优化F＃字符串操作

Question

I am just learning F# and have been converting a library of C# extension methods to F#. I am currently working on implementing a function called ConvertFirstLetterToUppercase based on the C# implementation below :

public static string ConvertFirstLetterToUppercase(this string value) {
    if (string.IsNullOrEmpty(value)) return value;
    if (value.Length == 1) return value.ToUpper();
    return value.Substring(0, 1).ToUpper() + value.Substring(1);
}

The F# implementation

[<System.Runtime.CompilerServices.ExtensionAttribute>]
module public StringHelper
    open System
    open System.Collections.Generic
    open System.Linq

    let ConvertHelper (x : char[]) =  
        match x with
            | [| |] | null -> ""
            | [| head; |] -> Char.ToUpper(head).ToString()
            | [| head; _ |] -> Char.ToUpper(head).ToString() + string(x.Skip(1).ToArray())

    [<System.Runtime.CompilerServices.ExtensionAttribute>]
    let ConvertFirstLetterToUppercase (_this : string) =
        match _this with
        | "" | null -> _this
        | _ -> ConvertHelper (_this.ToCharArray())

Can someone show me a more concise implementation utilizing more natural F# syntax?

Answer 1

open System

type System.String with
    member this.ConvertFirstLetterToUpperCase() =
        match this with
        | null -> null
        | "" -> ""
        | s -> s.[0..0].ToUpper() + s.[1..]

Usage:

> "juliet".ConvertFirstLetterToUpperCase();;
val it : string = "Juliet"

Answer 2

Something like this?

[<System.Runtime.CompilerServices.ExtensionAttribute>]
module public StringHelper = 
[<System.Runtime.CompilerServices.ExtensionAttribute>]
let ConvertFirstLetterToUppercase (t : string) =
    match t.ToCharArray() with
    | null -> t
    | [||] -> t
    | x -> x.[0] <- Char.ToUpper(x.[0]); System.String(x)

Answer 3

Try the following

[<System.Runtime.CompilerServices.ExtensionAttribute>]
module StringExtensions = 
    let ConvertFirstLetterToUpperCase (data:string) =
        match Seq.tryFind (fun _ -> true) data with
        | None -> data
        | Some(c) -> System.Char.ToUpper(c).ToString() + data.Substring(1)

The tryFind function will return the first element for which the lambda returns true . Since it always returns true it will simply return the first element or None . Once you've established there is at least one element you know data is not null and hence can call Substring

Answer 4

There's nothing wrong with using .NET library functions from a .NET language. Maybe a direct translation of your C# extension method is most appropriate, particularly for such a simple function. Although I'd be tempted to use the slicing syntax like Juliet does, just because it's cool.

open System
open System.Runtime.CompilerServices

[<Extension>]
module public StringHelper =

    [<Extension>]
    let ConvertFirstLetterToUpperCase(this:string) =
        if String.IsNullOrEmpty this then this
        elif this.Length = 1 then this.ToUpper()
        else this.[0..0].ToUpper() + this.[1..]