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TimSweeneyEpic's profile
Tim Sweeney
Tim Sweeney
Tim Sweeney
@TimSweeneyEpic

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Tim Sweeney

@TimSweeneyEpic

Epic Games founder & CEO

epicgames.com
Joined August 2013

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    1. Tim Sweeney‏ @TimSweeneyEpic 10 Dec 2018
      • Report Tweet
      • Report NetzDG Violation

      The key mathematical property of equality is that a=b implies f(a)=f(b) for all possible functions f. It was a mistake of C++, C#, and Java to violate this. This is the ultimate cause of the conundrum @BarryRevzin discusses in https://brevzin.github.io/c++/2018/12/09/mixed-comparisons/ …

      11 replies 38 retweets 151 likes
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      Tim Sweeney‏ @TimSweeneyEpic 10 Dec 2018
      • Report Tweet
      • Report NetzDG Violation

      Thus a programming-language comparison like 2==2.0f must either be false, or produce a compile-time error to report potential confusion when mismatching different data types. IEEE 754 makes the converse mistake by introducing an x such that x!=x.

      11:35 AM - 10 Dec 2018
      • 2 Retweets
      • 37 Likes
      • Eyal Lotem 🏴 dekaf Eric Gajnak Dave Nicponski ✍️ damian 🍥 Christian Steinert Romain Ruetschi Æliott (((RedRedSuit)))
      9 replies 2 retweets 37 likes
        1. Tim Sweeney‏ @TimSweeneyEpic 10 Dec 2018
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          Similarly, given value x of type t, if a compiler allows automatic conversion to type u, then we should expect to have f(a)==f(u(a)) for all functions f, but most languages also violate this. Haskell and ML stand out as being honest about equality and conversions.

          6 replies 3 retweets 30 likes
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        2. Paolo G. Giarrusso‏ @Blaisorblade 10 Dec 2018
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          Replying to @TimSweeneyEpic

          I wonder if Option<Double> has better semantics, tho IEEE754 fans repeat that came first and was worse. FWIW, making invalid elements unequal to themselves has precedents in math (PERs, or partial equivalence relations), tho learning PERs might not make floats less error-prone.

          1 reply 0 retweets 1 like
        3. Tim Sweeney‏ @TimSweeneyEpic 10 Dec 2018
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          Replying to @Blaisorblade

          Here's a neat trick. Use IEEE 754 with -0==+0, but treat -inf == +inf == NaN, and modify a/b to be b-b+a/b to stop production of infinities. Now you have floating point math that obeys the equality axioms.

          3 replies 0 retweets 3 likes
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        1. Edward Kmett‏ @kmett 11 Dec 2018
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          Replying to @TimSweeneyEpic

          It also has -0 = 0, but 1/(-0) != 1/0, so IEEE made both mistakes. Mind you they did it for what they felt were good reasons, but the result is still a sad state of affairs.

          0 replies 1 retweet 6 likes
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        1. shachaf‏ @shachaf 10 Dec 2018
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          Replying to @TimSweeneyEpic

          == is not equality but a decision procedure for equality. If floating point values are meant to represent reals, then inequality is only semidecidable, so you'd expect == to at best be able to tell you either "no" or "I don't know".

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        1. Federico Vaggi‏ @F_Vaggi 10 Dec 2018
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          Replying to @TimSweeneyEpic

          It's interesting how a totally defensible decision that seems to greatly improve ergonomics leads you down a horrible rabbit hole of having to pick and choose what consistency you want to preserve. Neat blogs.

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        1. Oleg Kovalov‏ @oleg_kovalov 10 Dec 2018
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          Replying to @TimSweeneyEpic

          Basically what Go does. I've ported C -> Go some time ago (mostly math code) and it was difficult enough, a lot of expressions were splitted into several to make them more clear and find where different types were used.

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        1. Solid Frog‏ @solidfrog87 10 Dec 2018
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          Replying to @TimSweeneyEpic

          Yes

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