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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 4 Jan 2018
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      Replying to @paniq

      Stages? I had to merge lexing and parsing to support markup. I had to merge typechecking and evaluation to support dependent types, and now macro evaluation is merged in there too. So there are only 2 stages between text and abstract interpretation.

      1 reply 0 retweets 1 like
    2. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @TimSweeneyEpic

      sure, lexing and parsing are also one pass in my system, but they're still clearly hierarchically separated; there's no backchannel from the parser to the lexer.

      1 reply 0 retweets 0 likes
    3. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @paniq @TimSweeneyEpic

      i too merged typechecking and evaluation into a specializer, but that doesn't invalidate the idea of stages; e.g. the specializer explicitly only instantiates IL instructions, it never generates new IL - because of this, macro expansion is a separate stage before that.

      1 reply 0 retweets 0 likes
    4. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @paniq @TimSweeneyEpic

      stages are useful - what, for example, if you just want to expand all macros and look at the unspecialized IL? if you do it all in the same stage, you'll have to add unappetizing modes just to support this, if at all - it's possible you can't even do this anymore.

      1 reply 0 retweets 0 likes
    5. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @paniq @TimSweeneyEpic

      but! i concealed the fact that it is possible to nest expand, compile & execute stages within the partial evaluator. but they're all separate passes. to merge them into one would make it increasingly difficult to reason about what's happening.

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    6. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @paniq @TimSweeneyEpic

      and! i failed to mention that it is possible for macros to interrogate types of constant values that occur in the expressions they are expanding, as well as create new types at that point. (macros themselves are small compiled routines, so very much like compiler plugins)

      1 reply 0 retweets 0 likes
    7. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @paniq @TimSweeneyEpic

      but at the moment the macro expands, the only environment it has is the lexical scope (and all parents), which it can also freely mutate / swap. the partial evaluation stack frame doesn't exist at that point.

      1 reply 0 retweets 0 likes
    8. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @paniq @TimSweeneyEpic

      so there's a clear definition of what a macro does: in goes S-expr and lexical scope, out comes generic IL (template functions in CPS style). i don't want programmable IL transforms. it feels like that would be too much.

      1 reply 0 retweets 0 likes
    9. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @paniq @TimSweeneyEpic

      there is a bit of cross-macro communication happening but it is either explicitly done via metadata in the expanded expression, or by setting new keys in the lexical scope. both ways are easy to inspect & debug, that was important to me.

      1 reply 0 retweets 0 likes
    10. L.  😷. Ritter‏ @paniq 4 Jan 2018
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      Replying to @paniq @TimSweeneyEpic

      maybe the key question is: how stateful is your system? how much hidden state does it carry? is that hidden state inspectable at compile time, at user level?

      1 reply 0 retweets 0 likes
      Tim Sweeney‏ @TimSweeneyEpic 4 Jan 2018
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      Replying to @paniq

      The whole system is conceptually stateless, but uses a pure-ish notion of futures and refinements, which is necessary to resolve recursive things that occur out-of-order.

      9:20 PM - 4 Jan 2018
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      • L. 😷. Ritter
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        2. Tim Sweeney‏ @TimSweeneyEpic 4 Jan 2018
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          Replying to @TimSweeneyEpic @paniq

          Some challenges arise out of the desire to support open-world programming with strong names and Java-style forward compatibility rules. So, a macro seeing a field reference "x" doesn't yet know what means. Could be Engine::Vector::x, or a local variable x from an outer context.

          2 replies 0 retweets 1 like
        3. Tim Sweeney‏ @TimSweeneyEpic 4 Jan 2018
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          Replying to @TimSweeneyEpic @paniq

          Other challenges arise from the need to obey Walder-style free theorems / universal properties, as a means of compiler-guaranteed erasure and data hiding. Runtime code can't cheat by asking a type for its metadata; you had to expose it at compile-time via macro.

          1 reply 0 retweets 2 likes
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