Type Alias

This is what I really wish to have in C# language but I always wonder why it's never happened.. In C# we can do type alias on file scope basis. If you ever write a thing like this..

using System;
using PersonAddressDictionary = System.Collections.Generic.Dictionary<string,MyProgram.Address>;
namespace MyProgram {
    public static class Program {
        PersonAddressDictionary personDict = new();  // C# 9 syntax
    }
}

Problem is you need that long line on every C# file that uses the same type. In F#, it has type alias 💖!

open System
module MyProgram
type PersonAddressDictionary = System.Collections.Generic.Dictionary<string,Address>
let personDict = PersonAddressDictionary()

This PersonAddressDictionary type can be reused in other F# files without needing to redeclare the type ever again. I hope C# has a plan for this some day..

Records

Yes, this is the same C# Record feature but in different syntax. While C# uses Kotlin style(which also probably steals from other language too), F#'s record is more verbose.

public record Person(string FirstName, string LastName, int Age);
// create a record value
var person = new Person("John", "Doe", 99);

In F#:

type Person = {
    FirstName: string
    LastName: string
    Age: int
}
// or
// type Person = { FirstName: string; LastName: string; Age: int }
// create a record value
let person = { FirstName = "John"; LastName = "Doe"; Age = 99 }

If you notice, create a record value in F# doesn't need to even specify the record name! F# recognizes the record type by the assignment names.

Just like C#'s record, F# automatically generates code for data structural equality (and hash calculation) of all fields in the record, and also generates code for data comparison too!

Tuple

I assume that our reader knows tuple pretty well from C#. But some may not know that in .NET we have tuple as reference type (System.Tuple) and tuple as value type (System.ValueTuple). In recent C# version, whenever you use the tuple form, it is always ValueTuple. In contrast, Tuple in F# is reference type by default.

type Sample1 = string * int         // equivalent to Tuple<string,int> in C#
type Sample2 = struct(string * int) // equivalent to (string, int) in C#

This may be useful in case you want to tune performance.

List

Anonymous Object

Tagged Union

Option vs Nullable

Result

Pattern Matching

Workflow (Computation Expressions)

Sequence

Query

Task

Async

Unit of Measurement