constraints

问题 I have this code: extern crate serde; use serde::de::DeserializeOwned; use serde::Serialize; trait Bar<'a, T: 'a> where T: Serialize, &'a T: DeserializeOwned, { } I would like to write this using an associated type, because the type T is unimportant to the users of this type. I got this far: trait Bar { type T: Serialize; } I cannot figure out how to specify the other bound. Ultimately, I want to use a function like this: extern crate serde_json; fn test<I: Bar>(t: I::T) -> String { serde

问题 This question is related to another question I asked. In my other question I ask peoples opinions about 3 different ways I could construct a database. The cleanest way I can think of doing it without (practically) repeating tables and strange notions such as "super tables" is option 2: Location [Table] - Id - Name - HasLogger - LoggerRFID - LoggerUpperLimit - LoggerLowerLimit Sensor [Table] - Id [PK] - LocationId [FK] - UpperLimit - LowerLimit SensorReading [Table] - Id [PK] - SensorId [FK] -

问题 After some playing around F# member constraints feature and writing function like this: let inline parse< ^a when ^a : (static member Parse: string -> ^a) > s = (^a: (static member Parse: string -> ^a) s) That works perfectly fine: let xs = [ "123"; "456"; "999" ] |> List.map parse<int> I'm trying to write other func tryParse , that uses static method TryParse and wraps the parse result into 'a option type for better support in F#. Something like this doesn't compiles: let inline tryParse s =

问题 I am looking for how to implement unique constraints with NULL check. MySQL shouldn't allow multiple null value. Employee: id | name ---|----- 1 | null 2 | null -> should give error during inserting 2nd row. 回答1: No, MySQL is doing the right thing, according to the SQL-99 specification. https://mariadb.com/kb/en/sql-99/constraint_type-unique-constraint/ A UNIQUE Constraint makes it impossible to COMMIT any operation that would cause the unique key to contain any non-null duplicate values.

问题 I was wondering if it is possible to add multiple generic constraints? I have an Add method that takes an Object (Either Email, Phone or Address), so i was thinking something like: public void Add<T>(T Obj) where T : Address where T : Email where T : Phone { if (Obj is Address) m_Address.Add(Obj as Address); else if (Obj is Email) m_Email.Add(Obj as Email); else m_Phone.Add(Obj as Phone); } But I keep getting: "A constraint clause has already been specified for type parameter 'T'. All of the

问题 I'm making test application, so in my tableviewCell in storyboard i have imageView & webView (for show html-text). I set constraints like top/left/right/height=200 for imageView, spacing=5 between them & left/right/bot for webView, so i want to calculate my webView height programmatically and then change cell's heigt to stretch my webView. But i got this : Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want. Try

问题 I use Entity Framework to access my SQL data. I have some constraints in the database schema and I wonder how to handle exceptions that are caused by these constraints. As example, I get the following exception in a case where two users try to add an (almost) identical entity to the DB concurrently. System.Data.UpdateException "An error occurred while updating the entries. See the InnerException for details." (inner exception) System.Data.SqlClient.SqlException "Violation of UNIQUE KEY

问题 I need to implement an MVC site with urls per below: category1/product/1/wiki category1/product/2/wiki category1/sub-category2/product/3/wiki category1/sub-category2/sub-category3/product/4/wiki etc. etc. where the matching criteria is that the url ends with "wiki". Unfortunately the below catch-all works only in the last part of the url: routes.MapRoute("page1", // Route name "{*path}/wiki", // URL with parameters new { controller = "Wiki", action = "page", version = "" } // Parameter

问题 I have 60 tables. I want to drop 10 tables where these 10 tables are Constraints(PK,FK) to other 20 tables. While dropping these 10 tables, I need to truncate or delete data from the other 20 tables. Finally I want to disable all 60 table Constraints(FK,PK) and then enable all 60 table constraints after I am done with my work of adding/dropping tables. Is this possible? When I drop a table it is asking for FK. When I truncate those FK dependencies it also is still showing the same. I don't

问题 I'm looking at the MySQL docs here and trying to sort out the distinction between FOREIGN KEYs and CONSTRAINTs. I thought an FK was a constraint, but the docs seem to talk about them like they're separate things. The syntax for creating an FK is (in part)... [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name, ...) REFERENCES tbl_name (index_col_name,...) So the "CONSTRAINT" clause is optional. Why would you include it or not include it? If you leave it out does MySQL create a