# How do ephermerons and weak hashes work? **URL:** \ **Category:** Questions & Answers\ **Created:** [May 27, 2022, 4:51pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036 "2022-05-27T16:51:56Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![dstorrs](https://avatars.discourse-cdn.com/v4/letter/d/898d66/32.png) [@dstorrs](https://racket.discourse.group/u/dstorrs)\ **Post date:** [May 27, 2022, 4:51pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/1 "2022-05-27T16:51:56Z")
I apparently don't understand the concept of unreachability as it applies to the garbage collector. ```scheme (define h1 (make-ephemeron-hash)) (define h2 (make-weak-hash)) (struct thing (val) #:transparent) (let () (hash-set! h1 (~a "foo" "bar") (thing 1) ; strings created with ~a are not interned ) (hash-set! h2 (~a "bar" "baz") (thing 2)) ; inside the `let`, each hash should contain one k/v pair (pretty-print h1) (pretty-print h2)) (sleep 2) ; give it time for unreachability to be noticed (collect-garbage) ; force a GC pass, just to be sure ; both hashes should be empty (pretty-print h1) (pretty-print h2) ; Output: ; (hash "foobar" (thing 1)) ; (hash "barbaz" (thing 2)) ``` I guess the values are being retained because they are accessible through h1 and h2? That surprises me, since I thought the entire point of these things was to ensure that those hashes _didn't_ count. ---
**Author:** ![samth](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/samth/32/3_2.png) [@samth](https://racket.discourse.group/u/samth)\ **Post date:** [May 27, 2022, 4:59pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/2 "2022-05-27T16:59:18Z")
I get empty hashes for the second printout when I try your program. Note that the sleep is not needed; nothing concurrent with your program is happening. However, additional garbage collections can cause more things to be unreachable via ephemerons, if the unreachable key was noticed to be unreachable _after_ the ephemeron was visited earlier by the GC. ---
**Author:** ![dstorrs](https://avatars.discourse-cdn.com/v4/letter/d/898d66/32.png) [@dstorrs](https://racket.discourse.group/u/dstorrs)\ **Post date:** [May 27, 2022, 5:03pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/3 "2022-05-27T17:03:03Z")
> [@samth](#): > > I get empty hashes for the second printout when I try your program. Ohhhkay... I reliably get NON-empty hashes. I'm on Racket 8.4 on macOS 10.15.5, would that matter? If not, where should I be looking? ---
**Author:** ![samth](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/samth/32/3_2.png) [@samth](https://racket.discourse.group/u/samth)\ **Post date:** [May 27, 2022, 6:02pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/4 "2022-05-27T18:02:20Z")
You should try adding a second (or third) call to `collect-garbage` and see if that changes the result. ---
**Author:** ![dstorrs](https://avatars.discourse-cdn.com/v4/letter/d/898d66/32.png) [@dstorrs](https://racket.discourse.group/u/dstorrs)\ **Post date:** [May 27, 2022, 6:07pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/5 "2022-05-27T18:07:48Z")
Tried it, no change. ---
**Author:** ![LiberalArtist](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/liberalartist/32/151_2.png) [@LiberalArtist](https://racket.discourse.group/u/LiberalArtist)\ **Post date:** [May 29, 2022, 4:37am UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/6 "2022-05-29T04:37:49Z")
With Racket 8.5 CS on x86\_64-linux, I also get consistently empty hashes. ---
**Author:** ![greghendershott](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/greghendershott/32/98_2.png) [@greghendershott](https://racket.discourse.group/u/greghendershott)\ **Post date:** [May 30, 2022, 12:53pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/7 "2022-05-30T12:53:50Z")
@dstorrs How are you running the program? Why I ask: I get the same result as everyone except you -- empty hash-tables -- when running using command-line Racket, DrRacket, and Racket Mode with `racket-pretty-print` set to `nil`. But running using Racket Mode with `racket-pretty-print` set to `t`, I see the non-empty hash-tables. What is retaining a reference to the keys? I suspect [this](https://github.com/greghendershott/racket-mode/blob/master/racket/print.rkt#L25-L37). I'll open an issue. ---
