Swift interestingly doesn’t require this, but only because Optionals are granted lots of extra syntax sugar in the language. It’s really wrapping it in .some(x) for you behind the scenes, but the compiler can figure this out on its own.
This means that in swift, changing a function from f(T) to f(T?) (ie. f(Optional<T>)) is a source-compatible change, albeit not an ABI-compatible one.
I'd call that explicit casting. Implicit casting would be
mOne = one
That is, are there some technical reasons it doesn't?
Or is it just (no pun inteded) a language choice?
It seems you're asking for code to be both null safe and not null safe simultaneously.
Having a language just decide that it would like to change the types of the values flowing through a system is wild. It's one of the reasons that JavaScript is a trash fire.
Of course with great power comes great responsibility, so it's a tool that should be used sparingly and deliberately.
Now as mentioned I don't use Haskell, but that's why I like it in other languages.
I asked as I was curious if there was something that prevented this in Haskell, beyond a design choice.
If you make a breaking change to your API, then you should want your tools to tell you loud and clear that it’s a breaking change.
I also don’t agree that keeping the structure around values internally logically consistent “doesn’t really contribute to anything”. On the contrary, I think this idea is hugely important. How would your idea generalise? The compiler should just know that my `Int` should be a `Maybe Int` and cast it for me. Should the compiler also know that my `[a]` should be cast to a `(a, b)` because incidentally we’re fairly confident that list should always have two elements in it?
I think if this way of thinking is unfamiliar, then it’s a good reason to learn Haskell (or Elm, which is at least as good, or maybe better, for driving this point home).
The compiler should not "just" know it. It would know it because we told it how.
Consider a function that takes a float and returns a complex number. I then change the function to take a complex number ("Complex Float" in Haskell if I read the docs right), and returns a float.
I could then tell the compiler, by implementing an implicit cast operator, how to cast float to complex. The implicit part is then that the compiler tries it without me telling it to use the cast explicitly.
Then any code that worked with the old function should work perfectly fine using the modified function without modifications, since per definition the reals are contained in the complex numbers.
This is how I do it in several languages I've used.
Again, I don't know Haskell, so from the outside it looks like much the same as the float -> complex conversion.
data Maybe a = Just a | Nothing
So to make an X a Maybe X, you'd put a Just before a value of type X.
For example:
one :: Int
one = 1
mOne :: Maybe Int
mOne = Just one -- alternatively, pure one since our type signature tells us what pure should resolve to.
Reason we can do this is because Maybe is also an Applicative and a Monad and so implements pure and return which takes an ordinary value and wraps it up into an instance of what we want.