According to
https://learn.microsoft.com/en-us/aspnet/core/web-api/action-return-types?view=aspnetcore-6.0#actionresultt-type
https://learn.microsoft.com/en-us/aspnet/core/web-api/action-return-types?view=aspnetcore-6.0#asynchronous-action-1
https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.actionresult-1?view=aspnetcore-6.0#operators
The following F# code should be legitimate:
[<HttpPost("post-data-2")>]
[<ProducesResponseType(StatusCodes.Status200OK)>]
[<ProducesResponseType(StatusCodes.Status500InternalServerError)>]
member this.PostData2(data: string): Task<ActionResult<int>> =
task {
try
return this.Ok(0)
with | x ->
return this.StatusCode(StatusCodes.Status500InternalServerError, -1)
}
Instead I get two compilation errors in the two 'return' lines
Error FS0193 Type constraint mismatch. The type
'OkObjectResult' is not compatible with type
'ActionResult'
and
Error FS0193 Type constraint mismatch. The type
'ObjectResult' is not compatible with type
'ActionResult'
This works however:
[<HttpPost("post-data-1")>]
[<ProducesResponseType(StatusCodes.Status200OK)>]
[<ProducesResponseType(StatusCodes.Status500InternalServerError)>]
member this.PostData1(data: string): Task<ActionResult<int>> =
task {
try
return ActionResult<int>(this.Ok(0))
with | x ->
return ActionResult<int>(this.StatusCode(StatusCodes.Status500InternalServerError, -1))
}
Why are the implicit cast operators not recognized by F#?
Not sure there can be much of an answer besides "because the language designers decided for it to be that way". Implicit conversions are only used in a narrow set of circumstances in F#.
In addition to calling the constructor as you have, you should also be able to explicitly call ActionResult.op_Implicit or define you own implicit conversion operator.
Related
Is there any difference between using an implicit cast to cast in dart vs the 'as' keyword? Will they result in the same (or similar) runtime error if the type is not as expected?
For example:
dynamic foo = "blah";
String boo = foo; // is this
String boo2 = foo as String; // the same as this?
No. And yes.
TL;DR: Don't worry about the difference, just do what reads the best.
If your program is correct, and the casts will succeed, then there is unlikely to be any difference.
When inferring types, String boo = foo; will infer the type of foo with a context type of String. If the resulting static type of foo then turns out to be dynamic then it implies an implicit downcast from dynamic to `String.
For String boo = foo as String;, the static type of foo is inferred with no context type. No matter what the resulting static type is, it will be cast to String at run-time.
You can see a difference between these two if you have a more complicated expression than just the variable foo:
T first<T extends dynamic>(List<T> list) => list.first;
String boo = first([1]); // <- compile-time error
String boo2 = first([1]) as String;
With this example, you get a compile-time error in the boo line because the compiler knows that the list should be a List<String>. There is no error in the boo2 line because the list only needs to be a List<dynamic>, and whatever first returns is then dynamically cast to String.
A more contrived example would be:
T firstOrDefault<T extends dynamic>(List<T> list) {
if (list.isEmpty) {
// Invent some default values for known types.
if (null is T) return null as T;
if (0 is T) return 0 as T;
if (0.0 is T) return 0.0 as T;
if ("" is T) return "" as T;
if (false is T) return false as T;
throw UnsupportedError("No default value for the needed type");
}
return list.first;
}
String boo = firstOrDefault([]); // <- returns "", with null safety.
String boo2 = firstOrDefault([]) as String; // <- returns null, throws.
(Doing that kind of type-parameter specialization is not a recommended programming style. It's too fragile precisely because it can be affected in unpredictable ways by subtle changes to static types.).
Ignoring inference and static checking, there is not much difference at run-time. If foo is just a simple expression with static type dynamic, then the language requires downcast to String in both situations.
However, the Dart2JS web compiler can enable unsound optimizations which basically omit implicit downcasts entirely (as an "optimization" assume that they would have succeeded) and the go on with potentially type-unsound values flowing around.
