Logs: liberachat/#haskell
| 2026-04-09 12:02:08 | <mauke> | import Debug.Trace (trace); x :: Int; x = trace "computing x" (4 + 1) |
| 2026-04-09 12:02:21 | <tomsmeding> | apparently GHC does decide to keep `x` in your print x >> print x example; click Core here: https://play.haskell.org/saved/4XJoAJ1A |
| 2026-04-09 12:02:41 | → | uli-fem joins (~uli-fem@115.128.112.118) |
| 2026-04-09 12:03:22 | <tomsmeding> | Milan_Vanca: the recursive function 'main' has a reference to x, and main stays alive (because it loops), so its reference to x also stays alive, so the GC does not remove x |
| 2026-04-09 12:03:37 | <mauke> | also, x is a global variable |
| 2026-04-09 12:03:43 | <mauke> | are those ever released? |
| 2026-04-09 12:03:50 | <tomsmeding> | they can be, yes |
| 2026-04-09 12:04:13 | <merijn> | mauke: Yes |
| 2026-04-09 12:04:37 | <merijn> | mauke: It's a CAF, those can be GCed just fine if nothing references them anymore |
| 2026-04-09 12:04:39 | <tomsmeding> | on a higher level: x is a monomorphic variable, so its value will live (once computed) until it goes out of scope -- or potentially for even longer if GHC decides so |
| 2026-04-09 12:05:08 | <tomsmeding> | here x is a global variable, so it never goes out of scope, so it lives forever (until the GC decides that it's pointless keeping it around because nothing references it any more) |
| 2026-04-09 12:07:42 | <Milan_Vanca> | Will https://paste.tomsmeding.com/XLBvdfd1 will x be in memory while one_hour_long computation runs? |
| 2026-04-09 12:07:44 | × | uli-fem quits (~uli-fem@115.128.112.118) (Ping timeout: 268 seconds) |
| 2026-04-09 12:08:02 | <Milan_Vanca> | Sorry to bother you with these stupid questions but I have found no tutorial on this matter. |
| 2026-04-09 12:09:04 | <Milan_Vanca> | I gues compiler sees that x is not used after one_hour_long and so releases it as soon as it finish printing of x? |
| 2026-04-09 12:09:12 | <merijn> | correct |
| 2026-04-09 12:09:38 | <merijn> | Milan_Vanca: But that's pretty much how it works in any garbage collected languages, that's not really specific to Haskell :) |
| 2026-04-09 12:09:58 | <mauke> | s/releases/makes available for release/ |
| 2026-04-09 12:10:09 | → | machinedgod joins (~machinedg@d172-219-48-230.abhsia.telus.net) |
| 2026-04-09 12:10:10 | <mauke> | haskell doesn't do timely destruction |
| 2026-04-09 12:10:24 | <merijn> | mauke: Well, if you wanna be pedantic it's "no longer makes unavailable for release" ;) |
| 2026-04-09 12:10:29 | <mauke> | :-) |
| 2026-04-09 12:10:58 | <Milan_Vanca> | I am aware of GC and their purpose.. and all these questions stem from this fact :) |
| 2026-04-09 12:11:36 | <merijn> | Milan_Vanca: Basically, GHC uses a "copy & compact" GC strategy. Which means it only cares (and scales in) live data |
| 2026-04-09 12:12:13 | <Milan_Vanca> | Guyz.. what about this example https://paste.tomsmeding.com/TvkbD7xi |
| 2026-04-09 12:12:16 | <merijn> | Milan_Vanca: Once the heap is full and GC is triggered it copies all live data (that is, data that is still reachable from the currently evaluating code) and copies that to a new heap. Then the old heap is reset |
| 2026-04-09 12:12:47 | <merijn> | Milan_Vanca: 'x' becomes unreachable (an thus GC-able) after the first 30 minutes elapse |
| 2026-04-09 12:14:28 | <Milan_Vanca> | So x is computed once and kept for first 30 mins..hmmmm.. interesting. |
| 2026-04-09 12:14:56 | <Milan_Vanca> | It knows it can reuse the same x even when it is in different function |
| 2026-04-09 12:16:05 | × | synchromesh quits (~john@2406:5a00:2412:2c00:8174:9ae9:efe0:c152) (Read error: Connection reset by peer) |
