4 ms·

Is there an example, in any language, showing how you would represent this at the type level? I can’t see how you would do it.

• lexicality 6y ago
  Typescript: type NormalisedFloat = number Admittedly it doesn't add any actual value checking, but it does convey the information when you look at the parameter definition.
• savanaly 6y ago
  In Elm (and many other languages, I assume, I'm just most familiar with Elm) there's a pattern called "opaque data type". [0] You make a file that contains the type and its constructor but you don't export the constructor. You only export the getter and setter methods. This ensures that if you properly police the methods of that one short file, everywhere else in your program that the type is used is guaranteed by the type system to have a number between zero and one. -- BetweenZeroAndOne.elm module BetweenZeroAndOne exposing (get, set) type BetweenZeroAndOne = BetweenZeroAndOne Float set : Float -> BetweenZeroAndOne set value = BetweenZeroAndOne (Basics.clamp 0.0 1.0 value) get : BetweenZeroAndOne -> Float get (BetweenZeroAndOne value) = value [0] https://en.wikipedia.org/wiki/Opaque_data_type#:~:text=In%20computer%20science%2C%20an%20opaque,access%20to%20the%20missing%20information https://en.wikipedia.org/wiki/Opaque_data_type#:~:text=In%20....
• hombre_fatal 6y ago
  You would just make the constructor return a possible error if it's not in range, or maybe some specialty constructors that may clamp it into range for you so they always succeed. It's the same question of, how can you convert a string to a Regexp type if not all strings are valid Regexps?
  • rco8786 6y ago
    Right, so there's no way to do this: > In other words, the parameter should not be a float, but a more constrained type that allows floats only in [0,1] It's a value check, not a type check.
• deleted 6y ago
  [deleted]