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How can I limit a number to a certain range in a struct/enum definition? (lemmy.world)

submitted 1 year ago by [email protected] to c/[email protected]

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Say I want to have a struct called Tile that will have a suit (let it be a string for simplicity's sake) and a number. This number can only exist in the range of 1-9.

In Nim, for example, you can do something like this:

type 
  Tile = object
    suit: string
    num: range[1..10]

var t = Tile(suit: "test", num: 9)
echo t.num # 9
t.num += 2 # OverflowDefect

How can I do such a thing in Rust? I can only think of one way: not allowing the number to be out of range in the "new" constructor and then adding auxiliary methods (add, sub, etc.) that do bounds checking. This solution seems too complex, though. There might be a way to do this using various range traits, but I can't seem to figure out how.

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[–] [email protected] 3 points 1 year ago (1 children)

You'll notice that new is not a keyword or any special rust thing. It's actually a software design pattern called a smart constructor, and validation is part of the intent. Pattern wise putting this into a smart constructor and encapsulating changes to this value is, I believe, idiomatic.

IIUC rust does not have a type system where you can give numeric bounds. Creating one would likely mean you'd need to define all the maths, which is likely not worth it.

[–] [email protected] 4 points 1 year ago* (last edited 1 year ago)

You reminded me of a deeply amusing story of solving the 4 queens problem with just the type system.

[–] [email protected] 1 points 1 year ago

You might be able to make num a newtype and make a custom implementation of the Add trait for it that checks that it is in bounds

[–] [email protected] 1 points 1 year ago

As far as I know, these are referred to as refinement/range/pattern types. There's some official discussion here (nothing in the language yet). There's a library called flux though that implements a form of it