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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. Jason Swartz‏ @swartzrock 11 Jun 2020
      Replying to @propensive

      .... all amounts in bitcoin, no rounding. Done!

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

      I actually really like the idea of arbitrary-precision money values, independent of bitcoin.

      6:00 AM - 11 Jun 2020 from Krakow, Poland
      1 reply 0 retweets 0 likes
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        1. Jason Swartz‏ @swartzrock 11 Jun 2020
          Replying to @propensive

          Oh? I actually prefer stripe’s model, everything as integers (in cents).

          1 reply 0 retweets 0 likes
        2. Jon Pretty‏ @propensive 11 Jun 2020
          Replying to @swartzrock

          That doesn't fix the issue that a 20%-tax-inclusive amount of 119 cents shouldn't exist: there's no net amount which should produce it.

          1 reply 0 retweets 0 likes
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