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All the Perl that's Practical to Extract and Report

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  • First, I appreciate the schadenfreude here as much as the next hacker, but it appears this bug was in the Freescale code for the platform, not the code Microsoft wrote for the Zune. So it effects more than just Zunes...

    Second, the "some type inference systems" is misleading. The one type inference system that Andrew Koenig and MJD discuss is the Hindley-Milner [] type system, which is used in ML and its descendants (OCaml, F#, Haskell, etc.). Koenig's example demonstrated that a recursive algorithm would

    • Thanks for the extra information.

      I think, ultimately, that part of the problem here could be alleviated by coupling things which must be coupled. For example, having a DayOfYear type with a type coercion system could partially help. Imagine the following type definition (in Perl 6, which doesn't do inferencing, of course, but bear with me):

      subset DayOfYear of Int where { 0 <= $_ <= 366 };

      Obviously, that still leaves edge cases, but I would imagine with inferencing, while you wouldn't get a compile

      • All of this, of course, if from a typing newbie, so if I'm talking complete bollocks, feel free to correct me!

        You're talking complete bollocks. :-)

        The kinds of things a strong static typing system can do is let you create types like DayOfYear, which let you specify a single calendar date, and prevents such nonsense like the 47th of April as being a "date". From there, you can create functions like addDays that take a DayOfYear and an integer number of days and return a new DayOfYear.

        What this code was doing was taking a count of days since the beginning of time, and figuring out the current date by subtracting one year's worth of days at a time, and producing the current year and the current count of days since the start of the year. If we were dealing with a calendar that didn't have leap days, it would be a simple matter of calculating day_count div days_in_year and day_count mod days_in_year. But date math is more complex than that, so...

        What type checking cannot do is infer specific magic properties about a simple counter used in a specific context. Adding and subtracting numbers is always just adding and subtracting numbers. If you want a type checker to help you out (in Java or in Haskell), then you need to define a new type with new behaviors that more closely matches the domain, whether you're talking about date math, tax math or statistical probabilities.