F # spoiled me or why I don’t want to write in C # anymore

C#

, , , , . Python Javascript ( ), Java , , value-, - Integer, . — C# .

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F#.

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, :

• , , .
• -, Discriminated Unions .
• Computation Expressions
• SRTP ( -)
• null, .
• type inference

null , , Task<IEnumerable<Employee>>. .

, POCO :

public class Employee
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public string Phone { get; set; }
    public bool HasAccessToSomething { get; set; }
    public bool HasAccessToSomethinElse { get; set; }
}

, , . , ?

F# :

type Employee =
{ Id: Guid
  Name: string
  Email: string
  Phone: string
  HasAccessToSomething: bool
  HasAccessToSomethingElse: bool }

. , , . C# public { get; set; }. , F# null.

, , C# , public :

public class Employee
{
    public Guid Id { get; }
    public string Name { get; }
    public string Email { get; }
    public string Phone { get; }
    public bool HasAccessToSomething { get; }
    public bool HasAccessToSomethinElse { get; }

    public Employee(Guid id, string name, string email, string phone, bool hasAccessToSmth, bool hasAccessToSmthElse)
    {
        Id = id;
        Name = name;
        Email = email;
        Phone = phone;
        HasAccessToSomething = hasAccessToSmth;
        HasAccessToSomethinElse = hasAccessToSmthElse;
    }
}

! , 3 : .

, , / , .

F# . .

:

let employee =
    { Id = Guid.NewGuid()
      Name = "Peter"
      Email = "peter@gmail.com"
      Phone = "8(800)555-35-35"
      HasAccessToSomething = true
      HasAccessToSomethinElse = false}

, . , — . , ? :

let employee2 = { employee with Name = "Valera" }

C#? , .

, { get; } — . ?

?

-. - , , true. , false. ? ? , - ? , .

— , , .

:

• , — ,
• ,
• / -,

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  , , .

  
  
  

  C# , , .

  
  
  

, . ? F# :

• Structural Equality
• Structural Comparison

:

if employee1 = employee2 then
//...

. Equals , Object.ReferenceEquals, .

- , , , Equals & GetHashCode , . , - — , . , : , HashSet<> & SortedSet<>, ( , , ), .

Discriminated Unions

, , . , try { i = Convert.ToInt32("4"); } catch()... int.TryParse.

, . ? ValidationException. ? IndexOutOfRangeException!

, , , - . — OutOfMemoryException, StackOverflowException, AccessViolationException .. — ? ? Int32, 2 32 . , 10000. . Int32, , , , "" !

— . .

, , : , " , , , ". ( ), string ErrorMessage & bool IsSuccess. C# — , .

-,

public class Result<TResult, TError>
{
    public bool IsOk { get; set; }
    public TResult Result { get; set; }
    public TError Error { get; set; }
}

, , , . , , IsOk, , .

F# :

type Result<'TResult, 'TError> =
    | Ok of 'TResult
    | Error of 'TError

type ValidationResult<'TInput> =
    | Valid of 'TInput
    | Invalid of string list

let validateAndExecute input =
    match validate input with //    
    | Valid input -> Ok (execute input) //   -  ""  
    | Invalid of messages -> Error messages //  ,     

, , , . xml doc, - , try/catch . — , .

, . BusinessException ApiException, , , , , - , , , 404 403 500. , .

F# , match . , DU. DU , :

type UserCreationResult =
    | UserCreated of id:Guid
    | InvalidChars of errorMessage:string
    | AgreeToTermsRequired
    | EmailRequired
    | AlreadyExists

, . AgreeToTermsRequired , , .

, ( ). . , , , , .

, if/else :

let doSmth myArray index =
    match Array.tryItem index myArray with
    | Some elem -> Console.WriteLine(elem)
    | None -> ()

Option:

type Option<'T> =
    | Some of 'T
    | None

, , , , - . , .

expression-based .

:

• — . .
• .

( ) , .

Expression-based design , , . :

let a = if someCondition then 1 else 2

, if, else.

C# :

int a = 0;
if(someCondition)
{
    a = 1;
}
else
{
    a = 2;
}

, a , , .

, — I/O, . . - , .

: , . , , . DI , - .

, , , 2 5, , 3-5 , , . 1-2 (?) , . , . — - . , , , . , . , , — - , . - , . . . . , . , , .

: - 1 , , , . , DI , . , , , . , , . , , , .

-. — , . , , , . , , , , . , ? , !

:

let getReport queryData =
    use connection = getConnection()
    queryData
    |> DataRepository.get connection //       ,    
    //         lifestyle'    
    |> Report.build

, |> , :

let gerReport queryData =
    use connection = getConnection()
    Report.build(DataRepository.get connection queryData)

C#:

public ReportModel GetReport(QueryData queryData)
{
    using(var connection = GetConnection())
    {
        // Report  --  .    F# 
        return Report.Build(DataRepository.Get(connection, queryData));
    }
}

, :

let getReport qyertData =
    use connection = getConnection()
    queryData
    |> (DataRepository.get connection >> Report.build)

, Report.build . . , FsCheck, , , . , , - .

, — 1 . ? , , , , .

, . , . , , EntityFramework Dapper, .

Statically Resolved Type Parameters (SRTP)

, .

let inline square
     (x: ^a when ^a: (static member (*): ^a -> ^a -> ^a)) = x * x

, . , , . !

let inline GetBodyAsync x = (^a: (member GetBodyAsync: unit -> ^b) x)

open System.Threading.Tasks
type A() =
    member this.GetBodyAsync() = Task.FromResult 1

type B() =
    member this.GetBodyAsync() = async { return 2 }

A() |> GetBodyAsync |> fun x -> x.Result // 1
B() |> GetBodyAsync |> Async.RunSynchronously // 2

, , , — ! C#.

Computation Expressions

Result. , , Result. , .

,

let res arg =
    match doJob arg with
    | Error e -> Error e
    | Ok r ->
        match doJob2 r with
        | Error e -> Error e
        | Ok r -> ...

type ResultBuilder() =
    member __.Bind(x, f) =
        match x with
        | Error e -> Error e
        | Ok x -> f x
    member __.Return x = Ok x
    member __.ReturnFrom x = x

let result = ResultBuilder()

:

let res arg =
    result {
        let! r = doJob arg
        let! r2 = doJob2 r
        let! r3 = doJob3 r2
        return r3
    }

let! Error e . , Ok r3.

, . DSL.

, , — task & async. , — Async. F#, , cold start, Tasks API. , . :

let myTask =
    task {
        let! result = doSmthAsync() //    await Task
        let! result2 = doSmthElseAsync(result)
        return result2
    }

let myAsync =
    async {
        let! result = doAsync()
        let! result2 = do2Async(result)
        do! do3Async(result2)
        return result2
    }

let result2 = myAsync |> Async.RunSynchronously

let result2Task = myAsync |> Async.StartAsTask

let result2FromTask = myTask |> Async.AwaitTask

(DTO, ) , . 1 , , - .

, F# — , . by design, , . — : , - . , , , C# .

, 7 , 130 .

, . , 1 1 . C# . — , . , , , -. , — pattern matching, , nullable reference types.

, -, , F#, -, . Pattern matching Discriminated unions & Record destruction — , , . Nullable reference types — , Option .

, F# — , "" .

F# — .

, . Property-based (, FsCheck) , QA . - , - . , , , - - . F# . .