| 2026-04-09 12:16:35 | → | synchromesh joins (~john@2406:5a00:2412:2c00:e029:fa8c:cb38:398f) |
| 2026-04-09 12:19:49 | <merijn> | Milan_Vanca: It helps to think of a haskell program as a data structure that is mutated |
| 2026-04-09 12:19:50 | <Milan_Vanca> | Does it matter if X is defined in another module? |
| 2026-04-09 12:19:55 | <merijn> | Milan_Vanca: No |
| 2026-04-09 12:21:16 | → | uli-fem joins (~uli-fem@115.128.112.118) |
| 2026-04-09 12:21:57 | <merijn> | Milan_Vanca: basically we start out with a graph of expressions with main as root, which references x and 'one_hour_long', where 'one_hour_long' in turn also references x (which is a subgraph of '4' '+' and '1'), as code gets evaluated these graphs expand and collapse as we force new computation. Starting to evaluate 'x' first materialises this subgraph of 4+1, once that evaluation is complete 'x' just |
| 2026-04-09 12:22:03 | <merijn> | refers to the result (regardless of how you reach x) |
| 2026-04-09 12:22:33 | <merijn> | Milan_Vanca: Eventually as evaluation procedes parts of the original graph become unreachable (and thus eligible for GC) |
| 2026-04-09 12:24:05 | <merijn> | a thunk is just "some computation that unfolds into a (sub)graph" and evaluation is "collapsing a graph into a result" |
| 2026-04-09 12:25:22 | × | CiaoSen quits (~Jura@p549cb690.dip0.t-ipconnect.de) (Ping timeout: 248 seconds) |
| 2026-04-09 12:25:24 | × | Googulator quits (~Googulato@94-21-172-213.pool.digikabel.hu) (Quit: Client closed) |
| 2026-04-09 12:25:26 | × | qqq quits (~qqq@185.54.23.237) (Remote host closed the connection) |
| 2026-04-09 12:25:32 | → | Googulator joins (~Googulato@94-21-172-213.pool.digikabel.hu) |
| 2026-04-09 12:27:29 | <ski> | @where lazy |
| 2026-04-09 12:27:30 | <lambdabot> | "Lazy Evaluation of Haskell" by monochrom at <http://www.vex.net/~trebla/haskell/lazy.xhtml>; "The Incomplete Guide to Lazy Evaluation (in Haskell)" by apfelmus in 2015-03-07 at <https://apfelmus. |
| 2026-04-09 12:27:30 | <lambdabot> | nfshost.com/articles/lazy-eval.html>; "Laziness, strictness, guarded recursion" by bitemyapp at <https://github.com/bitemyapp/learnhaskell/blob/master/specific_topics.md#user-content-laziness- |
| 2026-04-09 12:27:30 | <lambdabot> | strictness-guarded-recursion> |
| 2026-04-09 12:27:33 | <ski> | Milan_Vanca ^ |
| 2026-04-09 12:28:07 | <Milan_Vanca> | Ty all :) I am gonna also read that link |
| 2026-04-09 12:31:20 | → | tremon joins (~tremon@83.80.159.219) |
| 2026-04-09 12:34:21 | <merijn> | Milan_Vanca: What's your background? |
| 2026-04-09 12:34:38 | <merijn> | i.e. what languages are you experienced with, etc.? |
| 2026-04-09 12:35:53 | <Milan_Vanca> | merijn: Mostly procedural ones. Haskell is my first attempt at FP |
| 2026-04-09 12:36:32 | <merijn> | Milan_Vanca: If you're comfortable with C and some assembly I've always found that STG paper rather enlightening |
| 2026-04-09 12:37:39 | <merijn> | Milan_Vanca: The essence of that paper is "how do you compile Haskell's lazy semantics to x86 hardware", which covers a lot low level details related to how GHC compiled Haskell actually runs |
| 2026-04-09 12:37:43 | <merijn> | @where stg |
| 2026-04-09 12:37:44 | <lambdabot> | https://dl.acm.org/doi/pdf/10.1145/99370.99385 |
| 2026-04-09 12:38:51 | <Milan_Vanca> | I think reading code yields the most insights but one can pick implementation details that are more GHC than haskell |
| 2026-04-09 12:39:40 | <Milan_Vanca> | Btw haskell spec probably doesn't say anything about keeping values and reevaluating them right? If my implementation would always reevaluate everything then would it be valid haskell implementation? |
| 2026-04-09 12:46:03 | → | haritzondo joins (~hrtz@140.228.70.141) |
