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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. ■ numerik‏ @d6 11 Jun 2020
      Replying to @propensive

      Need to use subtraction for one of the values based on the other. You could catch this with a law that adds them back up and ensures totals match.

      1 reply 0 retweets 2 likes
      Jon Pretty‏ @propensive 11 Jun 2020
      Replying to @d6

      Yeah, it's difficult to encode in a type, so property testing is your next best friend.

      9:43 AM - 11 Jun 2020 from Krakow, Poland
      0 replies 0 retweets 0 likes

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