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F# Overview for C# developers

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This page intends to overview F# features to you. Once you finish this, you should be able to write simple F# code with functions :)

F# Introduction

The key difference between F# and C# is that F# is a function-first language, meaning it expects us to write code full of functions and lambdas, and less of classes or even none.

This article is not intended for introducing functional programming. I assume that the reader should have some knowledge about functional programming (i.e. its principles).

Let's start from the high-level view...

Project and file structure

Here's the list of F# file extensions:

File ExtensionDescription
.fsF# source file
.fsxF# script file
.fsprojF# project file

fsx is similar to .csx (C# Script File) but it has a nice feature where you can reference libraries from nuget.org with #r directive in the file, pretty handy for writing a power script.

In .NET Core, you can run a script by dotnet fsi script.fsx (and dotnet fsi alone is for F# interactive mode).

Project Structure

Basically, F# project is very like C# project, just a group of .fs files but the big difference is on their distinct ideologies. C# is OO language which usually has many classes, divided into namespaces or packages. It doesn't matter how many .cs files you have, when you compile them, it just becomes a pool of classes which can freely refer to each other (cyclic dependency!? no problem! 😂).

F# is based on top-down, modular design. Functions are often put in modules, each module should be in its .fs file, and the order of files are important! Order of functions in each .fs file is important as well, functions that are declared on top-most of file cannot refer to functions declared later.

For example, supposed that we have a F# project with two modules A and B and the project structure looks like following:

MyProject.fsproj
|- A.fs
|- B.fs

Any functions in module A.fs cannot refer to anything in B.fs.

Also, if module A has following functions:

let f() = g() // compilation error!
let g() = printfn "I'm G"

It won't work, f() won't recognize g(). Writing code in F# needs you to plan your code dependencies.. :)

Program entrypoint

In C# we need a static Main method defined as an entry point. Normally we often see this in a console program.

// Program.cs
using System;
class Program {
static void Main(string[] args) {
Console.WriteLine("Hello world!");
}
}

In F#, there are two ways to define an entrypoint...

Top-level statements

Does the topic sound familiar 🤭? Yes, it's the new feature in C# 9 that I believe taking from F#.

// Program.fs
printfn "Hello world!"

Similar to C#, top-level statement style can be used only in the main program file.

Explicitly declaring an "EntryPoint"

This one looks similar to C#'s.

// Program.fs
[<EntryPoint>]
let mymain args =
printfn "Hello world!"

To make it more alike, it can also be..

// Program.fs
module Program
open System
[<EntryPoint>]
let Main(args: string[]) =
Console.WriteLine("Hello world!")

F# vs C# syntaxes

Using module / namespace

Basically, F# open keyword is like C# using. We use using NameSpace.Name; to include everything in that namespace, in F# we use open NameSpace.Name instead. Note that F# languages doesn't use semi-colon to close statements like C#, though it has other meaning.

open in F# not only for including things in namespaces, it's also used for including everything in a "module". You can simply think of F# module as a static class, this analogy works for me.

.NET Attribute

F# uses [<Attribute>] style to apply an attribute, where C# just uses [Attribute]. For multiple attributes, you can use [<Attribute1; Attribute2; Attribute3(Param=123)>]. Another note, F# uses semi-colon for separating items, comma is preserved for tuple..

Simple Function Declaration

F# uses keyword let to define "variable" and function. Since in FP point-of-view, a function is just a value. Defining a function in F# is as same as defining a "variable". (I know using the word "variable" here is technically wrong, that's why I put double-quote around the word but I'll omit it from now on.)

Here, an example of defining a function and variable.

let PI = 3.141 // variable
let radius x = x * PI / 180.0 // function

F# type interference is more powerful than C#, I believe, due to language's simpler grammar, so type annotation is often an option. From the example, x will be known as float (which is double type in C#) because it deducts from type of PI and 180.0.. Note that you can't write let radius x = x * PI / 180 because 180 is a literal of int and it doesn't work with float type. F# is type-strict language! (and hence more powerful type interence)

But if you want to annotate a type, you can use name: type style (similar to TypeScript and many languages) to do it.

let PI: float = 3.141
let radius (x: float) :float = x * PI / 180.0

Technically, radius is a lambda, this is like, tho not exactly, doing this Func<double,double> radius = x => PI / 180; in C#.

printf, printfn

If you have C/C++ background, you will immediately familiar with the function name. It's quite same concept but with stronger-typed 😊

open System
Console.WriteLine("There are {0} people in {1}.", 10_000_000, "Bangkok")
printfn "There are %d people in %s." 10_000_000 "Bangkok"

Those statements have the same output. But printfn may look odd to ones who never heard of curry function style, which I will have another dedicated page for curry style.

Multi-variable Function (C# style)

C# Data Types Mappings

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