data Maybe a = Nothing | Just a
deriving (Eq, Ord)
>> Because then you would have to modify all the code which already uses the function, as "A" and "Maybe A" are incompatible types. Which is illogical.
I'm not so sure about your statement. If the type of the function changes, revising its usage at every use point is good for your sanity. I would go further and say that sometimes Maybe X is too weak, because it doesn't contain precise semantics for its Nothing and Just x alternatives. For example, sometimes you want a `Nothing` for a value that hasn't yet been found, but could potentially be filled up the evaluation chain, e.g. `NothingYet`, and a different Nothing for a value that is conclusively not there, e.g. `TerrifyingVoid`. If you fork your `Nothing` value into these two variants after you discover the need for it, you will have to revise each call anyway, and check what's the proper course of action. And this is a feature I wish I could use from Python.
More generally, in large Haskell code bases, having the type-checker help you track, at compile time, code that breaks far away from where you made your changes, is an incredible time-saver.
Yes, there are edge cases where you would like to have multiple different "kinds of null", but those use cases seem so uncommon in practice that they are mostly irrelevant.
>> Because then you would have to modify all the code which already uses the function, as "A" and "Maybe A" are incompatible types. Which is illogical.
I'm not so sure about your statement. If the type of the function changes, revising its usage at every use point is good for your sanity. I would go further and say that sometimes Maybe X is too weak, because it doesn't contain precise semantics for its Nothing and Just x alternatives. For example, sometimes you want a `Nothing` for a value that hasn't yet been found, but could potentially be filled up the evaluation chain, e.g. `NothingYet`, and a different Nothing for a value that is conclusively not there, e.g. `TerrifyingVoid`. If you fork your `Nothing` value into these two variants after you discover the need for it, you will have to revise each call anyway, and check what's the proper course of action. And this is a feature I wish I could use from Python.
More generally, in large Haskell code bases, having the type-checker help you track, at compile time, code that breaks far away from where you made your changes, is an incredible time-saver.