Setting Up Your Environment

Editor

• Visual Studio Code with the Ionide-fsharp extension — free, cross-platform, and the most popular choice. It gives autocompletion, inline type hints, error checking, and "send to F# Interactive".
• JetBrains Rider — a full .NET IDE with excellent F# support.
• Visual Studio (Windows) — install the ".NET desktop development" workload, which includes F#.

F# Interactive (the REPL)

Run dotnet fsi to get an interactive prompt. End each input with ;; and F# prints the value and its type:

$ dotnet fsi

Microsoft (R) F# Interactive version 15.2.401.0 for F# 10.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> 1 + 2;;
val it: int = 3

> let square x = x * x;;
val square: x: int -> int

> square 7;;
val it: int = 49

> #quit;;

it is the value of the last expression. square: x: int -> int means "a function taking an int called x and returning an int" — you'll read signatures like this constantly.

Scripts and projects

• Scripts (.fsx) run directly: dotnet fsi hello.fsx. Most examples in this course are scripts.
• Projects (.fsproj) are for applications and libraries: dotnet new console -lang F# creates one, dotnet run runs it.

Formatting

Fantomas is the standard F# code formatter; Ionide and Rider can run it on save.