Simple and helpful tips for F# Beginners

#functional #fsharp #beginners #tutorial

I wanted to learn functional programming, and since I'm a .NET guy, so I started learning F#.

It's funny to start learning something that different after years of programming in C#. The syntax itself seems so unintuitive, although design choices make sense.

And since I'm doing this in .NET Core 3.1 it's a bit different from many tutorials I managed to get. Like, no fsc (F# compiler) available in the commandline, and so on.

Therefore I decided to share my steps in the form of tips. If someone has problems with learning F# perhaps they can learned on my mistakes and save some time. By no means this post is to be considered different than a simple developer diary from learning something everyone else already does ;-)

My HelloWorld application

Using VS Code and the command line I created a project. First I executed this in the command line:

dotnet new console -lang F# -o SimpleExample

Then I added some code:

open System

[<EntryPoint>]
let main argv =
    let square x = x * x
    let squared = List.map square [1;2;3;5;7]
    0 // return an integer exit code

And run it:

dotnet run

You'll never guess... I never saw an output ;-)

Printfn cheatsheet

Since running this code with dotnet run did not view any results (I am using VSCode) I had to write a printfn to see what is happening:

open System

[<EntryPoint>]
let main argv =
    let square x = x * x
    let squared = List.map square [1;2;3;5;7]
    printfn "%s" squared
    0 // return an integer exit code

But this resulted in an error.

This expression was expected to have type 'string' but here has type 'int list'

The %s is ofcourse the simplest form of transforming value into message you can find in any tutorials. But, unlike C#, values are not converted to string by default. F# likes to brag that thanks to it's powerful type inference system you almost never have to specify the type of an object. Well, you learn the almost never quite fast to be honest ;-)

In this case you just had the use this:

printfn "%A" squared

This basically converts the entire array into strings - showing it's values like this:

[1; 4; 9; 25; 49]

But sometimes you might have different values to view in the output window. So I prepared a small cheatsheet of types I found and how to represent them:

Format	Type
%s	string
%i	integer
%u	unsigned integer
%f	float
%b	boolean
%O	other objects
%o	octal
%x	lowercase hex
%X	uppercase hex
%A	native F# types

What are native F# types? Non-primitive types. Like: arrays, tuples, records and union types. Probably much more - to be learned.

Spaces at the end matter.

Having the correct code resulted in a new error:

error FS0597: Successive arguments should be separated by spaces or tupled, and arguments involving function or method applications should be parenthesized

Pointing to the beginning of the word "squared".
Guess what... There was a single space after squared. Removing it fixed the bug. WHITESPACES MATTER, PEOPLE! ;-)

Now, let's get back to learning!