Logs: liberachat/#haskell
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| 2021-06-23 10:57:56 | <chisui> | Hey, has someone any clue why ghc produces this error? https://gist.github.com/chisui/7b558c2d63e3a7fcffc67ae714b69714 |
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| 2021-06-23 10:58:56 | <chisui> | The QuantifiedConstraint shouldn't imply that the instance of Typeable is user defined or am I missing something? This looks like a bug to me. |
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| 2021-06-23 11:05:54 | <boxscape> | chisui: that does look weird, from a quick look at the GHC source code I can only find that error message used in relation to checking instance heads |
| 2021-06-23 11:06:42 | <int-e> | chisui: https://paste.debian.net/1202126/ may be a viable workaround |
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| 2021-06-23 11:07:25 | <int-e> | but yeah, it does look like a bug |
| 2021-06-23 11:07:53 | <chisui> | Thanks. |
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| 2021-06-23 11:13:28 | <boxscape> | chisui: FWIW unless there really is a reason to not allow this it looks like `not quantified_constraint` is missing in this guard https://gitlab.haskell.org/ghc/ghc/-/blob/master/compiler/GHC/Tc/Validity.hs#L1539-1542 |
| 2021-06-23 11:14:47 | <chisui> | boxscape: I'll mention that in the issue. |
| 2021-06-23 11:15:05 | <boxscape> | 👍️ |
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| 2021-06-23 11:16:59 | <merijn> | Achievement unlocked: GHC bug ;) |
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| 2021-06-23 11:30:00 | <chisui> | If someones interested: https://gitlab.haskell.org/ghc/ghc/-/issues/20033 |
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| 2021-06-23 11:32:06 | <Franciman> | maerwald: you available? |
| 2021-06-23 11:32:33 | <maerwald> | barely |
| 2021-06-23 11:32:49 | <Franciman> | ok np I wanted to talk to you about ghcup satisfy |
| 2021-06-23 11:32:56 | <Franciman> | it can wait |
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| 2021-06-23 11:34:40 | <maerwald> | there is #haskell-ghcup |
| 2021-06-23 11:35:01 | <Franciman> | great |
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| 2021-06-23 12:03:58 | <shane> | I'm looking for a thing, but I don't know what it's called. I guess I'm looking for a "category", that's kind of like (->), except the output is always solely a "subproduct" of the input? |
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| 2021-06-23 12:07:52 | <shane> | So like, if we were to call this thing ~>, the only two valid implementations of `(a, a) -> a` would be `fst` and `snd` |
| 2021-06-23 12:08:35 | <shane> | I suppose that's true by parametricity if `a` is polymorphic |
| 2021-06-23 12:08:46 | <dminuoso> | shane: Mmm, Lens definitely fits that description. |
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| 2021-06-23 12:09:18 | <dminuoso> | i.e. `Lens' S T` gives you T as a sub-part of S |
| 2021-06-23 12:09:35 | <shane> | I suppose, if it's a law abiding lens |
| 2021-06-23 12:09:54 | <shane> | Actually it doesn't even have to be law breaking |
| 2021-06-23 12:10:16 | <dminuoso> | Well I guess its not entirely true |
| 2021-06-23 12:10:30 | <dminuoso> | Try and find a precise way of saying what "subproduct" exactly means |
| 2021-06-23 12:10:36 | <shane> | Yes, I'm trying :) |
| 2021-06-23 12:10:40 | <dminuoso> | Because one could argue that: |
| 2021-06-23 12:10:44 | <dminuoso> | split :: a -> (a, a) |
| 2021-06-23 12:10:48 | <chisui> | shane: do you mean a cartesian category? |
| 2021-06-23 12:10:53 | <dminuoso> | gives you a subproduct |
| 2021-06-23 12:11:03 | <dminuoso> | this gets a bit more interesting with impredicativity in the mix |
| 2021-06-23 12:11:12 | <shane> | chisui: Maybe, that sounds promising |
| 2021-06-23 12:12:01 | <dminuoso> | Mmm, come to think of it I think chisui is right. This is just a cartesian category |
| 2021-06-23 12:12:22 | <shane> | Let's forget about the `(a, a) ~> a` example, because that's true by parametricity. If you specialise it to `(Int, Int) ~> Int`, I want it to be such that the output `Int` has to be one of the two given ones. It can never just be `1` or `2` etc. Or even `negate a` or `a + 1` |
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| 2021-06-23 12:12:42 | <shane> | Okay, I will look up cartesian category |
| 2021-06-23 12:12:50 | <shane> | Are there any Haskell libraries that implement something like that? |
| 2021-06-23 12:12:55 | <dminuoso> | Or actually a precartesian category |
| 2021-06-23 12:12:57 | <dminuoso> | Yes |
| 2021-06-23 12:12:59 | <dminuoso> | https://hackage.haskell.org/package/category-extras-0.52.0/docs/Control-Category-Cartesian.html#t:PreCartesian |
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| 2021-06-23 12:13:36 | <shane> | Okay, thanks! |
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| 2021-06-23 12:17:13 | <chisui> | shane: what do you mean by "can never just be `1` or `2` etc. Or even `negate a` or `a + 1`"? wouldn't `fst (1, 2)` give you `1` in your example? |
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