UPDATE: This answer was written over ten years ago; since then the type inference specification and implementation have been updated several times including changes to how constraints are used during inference. This answer should be considered of historical interest only; consult a recent copy of the C# specification to see how type inference works in … Read more
UPDATE c++17 introduced “P0127R2 Declaring non-type template parameters with auto“, allowing to declare a non-type template parameter(s) with auto as a placeholder for the actual type: template <auto P> struct Ptr {}; That is, P is a non-type template parameter. Its type can be inferred with decltype(P). auto in a template parameter list is subject … Read more
The T of sneakyThrow is inferred to be RuntimeException. This can be followed from the langauge spec on type inference (http://docs.oracle.com/javase/specs/jls/se8/html/jls-18.html) Firstly, there’s a note in section 18.1.3: A bound of the form throws α is purely informational: it directs resolution to optimize the instantiation of α so that, if possible, it is not a … Read more
It’s an open issue. Issue 6842: spec: Assigning to fields with short declaration notation
Type inference works based on an all or nothing principle. In the first case: fn({ a: 1 }) has no generic type parameters provided so it will infer both: B will be inferred as { a: number } based on the function argument; and A will be inferred as unknown since it’s not used anywhere … Read more
Building on Brian’s and Stephen’s answers, here’s some complete code: module NumericLiteralG = let inline FromZero() = LanguagePrimitives.GenericZero let inline FromOne() = LanguagePrimitives.GenericOne let inline FromInt32 (n:int) = let one : ^a = FromOne() let zero : ^a = FromZero() let n_incr = if n > 0 then 1 else -1 let g_incr = if … Read more
If you want to check against a specific type you can do the following: if let stringArray = obj as? [String] { // obj is a string array. Do something with stringArray } else { // obj is not a string array } You can use “as!” and that will throw a runtime error if … Read more
Q: Why doesn’t toSet do what I want? A: That would be too easy. Q: But why doesn’t this compile? List(1).toSet.map(x => …) A: The Scala compiler is unable to infer that x is an Int. Q: What, is it stupid? A: Well, List[A].toSet doesn’t return an immutable.Set[A]. It returns an immutable.Set[B] for some unknown … Read more
A few things are going on here. The first (minor) issue is that, with your Student interface, the compiler will not treat checking the isValid property as a type guard: const s = students[Math.random() < 0.5 ? 0 : 1]; if (s.isValid) { foo([s]); // error! // ~ // Type ‘Student’ is not assignable to … Read more
Use auto&: auto& ref_bigclass = GetBigClass(); References are supposed to be transparent: any operation on them happens on the object they refer to, there is no way to ‘get’ reference itself. UPD: This is covered in 7.1.6.4/6: Once the type of a declarator-id has been determined according to 8.3, the type of the declared variable … Read more