Haskell/GHC tells you what the types are. Proper, global, most-general type inference. Not that local inference crap [1] that the sour-grapes types will say is better.
You lose this ability if you start letting the compiler decide that `Int` is as good as `Maybe Int`. Or if an `Async (Async String)` may as well be an `Async String`.
That's not to say it's not easy to transform (just a few keystrokes), but explicit beats implicit when it comes to casting (in any language).
[1] Does this work?
var x = 1 == 2 ? Optional.of(2L) : Optional.empty().map(y -> y + y);
// Operator '+' cannot be applied to 'java. lang. Object', 'java. lang. Object'
How about
Optional<Long> x = 1 == 2 ? Optional.of(2L) : Optional.empty().map(y -> y + y);
// Operator '+' cannot be applied to 'java. lang. Object', 'java. lang. Object'
or even
Optional<Long> x = 1 == 2 ? Optional.of(2L) : Optional.empty().map((Long y) -> y + y);
// Cannot infer functional interface type
No, we needed Optional.<Long>empty() instead of Optional.empty() to make it work
> letting the compiler decide that `Int` is as good as `Maybe Int`
I was thinking more like explicitly telling the compiler that an implicit cast is OK, in other languages done by implementing the implicit cast operator for example.
edit: but if I understood you correctly, Haskell just doesn't support any implicit casting?
No, the casting is still done implicitly. That is I can make the following compile fine in Delphi if I add an implicit cast operator to either Foo or Bar:
Foo x := Foo.Create();
Bar y := x;
If neither of them have a suitable implicit cast operator defined, it will of course fail to compile.
Just an example, nothing unique about Delphi. You can see an example of the operator definition here[1].
It can using pure or return or if working with just Maybe specifically then Maybe is defined like so:
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.
Sounds similar to how you need to do Some(x) when passing x to something expecting an Option in rust.
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
Compiler already knows what "one" is, it could insert the "Just" itself, no? Possibly due to an operator defined on Maybe that does this transformation?
That is, are there some technical reasons it doesn't?
Better question: Why would you want your call site code to break when your type signature gets changed in a way that doesn't necessitate breaking anything?
Because what you're asking for precludes the concept of mathematical guarantees. I'm not taking your question at face value, because you could be asking why call site code should break when the type signature generalises (which is a useful thing), but that's not what you're asking.
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.
Because it otherwise forces the caller to have an extra explicit step that doesn't really contribute to anything. It's a trivial transform, and as such just gets in the way of what the code actually does.
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.
A good reason to use Haskell is that it generally guides the programmer away from doing things like this.
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 just know that my `Int` should be a `Maybe Int` and cast it for me.
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.
But now you’re talking about something else aren’t you? Now you’re talking about generalising. You can generalise, for example, from Float to Floating a => a. But I don’t understand how the original Int to Maybe Int change could be sensible. How does that work?
