f#

问题 I have seen plenty of examples on how to query the database but nothing on how to update records. Below is the simple code that I wrote to retrieve a table, but can someone explain me how can I modify a field, say lastActiveDate, and update the table on the database Thank you, suday open System open Microsoft.FSharp.Linq let connString = "Server=localhost;Database=myDb;Trusted_Connection=True;" let db = new MyDb(connString) db.Log <- System.Console.Out let res = Query.query <@ seq { for users

问题 The following code let inline foo< ^T, ^U when ^T : (member foo : (^U -> ^T) -> ^T -> ^T)> (f:(^U -> ^T)) (t:^T) : ^T = (^T : (member foo : (^U -> ^T) -> ^T -> ^T) f,t ) yields this error let inline foo< ^T, ^U when ^T : (member foo : (^U -> ^T) -> ^T -> ^T)> (f:^U) (t:^T) : ^T = (^T : (member foo : (^U -> ^T) -> ^T -> ^T) f,t );; -----------------------------------------------------------------------------------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ /stdin(45,96):

问题 Can anyone please explain why this results in an error: let xs = [| "Mary"; "Mungo"; "Midge" |] Array.iter printfn xs Whilst this does not: Array.iter printfn [| "Mary"; "Mungo"; "Midge" |] 回答1: The signature of printfn is Printf.TextWriterFormat<'a> -> 'a . The compiler infers literal values of strings as Printf.TextWriterFormat<unit> but cannot do so with dynamic strings. You can help the compiler in the first example by adding correct type annotation: let xs: Printf.TextWriterFormat<unit>

问题 Hi i'm a noob and asking this newbie question, please forgive me. I've installed successfully FSharpChart in my local directory ... Added package 'MSDN.FSharpChart.dll.0.60.0' to folder 'C:\Users\Fagui\Documents\GitHub\Learning Fsharp\Expert in F\packages' Added package 'MSDN.FSharpChart.dll.0.60.0' to 'packages.config' Successfully installed 'MSDN.FSharpChart.dll 0.60.0' to Expert in F now, if i do #load "FSharpChart.fsx";; ^^^^^^^^^^^^^^^^^^^^^^^ stdin(4,1): error FS0078: Unable to find the

问题 I'm accessing an F# discriminated union using C# and trying to use a switch statement over the union's cases. This works fine for values that have at least one field but not for empty values as these don't have a corresponding class generated, only a property. Consider the following F# discriminated union. type Letter = A of value:int | B of value:string | C | D In C# I have the following switch statement inside a function that has an argument letter of type Letter: switch (letter) { case A a

问题 I have the following problem: Within my type provider, I need to extend a generic type, that I have previously defined, with a method that will return an instance of this generic type. What I mean is, let's say we have : type receiveType<'a> = class val Next:int val Type:string new (state,stateType) = {Next = state;Type = stateType;} end later within my typeprovider I want to extend the type with the following function: [<TypeProvider>] type ProviderTest() as this= ( inherit

问题 I'm trying to use the FSharp.Data third party library but am getting an error The type 'XmlProvider' is not defined on the XmlProvider class. namespace KMyMoney open FSharp.Data module Read = let xml = File.ReadAllText("KMyMoneySampleFile.xml") type KMyMoneySource = XmlProvider<xml> I'm using NuGet to get the library. Library is 'FSharp.Data 1.1.8' When I type FSharp.Data. There are four options given: Csv , FreebaseOperators , Json , and RuntimeImplementation . Am I missing something? I'm

问题 I'm working through the Embedding F# Interactive example from here but like this post, I'm having an issue with the following line throwing an exception: let fsiSession = FsiEvaluationSession.Create(fsiConfig, allArgs, inStream, outStream, errStream) The exception thrown is: "System.Exception: 'Error creating evaluation session: StopProcessingExn None'" My project is being run from VS2017 Enterprise, setup as a simple F# console app, with the Target Framework as .NET Core 2.0 . The version of

问题 It is easy to call f:Func<'T, 'T> from F# as 'T -> 'T by using f.Invoke But how should I call f:Func<'T, 'T, 'T> from F# as 'T -> 'T -> 'T ? When I use f.Invoke I get 'T * 'T -> 'T , i.e. a tuple instead of two arguments. 回答1: Try: let g = f.Invoke |> FuncConvert.FuncFromTupled 回答2: Note that you do not need to turn the function into a function taking two arguments if you just want to call it. If you already know the arguments for the function, then you can just call it as a function taking a

问题 A common surprise for beginning F# programmers is the fact that the following is an incomplete match: let x, y = 5, 10 match something with | _ when x < y -> "Less than" | _ when x = y -> "Equal" | _ when x > y -> "Greater than" But I just encountered a situation that surprised me. Here's a small bit of sample code to demonstrate it: type Tree = | Leaf of int | Branch of Tree list let sapling = Branch [Leaf 1] // Small tree with one leaf let twoLeafTree = Branch [Leaf 1; Leaf 2] let describe