Creating a map entry

[Khalid Jebbari]

Is it possible to create a map entry directly with the MapEntry type? I see in the source code of CLJS that the type expects 3 parameters, key val and hash. I've glanced quickly at usages and it seems most usages revolves to (MapEntry. k v nil). Is it ok to use in app? Is it too low-level? I'd like to use it in situations where I populate a map from vectors, like (silly example)

(into {} (map-indexed (fn [idx v] (MapEntry. idx val nil) [1 2 3]))

And what does it mean in cljc code ? Is this ok in Clojure too?

[Khalid Jebbari]

I've found the time to experiment myself and report my findings below.

In CLJS, MapEntry seems to work fine, and is slightly faster than using a vector to represent it. Below are dumb micro benchmarks, executed in fresh repls void of any other dependencies:

```

> clj -Sdeps "{:deps {org.clojure/clojurescript {:mvn/version \"1.10.339\"}}}" -m cljs.main

ClojureScript 1.10.339

(simple-benchmark [rng (range 1000)]

                  (into {} (map (fn [n] [n n]) rng))

                  1000)

[rng (range 1000)], (into {} (map (fn [n] [n n]) rng)), 1000 runs, 923 msecs

nil

(simple-benchmark [rng (range 1000)]

                  (into {} (map (fn [n] (MapEntry. n n nil)) rng))

                  1000)

[rng (range 1000)], (into {} (map (fn [n] (MapEntry. n n nil)) rng)), 1000 runs, 840 msecs

nil

```

In this example, using MapEntry yields a 9% improvements.

In the Clojure side, there are 2 important differences. #1 MapEntry is not part of clojure.core in clojure.lang, so using it requires a JVM-specific import. #2 The MapEntry constructor doesn't need the third "hash" parameter like CLJS, only the key and value. So using MapEntry in cljc code will require reader conditional in `ns` and in each call-site.

```

(import clojure.lang.MapEntry)

(criterium/bench (let [rng (range 1000)]

                   (into {} (map (fn [n] [n n]) rng))))

Evaluation count : 317760 in 60 samples of 5296 calls.

             Execution time mean : 189.335048 µs

    Execution time std-deviation : 613.930267 ns

   Execution time lower quantile : 188.569507 µs ( 2.5%)

   Execution time upper quantile : 191.256133 µs (97.5%)

                   Overhead used : 1.867366 ns

Found 6 outliers in 60 samples (10.0000 %)

low-severe 3 (5.0000 %)

low-mild 3 (5.0000 %)

 Variance from outliers : 1.6389 % Variance is slightly inflated by outliers

=> nil

(criterium/bench (let [rng (range 1000)]

                   (into {} (map (fn [n] (MapEntry. n n)) rng))))

Evaluation count : 401880 in 60 samples of 6698 calls.

             Execution time mean : 149.182354 µs

    Execution time std-deviation : 255.340674 ns

   Execution time lower quantile : 148.692324 µs ( 2.5%)

   Execution time upper quantile : 149.779548 µs (97.5%)

                   Overhead used : 1.867366 ns

Found 3 outliers in 60 samples (5.0000 %)

low-severe 3 (5.0000 %)

 Variance from outliers : 1.6389 % Variance is slightly inflated by outliers

=> nil

```

This other dumb benchmarks shows a 26% speed improvements.

In my case, since I'm coding in cljc file I'm not totally confortable with the reader conditional. Furthermore I don't understand yet all the implications, apart from making the code that produce the map entries less flexible (a sequence of vectors is less generic than a sequence of map entries).

I'd love to hear from you specialists.

PS: I couldn't get clojure cli to download criterium to run the benchmark, so I ended up using the REPL provided by in my current app setup (based on Clojure 1.9.0, Boot 2.8.2 and criterium 0.4.4, the latest stable versions). 