| 2026-04-09 12:46:08 | × | haritz quits (~hrtz@user/haritz) (Ping timeout: 252 seconds) |
| 2026-04-09 12:49:37 | → | karenw joins (~karenw@user/karenw) |
| 2026-04-09 12:51:52 | → | castan joins (~castan@188.26.38.50) |
| 2026-04-09 12:55:06 | × | castan quits (~castan@188.26.38.50) (Client Quit) |
| 2026-04-09 12:55:44 | × | Enrico63 quits (~Enrico63@host-212-171-80-94.pool212171.interbusiness.it) (Quit: Client closed) |
| 2026-04-09 12:55:45 | × | poscat quits (~poscat@user/poscat) (Remote host closed the connection) |
| 2026-04-09 12:56:09 | → | poscat joins (~poscat@user/poscat) |
| 2026-04-09 12:56:36 | <janus> | yay, gershom merged my patch for hackage, now only authorized users can update distro information |
| 2026-04-09 12:57:41 | × | uli-fem quits (~uli-fem@115.128.112.118) (Ping timeout: 252 seconds) |
| 2026-04-09 13:11:39 | → | uli-fem joins (~uli-fem@115.128.112.118) |
| 2026-04-09 13:16:31 | × | uli-fem quits (~uli-fem@115.128.112.118) (Ping timeout: 264 seconds) |
| 2026-04-09 13:17:31 | × | lbseale quits (~quassel@user/ep1ctetus) (Quit: https://quassel-irc.org - Chat comfortably. Anywhere.) |
| 2026-04-09 13:17:59 | → | lbseale joins (~quassel@user/ep1ctetus) |
| 2026-04-09 13:29:47 | → | uli-fem joins (~uli-fem@115.128.112.118) |
| 2026-04-09 13:30:39 | × | gmg quits (~user@user/gehmehgeh) (Remote host closed the connection) |
| 2026-04-09 13:33:29 | → | gmg joins (~user@user/gehmehgeh) |
| 2026-04-09 13:34:30 | × | uli-fem quits (~uli-fem@115.128.112.118) (Ping timeout: 255 seconds) |
| 2026-04-09 13:46:27 | → | fp1 joins (~Thunderbi@staff236.kora-dyn.aalto.fi) |
| 2026-04-09 13:46:39 | <merijn> | Milan_Vanca: Yes, it would be |
| 2026-04-09 13:50:14 | → | danza joins (~danza@user/danza) |
| 2026-04-09 13:59:26 | → | uli-fem joins (~uli-fem@115.128.112.118) |
| 2026-04-09 14:05:44 | × | uli-fem quits (~uli-fem@115.128.112.118) (Ping timeout: 245 seconds) |
| 2026-04-09 14:12:09 | → | synchrom1 joins (~john@2406:5a00:2412:2c00:e029:fa8c:cb38:398f) |
| 2026-04-09 14:15:13 | × | synchromesh quits (~john@2406:5a00:2412:2c00:e029:fa8c:cb38:398f) (Ping timeout: 276 seconds) |
| 2026-04-09 14:17:57 | → | uli-fem joins (~uli-fem@115.128.112.118) |
| 2026-04-09 14:22:31 | → | sam113102 joins (~sam@modemcable200.189-202-24.mc.videotron.ca) |
| 2026-04-09 14:22:57 | × | uli-fem quits (~uli-fem@115.128.112.118) (Ping timeout: 265 seconds) |
| 2026-04-09 14:24:42 | <Freakie> | does anyone know of a way to redirect output to stdout? QuickCheck seems to only want to report results directly to the terminal but I'd like to save the results to a file instead |
| 2026-04-09 14:26:47 | → | absence_ joins (torgeihe@hildring.pvv.ntnu.no) |
| 2026-04-09 14:26:58 | → | lantti_ joins (~lantti@xcalibur.cc.tut.fi) |
| 2026-04-09 14:27:45 | <danza> | > and 2> in bash? |
| 2026-04-09 14:27:46 | <lambdabot> | <hint>:1:8: error: parse error on input `in' |
| 2026-04-09 14:27:58 | → | nurupo_ joins (~nurupo.ga@user/nurupo) |
| 2026-04-09 14:28:07 | → | thaumavorio joins (~thaumavor@thaumavor.io) |
| 2026-04-09 14:29:35 | <Freakie> | danza if it works it's going to include a lot of noise from e.g. compiler output as well |
| 2026-04-09 14:30:54 | × | bitdex quits (~bitdex@gateway/tor-sasl/bitdex) (Quit: = "") |
| 2026-04-09 14:31:00 | × | karenw quits (~karenw@user/karenw) (Ping timeout: 245 seconds) |
| 2026-04-09 14:31:18 | <danza> | yep. Maybe the answer depends on the test framework you run quickcheck into |
| 2026-04-09 14:31:39 | × | fp1 quits (~Thunderbi@staff236.kora-dyn.aalto.fi) (Ping timeout: 255 seconds) |
| 2026-04-09 14:32:23 | <danza> | my test suite based on Hspec saves a log when successful |
All times are in UTC.