re Haskell: I just looked at a few arbitrary files in arbitrary Haskell projects on Github, and I retract that part. I was either thinking of another language, or had seen a lot of bad Haskell code before that I cannot find now.
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Isn't this just the nullability thing again that most people have decided was a bad idea, but with a different name? Instead of two ints, now your function takes two maybe-ints and crashes at runtime if they're not there.
I do think the thing you're asking for (interfaces but for fields instead of functions) is a missing feature in most languages. I understand why it's there, but it's still annoying.
The most well-known language I can think of that explicitly supports it in an otherwise static type system is TypeScript, where a type can be "object containing non-nullable ints called start and end, but with no constraints on other fields that may be there"[0]. It's technically a hash map at runtime, but it's a compile-time type-checked hash map.
In other languages, you could bodge it with a bunch of Java-esque getX/setX functions. I don't blame people for not doing so.
> Instead of two ints, now your function takes two maybe-ints and crashes at runtime if they're not there.
The idea in Haskell (and many more, I'm sure), is that if you take a maybe-int, you can't get the int without unpacking it (or using a function that only operates on the int if its there), so then when you pass the unpacked int around, you are 100% sure it's there at all times. No worrying about a JS-style undefined, or some other function altering the value to become null again. It's a guarantee, which is a huge part of why all this FP stuff is so great.
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Isn't this just the nullability thing again that most people have decided was a bad idea, but with a different name? Instead of two ints, now your function takes two maybe-ints and crashes at runtime if they're not there.
I do think the thing you're asking for (interfaces but for fields instead of functions) is a missing feature in most languages. I understand why it's there, but it's still annoying.
The most well-known language I can think of that explicitly supports it in an otherwise static type system is TypeScript, where a type can be "object containing non-nullable ints called start and end, but with no constraints on other fields that may be there"[0]. It's technically a hash map at runtime, but it's a compile-time type-checked hash map.
In other languages, you could bodge it with a bunch of Java-esque getX/setX functions. I don't blame people for not doing so.
[0] https://www.typescriptlang.org/docs/handbook/2/objects.html