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2020-11-02 05:46:24 × day quits (~Unknown@unaffiliated/day) (Ping timeout: 240 seconds)
2020-11-02 05:46:24 day_ is now known as day
2020-11-02 05:47:20 <jackdk> so the Costar thing is a pun, like Biff/Tannen?
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2020-11-02 05:51:24 <int-e> jackdk: well, maybe :)
2020-11-02 05:51:38 <int-e> but there is also an actual duality
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2020-11-02 06:11:09 <danso> i see this pattern often in my code where i end up in many nested `case` expressions
2020-11-02 06:11:13 <danso> https://github.com/ninedotnine/soucc/blob/main/src/TypeChecker/TypeChecker.hs#L61-L75
2020-11-02 06:11:43 <danso> usually `do` notation makes these go away, but here it only works for the outer monad (State)
2020-11-02 06:11:51 <danso> and i would like it to work with Either as well
2020-11-02 06:12:09 <danso> i'm sure i've read about the cure for this problem on a blog somewhere, but i don't know how to find it now. help?
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2020-11-02 06:12:37 × knupfer quits (~Thunderbi@200116b82c45b30000b473fffe3f50e6.dip.versatel-1u1.de) (Client Quit)
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2020-11-02 06:13:24 <jackdk> danso: sounds like a job for monad transformers, specifically ExceptT
2020-11-02 06:13:58 <jackdk> if I understand you correctly, you can temporarily work with a transformer, you don't have to use it all over your program
2020-11-02 06:14:35 <jackdk> `run_globals defns = runExceptT $ do ...` etc
2020-11-02 06:15:31 <danso> thanks, i will start reading
2020-11-02 06:15:48 <danso> this occurs in many places in my program; maybe it would be appropriate to use ExceptT throughout
2020-11-02 06:16:25 × merijn quits (~merijn@83-160-49-249.ip.xs4all.nl) (Ping timeout: 264 seconds)
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2020-11-02 06:38:02 <danso> does it make a difference whether i use `StateT s (Either e a)` or `ExceptT e (State s) a` ?
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2020-11-02 06:52:14 <int-e> danso: They're isomorphic as types, but the latter has a handy Monad instance that propagates errors (approximately what your `case`s do)
2020-11-02 06:52:36 <int-e> (or exceptions, whatever)
2020-11-02 06:54:08 <int-e> danso: Err
2020-11-02 06:54:52 <int-e> danso: Sorry, I misread because the former is wrong... you must have meant StateT s (Either e) a.
2020-11-02 06:55:29 <int-e> danso: In which case, the main difference is that the former will forget the state on an exception/error.
2020-11-02 06:55:50 <danso> ah i see i mistyped
2020-11-02 06:55:56 <danso> oops
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2020-11-02 06:56:12 <danso> okay, i definitely want to keep the state
2020-11-02 06:56:15 <danso> thanks int-e
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2020-11-02 06:56:52 <danso> curious, why does that happen?
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2020-11-02 06:59:46 <int-e> @unmtl StateT s (Either e a)
2020-11-02 06:59:46 <lambdabot> err: `StateT s (Either e a)' is not applied to enough arguments, giving `/\A. s -> Either e a (A, s)'
2020-11-02 06:59:51 <int-e> @unmtl StateT s (Either e) a
2020-11-02 06:59:51 <lambdabot> s -> Either e (a, s)
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2020-11-02 07:00:29 <int-e> Because StateT puts the resulting state into the monadic return value and Either's Left doesn't have one.
2020-11-02 07:00:41 <int-e> @unmtl ErrorT e (State s) a
2020-11-02 07:00:41 <lambdabot> s -> (Either e a, s)
2020-11-02 07:01:05 × yinfeng quits (~yinfeng@2001:250:5002:8100::1:851e) (Quit: WeeChat 2.9)
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2020-11-02 07:01:51 <int-e> Whereas this always has a state. (ErrorT is a bit weird and deprecated, but @unmtl doesn't know ExceptT)
2020-11-02 07:02:45 <danso> ah thanks, the ErrorT left me confused
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2020-11-02 07:21:13 <energizer> what is the name of the function that takes f : X -> Y and Either X Null, if a is Null returns Null otherwise `f a`?
2020-11-02 07:22:31 <energizer> (forgive my word choice, i'm not experienced with this kind of stuff)
2020-11-02 07:22:40 × christo quits (~chris@81.96.113.213) (Remote host closed the connection)
2020-11-02 07:23:59 <energizer> for example, it takes addOne and either a number or null, and if it's null returns null, but otherwise increments the number
2020-11-02 07:24:48 <energizer> f addOne null = null; f addOne 3 = 4
2020-11-02 07:25:17 <danso> what you're talking about is basically fmap
2020-11-02 07:25:35 <danso> but you seem to be mixing a few different ideas here
2020-11-02 07:25:41 <energizer> oh?
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2020-11-02 07:26:03 <danso> @t fmap
2020-11-02 07:26:04 <lambdabot> Maybe you meant: tell thank you thanks thesaurus thx tic-tac-toe ticker time todo todo-add todo-delete type v @ ? .
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