and ran with them. If you think that gen tests are required for this patch then I am happy to explore further.

Alex Miller September 7, 2021 at 11:29 PM
Edited

In the answered questions section "Is this generic across associative types? What does it mean to update the keys of a vector? (see below)" - I don't see an explicit answer to this "below". Can you answer that explicitly? Seems like update-keys explicitly makes a map so it “works” to take a vector but always emits a map.

The check using count could be slow if, for example, the incoming m is actually a seq of map-entries (count there will be O(n)). Were other approaches considered or perf tested?

Did you think about a generative test for this?

Michael Fogus August 2, 2021 at 1:32 PM

Original ticket text:

Many people have been writing a function to map values in HashMap:

Proposal: Add map-keys and map-values which: maps keys in HashMap, and map values in HashMap. They return HashMap as a result.

Workaround: Using function reduce-kv or ordinary map and into is a common solution, but they are confusing and types change, which makes it tricky and tedious.

Fixed

Details

Assignee

Reporter

Approval

Ok

Patch

Code and Test

Priority

Fix versions

Created June 14, 2016 at 5:19 PM
Updated September 14, 2021 at 3:33 PM
Resolved September 14, 2021 at 3:33 PM

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