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propensive's profile
Jon Pretty
Jon Pretty
Jon Pretty
@propensive

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Jon Pretty

@propensive

Supporting Scala through professional training and open-source software. Responsible for Magnolia, Fury, Scala World and Functional Africa.

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propensive.com
Joined July 2010

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    1. Jon Pretty‏ @propensive 11 Jun 2020

      Sometimes the simplest problems are harder than they first seem. A client once specified a requirement to me: we'll give you the VAT-inclusive (20%) price; you need to generate invoices showing the net and VAT amounts. Sounds easy, right? net = round(tot*5/6) vat = round(tot/6)

      3 replies 0 retweets 6 likes
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    2. Jon Pretty‏ @propensive 11 Jun 2020

      Some time passed. The client came back to me: "You've been calculating the invoices wrong! Our accountant has had to work an extra few days because about one in five of your invoices has an error of £0.01, and our accounts don't balance!"

      1 reply 0 retweets 3 likes
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    3. Jon Pretty‏ @propensive 11 Jun 2020

      Did you spot my error? So, what's the moral of this story? I'm not sure, because it's quite subtle, but I could claim that it's a case of "make impossible states unrepresentable". The challenge was in recognising that we were representing impossible states. Suggestions welcome.

      10 replies 0 retweets 3 likes
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    4. John Azariah #BlackLivesMatter‏ @johnazariah 11 Jun 2020
      Replying to @propensive

      AHHHH - (sub-)cent/penny rounding has different rules in accounting than it does in maths :) The accounting domain quite literally has its own algebra. We spent a lot of time testing these cases out and getting it right when I worked at @MYOB! Add in currency, and *phoar!*pic.twitter.com/Wsgn7HIwEv

      1 reply 0 retweets 3 likes
      Jon Pretty‏ @propensive 11 Jun 2020
      Replying to @johnazariah @MYOB

      It's an arithmetic problem too, though... Imagine every whole value from 1 to 100, and add 20% to each. You now have 100 different numbers in the range 1-120. 20 numbers in that interval don't occur. Pick one. What whole number, when you add 20% to it, produces your number? None!

      8:52 AM - 11 Jun 2020 from Krakow, Poland
      1 reply 0 retweets 0 likes
        1. New conversation
        2. John Azariah #BlackLivesMatter‏ @johnazariah 11 Jun 2020
          Replying to @propensive @MYOB

          That's true - the problem is actually exacerbated mathematically because operations aren't over Z or R, but over some arbitrary degree of fixed-point precision. (It's arbitrary because currency-exchange rates, unit-pricing and all that). And that's where the difficulty creeps in!

          2 replies 0 retweets 1 like
        3. Jon Pretty‏ @propensive 11 Jun 2020
          Replying to @johnazariah @MYOB

          I think I'm beginning to imagine the amount of work you must have put in at @MYOB...

          1 reply 0 retweets 0 likes
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