**Author:** ![greghendershott](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/greghendershott/32/98_2.png) [@greghendershott](https://racket.discourse.group/u/greghendershott)\ **Post date:** [May 30, 2022, 1:53pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/8 "2022-05-30T13:53:43Z")
> > > Arising from https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes…-work/1036/5. > > Given this program: > > > \`\`\`racket > \#lang racket/base > > (require racket/format > racket/pretty) > > (define h1 (make-ephemeron-hash)) > (define h2 (make-weak-hash)) > > (struct thing (val) #:transparent) > > (let () > ;; strings created with ~a are not interned > (hash-set! h1 (~a "foo" "bar") (thing 1)) (hash-set! h2 (~a "bar" "baz") (thing 2)) > > ;; inside the \`let\`, each hash should contain one k/v pair > (pretty-print h1) > (pretty-print h2)) > > (collect-garbage 'major) > > ;; Both hashes should be empty > (pretty-print h1) > (pretty-print h2) > \`\`\` > > When run using command-line Racket, DrRacket, or Racket Mode with \`racket-pretty-print\` set to \`nil\`, it correctly prints: > > \`\`\` > (hash "foobar" (thing 1)) > (hash "barbaz" (thing 2)) > '#hash() > '#hash() > \`\`\` > > When run using Racket Mode with \`racket-pretty-print\` set to \`t\`, it incorrectly prints: > > \`\`\` > (hash "foobar" (thing 1)) > (hash "barbaz" (thing 2)) > (hash "foobar" (thing 1)) > (hash "barbaz" (thing 2)) > \`\`\` > > \--- > > The fix might be as simple as using a weak hash-table ourselves: > > \`\`\`diff > modified racket/print.rkt > @@ -34,7 +34,7 @@ > ;; (Note: Although I had tried using the pre-print and post-print > ;; hooks, they seemed to be called inconsistently.) > > \-(define ht (make-hasheq)) > +(define ht (make-weak-hasheq)) > > (define (make-pretty-print-size-hook pixels/char) > (define (racket-mode-size-hook value \_display? \_port) > \`\`\` The fix _might_ be as simple as using a weak hash-table there. But I'll give it a think. ---
**Author:** ![dstorrs](https://avatars.discourse-cdn.com/v4/letter/d/898d66/32.png) [@dstorrs](https://racket.discourse.group/u/dstorrs)\ **Post date:** [May 31, 2022, 1:28pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/9 "2022-05-31T13:28:06Z")
Aha. Yes, that was the issue. When I run it from the command line it works as expected and as soon as I set `racket-pretty-print` to `nil` it also worked. Thanks, Greg. ---
**Author:** ![greghendershott](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/greghendershott/32/98_2.png) [@greghendershott](https://racket.discourse.group/u/greghendershott)\ **Post date:** [May 31, 2022, 6:21pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/10 "2022-05-31T18:21:33Z")
I merged a fix yesterday, so that `racket-pretty-print` enabled works, too. It should already be available on MELPA, whenever you might want to [update Racket Mode](https://www.racket-mode.com/#Update). ---
**Author:** ![dstorrs](https://avatars.discourse-cdn.com/v4/letter/d/898d66/32.png) [@dstorrs](https://racket.discourse.group/u/dstorrs)\ **Post date:** [May 31, 2022, 9:57pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/11 "2022-05-31T21:57:17Z")
I uninstalled racket-mode as per the directions, then went to reinstall it, and emacs told me "Hey, I can't connect to elpa! Stinks to be you!" This sent me down a rabbit hole of googling to figure out why emacs wasn't able to connect when Firefox could. One solution said that it's an issue with the user-agent string. Another said that I needed to downgrade (!) my OpenSSL library, which is a thing that macOS makes extremely difficult, which is sad since the one that comes on macOS 10.15 is `OpenSSL 1.1.0-pre5-dev xx XXX xxxx` which holy heckfire, the current version is 3.0!! In the end I simply copied the racket-mode-20211130.1748 directory back into .emacs.d/elpa and called it a day. 😕 It seems to be working, which is bizarre since I reverted the `racket-pretty-print` to be non-nil. I'm going to chalk this one up to "computers are weird, networks are awful, GNU software is user-hostile, and ain't nobody got time for this." Thanks for pointing me to the fix. If I wanted to git clone the latest racket-mode, what would the appropriate version string be so that I could drop it into .../elpa ? ---
**Author:** ![greghendershott](https://yyz2.discourse-cdn.com/free1/user_avatar/racket.discourse.group/greghendershott/32/98_2.png) [@greghendershott](https://racket.discourse.group/u/greghendershott)\ **Post date:** [May 31, 2022, 10:26pm UTC](https://racket.discourse.group/t/how-do-ephermerons-and-weak-hashes-work/1036/12 "2022-05-31T22:26:37Z")
That sounds frustrating. Emacs and TLS isn't my wheelhouse, so my advice for a problem connecting with MELPA would be [their guide](https://melpa.org/#/getting-started), and if still no joy, open an issue [there](https://github.com/melpa/melpa/issues)?