For that reason, some people (mainly those coding for the web) may prefer to use implicit downcasts over explicit downcasts.
Dart with null safety only has implicit downcasts from dynamic.
You can always force an implicit downcast from any type by doing:
String boo3 = foo as dynamic;
The as dynamic is a free up-cast, it has no effect at run-time (it can't fail and the compiler knows that), so all it does is change the static type of the expression ... to something which introduces an implicit downcast, which the dart2js compiler will then (unsoundly) ignore as well.
(Use with caution, as with everything involving dynamic. Also, the analyzer might warn about an "unnecessary up-cast".)
I am trying to add a F#-style interface to a type, that has a byref return method.
Here's the code:
type IPool<'P, 'T when 'T: struct> =
abstract member GetReference: ITypedPointer<'P, 'T> -> byref<'T>
let Ref<'TPool, 'P, 'T when 'TPool :> IPool<'P, 'T>> (pool: 'TPool) pointer =
pool.GetReference pointer
Now to my surprise, a similar thing worked fine until I introduced IPool interface. Before that Ref itself contained an implementation like &pool.data.[idx], and worked fine.
I tried installing nightly build of F# Tools, cause latest release does not officially support byref returns, and PR to introduce them was recently completed: https://github.com/Microsoft/visualfsharp/pull/4888
However, I still get error FS3209: The address of the variable 'copyOfStruct' cannot be used at this point. A method or function may not return the address of this local value. in Visual Studio. Type outref<T> still does not seem to be available either. Am I missing something?
I also tried to drop the pointer parameter, and just return pool.GetReference to only get a different error message.
Addition: the ultimate goal is to be able to do
let aref = Ref pool ptr
let bref = Ref pool ptr
aref <- 42
assert(aref = bref)
e.g. give caller a direct reference to an internal memory, usually backed by an array, similar to Span<T>. I am making this for performance reasons, so it is not OK to allocate on every call to Ref.
For some reason, reducing generalization helped to get rid of the error:
let Ref<'P, 'T when 'T: struct> (pool: IPool<'P, 'T>) pointer = pool.GetReference pointer
Solution provided by
https://github.com/Microsoft/visualfsharp/issues/5366#issuecomment-407521220
Though it does not explain why the original code does not compile.
I don't think it's standard practice to return a byref type. This type is really meant for method parameters, mostly for C# interop with out or ref parameters. Take a look at this StackOverflow question for a good explanation.
What you can do is change the method on your interface to take a tuple of ITypedPointer<'P,'T> and byref<'T> (usage of byref is not allowed with curried parameters) and return unit instead. Then you can call GetReference like any standard .NET method with an out parameter in C#. That would look something like this:
type ITypedPointer<'P, 'T> = interface end
type IPool<'P, 'T when 'T: struct> =
abstract member GetReference: ITypedPointer<'P, 'T> * byref<'T> -> unit
let Ref<'TPool, 'P, 'T when 'TPool :> IPool<'P, 'T>> (pool: 'TPool) pointer =
let mutable value = Unchecked.defaultof<'T>
pool.GetReference(pointer, &value)
value
The following yields
This construct causes code to be less generic than indicated by the type annotations. The type variable 'P has been constrained to be type 'bool'.
for the right side of the let myValue = expression, and
This code is less generic than required by its annotations because the explicit type variable 'P' could not be generalized. It was constrained to be 'bool'.
for the generic <'P> in the Value method:
type MyTypeA<'T> (myObject : 'T) as this =
let myValue = this.Value<bool> "SomeBooleanProperty"
member this.Value<'P> name = typeof<'T>.GetProperty(name, typeof<'P>).GetValue(myObject, null) :?> 'P`
However, this compiles just fine and yields no warnings or errors:
type MyTypeB<'T> (myObject : 'T) as this =
member this.Value<'P> name = typeof<'T>.GetProperty(name, typeof<'P>).GetValue(myObject, null) :?> 'P
member this.Method<'P> name = this.Value<'P> name
What's going on, here? Why, in the first example, is the method recognized in the assignment of the private value, but not as a legitimately generic method?
