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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 May 26
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      Here's a neat test of programming language expressiveness: Can you write a function PrintXY taking an integer n>=0 that prints all strings of length n containing only the characters 'X' and 'Y'? Can you do it without recursion, and without assuming n<=64? Is it readable?

      83 replies 29 retweets 366 likes
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    2. Tim Sweeney‏ @TimSweeneyEpic May 26
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      About two years into Apple BASIC programming, I was befuddled by this. I only knew loops and not recursion, so I wanted to write an outer loop that maintained a variable number of inner loops, but that wasn't supported:pic.twitter.com/2z6FsTnB6d

      3 replies 2 retweets 71 likes
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    3. Tim Sweeney‏ @TimSweeneyEpic May 26
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      Because BASIC supported GOSUB recursion only, without a parameter stack, I eventually abandoned it in frustration and wrote a compiler for a Pascal style language. The recursive solution is easy, but the question still stands: why are for-loops too weak to express this?

      5 replies 1 retweet 54 likes
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    4. Tim Sweeney‏ @TimSweeneyEpic May 26
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      A Haskell programmer will use some higher-order library functions to build a list comprehension over a variable number of permutations, but that's tricky and just moves the recursion to a library function.

      4 replies 1 retweet 40 likes
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    5. Tim Sweeney‏ @TimSweeneyEpic May 26
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      There's actually a deep answer to this: looping over just mutable loop variables isn't very powerful. A set of n simple nested loops of k iterations each can only explore k^n possibilities, and n must be static. We can't handle n being a variable.

      2 replies 1 retweet 43 likes
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    6. Tim Sweeney‏ @TimSweeneyEpic May 26
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      This is another area where the functional logic language family is more powerful. There, we can loop over multi-valued choices (like little forks whose scope is limited to the enclosing loop) and gather all of their results in sequence through backtracking.

      1 reply 1 retweet 37 likes
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      Tim Sweeney‏ @TimSweeneyEpic May 26
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      Here, let "a..b" represent the choice of all values in the range from a to b, and "for(a) do b" loop over all choices in a, producing an array containing the values of b. Then we can write a simple loop like:pic.twitter.com/yVYAUSLSR3

      6:34 PM - 26 May 2020
      • 2 Retweets
      • 31 Likes
      • Nick Gerace Caner Serbest 𝗿 𝗲 𝘁 𝗿 𝗼 The_RealTajgames gta onlin Mediocre Anelsy david doll ADK Aidan whisprr
      3 replies 2 retweets 31 likes
        1. New conversation
        2. Tim Sweeney‏ @TimSweeneyEpic May 26
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          New expressive power comes from our ability to nest choices inside of loops, where each iteration explores all choices. Then we can variably "exponentiate", and write PrintXY as:pic.twitter.com/loFkBD2RaC

          6 replies 4 retweets 41 likes
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        3. Tim Sweeney‏ @TimSweeneyEpic May 26
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          Is this a quirky, special-case language feature? Definitely not; it has well-defined semantics and increases the expressive power of loops. And it's what 14-year-old me intuitively tried to do in Apple BASIC.

          5 replies 0 retweets 35 likes
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        4. Tim Sweeney‏ @TimSweeneyEpic May 26
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          Tim Sweeney Retweeted Tikhon Jelvis

          There are many interesting solutions in this thread. The C and JavaScript solutions seen akin to mechanical devices that crank out solutions. One is an astonishingly short Haskell composition of library functions:https://twitter.com/tikhonjelvis/status/1265453175219183616?s=20 …

          Tim Sweeney added,

          Tikhon Jelvis @tikhonjelvis
          Replying to @TimSweeneyEpic
          printXY n = mapM_ print $ replicateM n "xy"
          3 replies 4 retweets 48 likes
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        5. Tim Sweeney‏ @TimSweeneyEpic May 26
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          There is generally a barrier to reading these solutions. In one case you have to run through the behavior of imperative code in your head to understand what it does. In another you have to understand the near magical incantation of pointfree function compositions.

          6 replies 0 retweets 44 likes
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        6. Tim Sweeney‏ @TimSweeneyEpic May 26
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          The more a solution can resemble a simple recitation of the problem to be solved, and rely on the compiler to generate the code, the better for ease of reading and writing code.

          10 replies 2 retweets 68 likes
          Show this thread
        7. End of conversation
        1. New conversation
        2. Ben Adams‏ @ben_a_adams May 26
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          Replying to @TimSweeneyEpic

          Is `a...b` inclusive or exclusive of `b`?

          1 reply 0 retweets 1 like
        3. Tim Sweeney‏ @TimSweeneyEpic May 26
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          Replying to @ben_a_adams

          It has to be inclusive if it's to be non-confusing and to work for any finite ordered values, such as 'A'..'Z'. Thus if used on loops, you'd write for(i=0..count-1).., which is ugly but fortunately uncommon if loops also support for(a:MyArray) iteration without indices.

          1 reply 0 retweets 1 like
        4. 5 more replies
        1. Longplay Games  🎮 🕹️‏ @Longplay_Games May 26
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          Replying to @TimSweeneyEpic

          You mean like the GCC-supported case range? https://gcc.gnu.org/onlinedocs/gcc/Case-Ranges.html … Those are very handy in some cases.

          0 replies 0 retweets 0 likes
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