f#

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'm attempting to set up a low-level keyboard hook using P/Invoke in an F# application. The Win32 function SetWindowsHookEx takes a HOOKPROC for its second argument, which I've represented as a delegate of (int * IntPtr * IntPtr) -> IntPtr , similar to how this would be handled in C#. When calling the method, I get a MarshalDirectiveException stating that the delegate parameter cannot be marshaled because Generic types cannot be marshaled I'm not sure how generics are involved, as all types

问题 I'm trying to implement a tree in F# using a list of tuples. [a] where a = (string, [a]) Each node has a list of their children and leaf nodes would be (name, []) I want to be able to recursively iterate through each level of the list like this. a b e c d f g They wont always be binary trees however. let t2 = [("a", [("b", [("c", []), ("d", [])]), ("e", [("f", []), ("g", [])])])] let rec checkstuff tple = match tple with | (_, []) -> true | (node, children) -> List.fold ( || ) false (List.map

问题 Is there an F# equivalent to eval? My intent is to have my app load a small code sample from a file and essentially let file = "c:\mysample" let sample = loadFromFile file let results = eval(sample) I am new to F# and trying to figure out some of the limitations before I apply it to a project. Thank you 回答1: There is no function that would allow you to do this directly. However, when you want to compile an F# source file programatically, you can invoke the F# compiler from your application.

问题 I have a need to capture the input and output of F# functions when using F# Interactive. I am able to get NLog to work just fine when the program is run under Visual Studio using F5 or Ctrl-F5. Also the same methods that contain statements to output to the log work just fine and are called when invoked via F# Interactive; just nothing in the log file. I also tried the following with F# Interactive to setup references to NLog and still nothing in the log when run from F# Interactive. #I @"..

问题 I'd like to write some code that runs a sequence of F# scripts (.fsx). The thing is that I could have literally hundreds of scripts and if I do that: let shellExecute program args = let startInfo = new ProcessStartInfo() do startInfo.FileName <- program do startInfo.Arguments <- args do startInfo.UseShellExecute <- true do startInfo.WindowStyle <- ProcessWindowStyle.Hidden //do printfn "%s" startInfo.Arguments let proc = Process.Start(startInfo) () scripts |> Seq.iter (shellExecute "fsi") it

问题 Is there a way to define your own operators in F#? If so can someone give me an example for this? I searched briefly, but couldn't find anything. 回答1: Yes: let (+.) x s = [for y in s -> x + y] let s = 1 +. [2;3;4] The characters that can be used in an F# operator are listed in the docs. They are !%&*+-./<=>@^|~ and for any character after the first, ? . Precedence and fixity are determined by the first character of the operator (see the spec). You can create your own let-bound operators as I

问题 F# is giving me some trouble with its type inference rules. I'm writing a simple computation builder but can't get my generic type variable constraints right. The code that I would want looks as follows in C# : class FinallyBuilder<TZ> { readonly Action<TZ> finallyAction; public FinallyBuilder(Action<TZ> finallyAction) { this.finallyAction = finallyAction; } public TB Bind<TA, TB>(TA x, Func<TA, TB> cont) where TA : TZ { // ^^^^^^^^^^^^^ try // this is what gives me a headache { // in the F#

问题 StringBuiler is a mutable object, F# encourages employing immutability as much as possible. So one should use transformation rather than mutation. Does this apply to StringBuilder when it comes to building a string in F#? Is there an F# immutable alternative to it? If so, is this alternative as efficient? A snippet 回答1: I think that using StringBuilder in F# is perfectly fine - the fact that sb.Append returns the current instance of StringBuilder means that it can be easily used with the fold

问题 I've used the Scrap Your Boilerplate and Uniplate libraries in the Haskell programming language, and I would find that form of generic programming over discriminated unions to be really useful. Is there an equivalent library in the f# programming language? 回答1: Not that I know of; without support built-in to the language/compiler, I expect the only alternative is a reflection-based version. (I don't know how Uniplate is implemented - do you?) Here's the code for a reflection-based version