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See-sharp

A slightly faster implementation of Clojure's `select-keys`

Quick summary

This library supplies a single function, fselect-keys, an alternative to Clojure's core select-keys that performs 5–20% faster on hashmaps containing millions of entries.

Setup

Leiningen/Boot

[com.sagevisuals/see-sharp "0-SNAPSHOT0"]

Clojure CLI/deps.edn

com.sagevisuals/see-sharp {:mvn/version "0-SNAPSHOT0"}

Require

(require '[see-sharp.core :refer [fselect-keys]])

Recommended

Feel free to directly copy-paste the code (license terms apply).

Motivation & usage

Clojure's select-keys is implemented with a recursive first/next idiom. Could an alternative implementation using some other standard Clojure idiom perform measurably better across a wide range of inputs?

Yes, an idiomatic variant composed of reduce, conj!, and transients performs 5–20% faster than Clojure's select-keys on hashmaps containing up to one-million entries.

Here's how we use fselect-keys. We supply a hashmap and a key sequence.

(fselect-keys {:a 11, :b 22, :c 33, :d 44, :e 55} [:a :c :e])
;; => {:a 11, :c 33, :e 55}

Exactly like select-keys, fselect-keys yields a hashmap with only those entries, but returns a smidge faster.

More examples.

Commentary

It probably only makes sense to use this variant if we're in some unusually performance-sensitive context where 5–20% is a significant. Otherwise, stick with Clojure's version.

Meta-commentary

If you ever think to yourself

I've got this totally awesome idea on how to rewrite some-function,

Beware that much later you'll likely realize

Good grief! Proving that some-function is faster takes a ton of time and grinding effort. And ultimately, it's merely 5% faster. And only on synthetic benchmarks.

Alternatives

• Clojure's core select-keys. No dependencies. Widely-used and battle-tested.

• Ben Sless' keys. Select and rename keys as fast as possible with idiomatic Clojure.

Glossary

key sequence

A sequence of keys to be extracted from a hashmap. Keys need not be keywords; they may be any valid Clojure value. A vector is convenient for manually supplying.

License

This program and the accompanying materials are made available under the terms of the MIT License.

Copyright © 2024–2026 Brad Losavio.
Compiled by ReadMoi on 2026 April 15.
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