Here's the command I ran and the error:

```

> clj -Sdeps "{:deps {org.clojure/clojure {:mvn/version \"1.9.0\"} criterum {:mvn/version \"0.4.4\"}}}"

Error building classpath. Could not find artifact criterum:criterum:jar:0.4.4 in central (https://repo1.maven.org/maven2/)

org.eclipse.aether.resolution.ArtifactResolutionException: Could not find artifact criterum:criterum:jar:0.4.4 in central (https://repo1.maven.org/maven2/)

at org.eclipse.aether.internal.impl.DefaultArtifactResolver.resolve(DefaultArtifactResolver.java:422)

at org.eclipse.aether.internal.impl.DefaultArtifactResolver.resolveArtifacts(DefaultArtifactResolver.java:224)

at org.eclipse.aether.internal.impl.DefaultArtifactResolver.resolveArtifact(DefaultArtifactResolver.java:201)

at org.eclipse.aether.internal.impl.DefaultRepositorySystem.resolveArtifact(DefaultRepositorySystem.java:260)

at clojure.tools.deps.alpha.extensions.maven$get_artifact.invokeStatic(maven.clj:95)

at clojure.tools.deps.alpha.extensions.maven$get_artifact.invoke(maven.clj:91)

at clojure.tools.deps.alpha.extensions.maven$eval698$fn__700.invoke(maven.clj:112)

at clojure.lang.MultiFn.invoke(MultiFn.java:243)

at clojure.tools.deps.alpha$lib_paths$fn__1068.invoke(alpha.clj:203)

at clojure.core.protocols$iter_reduce.invokeStatic(protocols.clj:49)

at clojure.core.protocols$fn__7839.invokeStatic(protocols.clj:75)

at clojure.core.protocols$fn__7839.invoke(protocols.clj:75)

at clojure.core.protocols$fn__7781$G__7776__7794.invoke(protocols.clj:13)

at clojure.core$reduce.invokeStatic(core.clj:6748)

at clojure.core$reduce.invoke(core.clj:6730)

at clojure.tools.deps.alpha$lib_paths.invokeStatic(alpha.clj:199)

at clojure.tools.deps.alpha$lib_paths.invoke(alpha.clj:197)

at clojure.tools.deps.alpha$resolve_deps.invokeStatic(alpha.clj:229)

at clojure.tools.deps.alpha$resolve_deps.invoke(alpha.clj:210)

at clojure.tools.deps.alpha.script.make_classpath$create_classpath.invokeStatic(make_classpath.clj:59)

at clojure.tools.deps.alpha.script.make_classpath$create_classpath.invoke(make_classpath.clj:52)

at clojure.tools.deps.alpha.script.make_classpath$run.invokeStatic(make_classpath.clj:70)

at clojure.tools.deps.alpha.script.make_classpath$run.invoke(make_classpath.clj:64)

at clojure.tools.deps.alpha.script.make_classpath$_main.invokeStatic(make_classpath.clj:109)

at clojure.tools.deps.alpha.script.make_classpath$_main.doInvoke(make_classpath.clj:84)

at clojure.lang.RestFn.applyTo(RestFn.java:137)

at clojure.lang.Var.applyTo(Var.java:702)

at clojure.core$apply.invokeStatic(core.clj:657)

at clojure.main$main_opt.invokeStatic(main.clj:317)

at clojure.main$main_opt.invoke(main.clj:313)

at clojure.main$main.invokeStatic(main.clj:424)

at clojure.main$main.doInvoke(main.clj:387)

at clojure.lang.RestFn.applyTo(RestFn.java:137)

at clojure.lang.Var.applyTo(Var.java:702)

at clojure.main.main(main.java:37)

Caused by: org.eclipse.aether.transfer.ArtifactNotFoundException: Could not find artifact criterum:criterum:jar:0.4.4 in central (https://repo1.maven.org/maven2/)

at org.eclipse.aether.connector.basic.ArtifactTransportListener.transferFailed(ArtifactTransportListener.java:48)

at org.eclipse.aether.connector.basic.BasicRepositoryConnector$TaskRunner.run(BasicRepositoryConnector.java:365)

at org.eclipse.aether.util.concurrency.RunnableErrorForwarder$1.run(RunnableErrorForwarder.java:75)

at org.eclipse.aether.connector.basic.BasicRepositoryConnector$DirectExecutor.execute(BasicRepositoryConnector.java:583)

at org.eclipse.aether.connector.basic.BasicRepositoryConnector.get(BasicRepositoryConnector.java:259)

at org.eclipse.aether.internal.impl.DefaultArtifactResolver.performDownloads(DefaultArtifactResolver.java:498)

at org.eclipse.aether.internal.impl.DefaultArtifactResolver.resolve(DefaultArtifactResolver.java:399)

... 34 more

```

[Alex Miller]

On Thursday, October 18, 2018 at 4:07:32 AM UTC-5, Khalid Jebbari wrote:

PS: I couldn't get clojure cli to download criterium to run the benchmark, so I ended up using the REPL provided by in my current app setup (based on Clojure 1.9.0, Boot 2.8.2 and criterium 0.4.4, the latest stable versions). 

Here's the command I ran and the error:

```

> clj -Sdeps "{:deps {org.clojure/clojure {:mvn/version \"1.9.0\"} criterum {:mvn/version \"0.4.4\"}}}"

Error building classpath. Could not find artifact criterum:criterum:jar:0.4.4 in central (https://repo1.maven.org/maven2/)

You misspelled "criterium" in your deps.

[Khalid Jebbari]

Ooops, silly me... Thanks for catch.

--
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