The warning (FS0064) is raised by a call to CheckWarnIfRigid function inside SolveTyparEqualsTyp fun from ConstraintSolver.fs.
After the warning is raised, SolveTyparEqualsTyp will continue (since there is no error so far) to solve type constraints.
The comment of SolveTyparEqualsTyp is :
/// Add the constraint "ty1 = ty" to the constraint problem, where ty1 is a type variable.
/// Propagate all effects of adding this constraint, e.g. to solve other variables
This leads to error FS0663 for member Value definition in OP's example. Followed by error FS0660.
For some reason I ignore, some propagation occurs.
Maybe type inference is too aggressively performed.
#jpe and other comments below OP's question contain more interesting clues.
Good day,
I have problem. I want to simulate some errors in hacklang.
<?hh
namespace Exsys\HHVM;
class HHVMFacade{
private $vector = Vector {1,2,3};
public function echoProduct() : Vector<string>{
return $this->vector;
}
public function test(Vector<string> $vector) : void{
var_dump($vector);
}
}
Function echoProduct() returns Vector of strings. But private property $vector is Vector of integers. When I call echoFunction and returning value use as argument for function test(). I get
object(HH\Vector)#35357 (3) { [0]=> int(1) [1]=> int(2) [2]=> int(3) }
Why? I am expecting some error because types mismatch.
There's two things at play here:
Generics aren't reified, so the runtime has no information about them. This means the runtime is only checking that you're returning a Vector.
$this->vector itself isn't typed. This means the type checker (hh_client) treats it as a unknown type. Unknown types match against everything, so there's no problem returning an unknown type where a Vector<string> is expected.
This is to allow you to gradually type your code. Whenever a type isn't known, the type checker just assumes that the developer knows what's happening.
The first thing I'd do is change the file from partial mode to strict mode, which simply involves changing from <?hh to <?hh // strict. This causes the type checker to complain about any missing type information (as well as a couple of other things, like no superglobals and you can't call non-Hack code).
This produces the error:
test.hh:6:13,19: Please add a type hint (Naming[2001])
If you then type $vector as Vector<int> (private Vector<int> $vector), hh_client then produces:
test.hh:9:16,28: Invalid return type (Typing[4110])
test.hh:8:44,49: This is a string
test.hh:6:20,22: It is incompatible with an int
test.hh:8:44,49: Considering that this type argument is invariant with respect to Vector
Which is the error you expected. You can also get this error simply by adding the type to $vector, without switching to strict mode, though I prefer to write my Hack in the strongest mode that the code supports.
With more recent versions of HHVM, the type checker is called whenever Hack code is run (there's an INI flag to turn this off), so causing the type mismatch will also cause execution of the code to fail.
I am wondering why F# compiler allows the following
type MyMath() =
member this.Add a b = a + b
What would be the type of Add method and its arguments ? If I compiled this into the Library and try to use it in C# what types of arguments it is going to expect ?
Shouldn't the F# require you to explicitly specify types when it comes to Methods of classes ?
You can enter the code in F# interactive and see the inferred type yourself:
> type MyMath() =
member this.Add a b = a + b;;
type MyMath =
class
new : unit -> MyMath
member Add : a:int -> b:int -> int
end
Here, the compiler uses default type for the + operator which is int. The operator can be used with other types, but the inference uses int as the default. You can use type annotations, but you are not required to do that if you are happy with the inferred type.
In general, you can use type annotations in F# to specify types if you want to, but in many cases, the inferred type will be exactly what you want, so you do not have to make the code more verbose, if the inference behaves as expected.
Of course, if you were writing some library and wanted to be super careful about changing the API, then you might want to use type annotations (or you can add F# Interface file .fsi)
F# Interactive is your friend:
type MyMath =
class
new : unit -> MyMath
member Add : a:int -> b:int -> int
end