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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. Jarek Ratajski‏ @jarek000000 11 Jun 2020
      Replying to @propensive

      I do not get it... Have you used `double` for monetary calculation? But that would not be subtle...

      2 replies 0 retweets 1 like
      Jon Pretty‏ @propensive 11 Jun 2020
      Replying to @jarek000000

      Certain whole numbers can't ever be the result of adding 20% to another whole number. If VAT were 100% instead of 20%, you couldn't have a VAT-inclusive price be odd, for example.

      6:42 AM - 11 Jun 2020 from Krakow, Poland
      1 reply 0 retweets 0 likes
        1. New conversation
        1. Jarek Ratajski‏ @jarek000000 11 Jun 2020
          Replying to @propensive

          Now I see, nice one. I would have fallen into this trap with 80% probability. There is a small chance I would think of assert / postcondition or a property base test.... a small chance.

          1 reply 0 retweets 1 like
        2. Jarek Ratajski‏ @jarek000000 11 Jun 2020
          Replying to @jarek000000 @propensive

          I see you think of making impossible state not representable - but since the prices were as they were - that was in fact the real state - not impossible one.

          0 replies 0 retweets 1 like
          3. End of conversation

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