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2021-08-09 19:03:37 wroathe joins (~wroathe@c-68-54-25-135.hsd1.mn.comcast.net)
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2021-08-09 19:09:57 <Gurkenglas> Is there a programming language that lets you implement ((a -> Void) -> Void) -> a? It would have to go "it must somewhere apply the (a -> Void) to an a, so lets take that a and return it instead."
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2021-08-09 19:11:35 <davean> Gurkenglas: but thats not true? With a forall it might be
2021-08-09 19:12:24 <tomsmeding> Gurkenglas: why must it apply that particular function?
2021-08-09 19:12:25 <Gurkenglas> davean, it would be true in that language, hopefully. how would it be false?
2021-08-09 19:12:58 <Gurkenglas> tomsmeding, because that language does not allow one to actually have a Void
2021-08-09 19:13:16 <Gurkenglas> so the only place to get one is to use the given (a -> Void)
2021-08-09 19:13:33 <tomsmeding> because it has no knowledge of what 'a' is?
2021-08-09 19:13:56 <Gurkenglas> wut? no this works for String to
2021-08-09 19:14:02 <tomsmeding> then davean's suggestion is correct, I think: it should at least be (forall a. (a -> Void) -> Void) -> a
2021-08-09 19:14:06 <Gurkenglas> *too
2021-08-09 19:14:17 <tomsmeding> oh right
2021-08-09 19:14:29 <tomsmeding> but there might be another function in the environment with type Int -> Void
2021-08-09 19:14:33 <Gurkenglas> ((String -> Void) -> Void) -> Void, by inspecting the argument for what string it feeds into the (String -> Void)
2021-08-09 19:14:38 <tomsmeding> and then for a ~ Int it could just call that
2021-08-09 19:15:04 <tomsmeding> yeah my forall remark was incorrect
2021-08-09 19:15:37 <tomsmeding> heh this is double-negation elimination, which implies law of the excluded middle if I remember correctly
2021-08-09 19:15:45 <Gurkenglas> indeed, thats the point :)
2021-08-09 19:15:48 <tomsmeding> so this won't work in any language based on pure type theory
2021-08-09 19:17:23 <Gurkenglas> tomsmeding, if the environment contains another source of Void you'd only get a Maybe String
2021-08-09 19:17:46 <davean> I'd meant (forall b . (a -> b) -> b) -> a
2021-08-09 19:18:03 <davean> because it knows what Void is, but with forall it doesn't, and we can inject whatever b we want
2021-08-09 19:18:24 <tomsmeding> in which case, ($ id) suffices
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2021-08-09 19:20:08 <tomsmeding> I wonder, given that 'a -> Void' is the usual encoding of "NOT a" in type theory, would 'forall b. a -> b' also be a valid encoding?
2021-08-09 19:20:35 <tomsmeding> it's at least as powerful, by substituting b = Void
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2021-08-09 19:21:58 <tomsmeding> i.e. is it true that for all propositions that you can curry-howard implement using the 'a -> Void' convention, you can also implement them using the 'forall b. a -> b' convention?
2021-08-09 19:22:17 <tomsmeding> yes, by post-composing with 'absurd'
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2021-08-09 19:23:09 <tomsmeding> so with that alternative encoding of "NOT a", the encoding of "NOT (NOT a) => a" in a type would be (forall b. (forall c. a -> c) -> b) -> a)
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2021-08-09 19:24:16 <tomsmeding> heh and then ($ id) doesn't work anymore
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2021-08-09 19:26:42 <tomsmeding> of course it doesn't because that would be a proof that ~~a => a in type theory, and that's impossible
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2021-08-09 20:21:26 <myShoggoth> Gil Mizrahi has posted the Haskell performance tuning book proposal, please check it out, give feedback, and volunteer your expertise!
2021-08-09 20:21:26 <myShoggoth> https://github.com/haskellfoundation/tech-proposals/pull/9
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