Setting up Visual Studio Code for F#

  • Cool: you can open a Terminal in VSCode with ^ + `
  • Do I need to install an extension to use F#? Syntax colouring is built in, looks like
  • Let’s try creating an FSX script file
    • Ok, I can do that, it has syntax highlighting, but I have no way to, you know, evaluate the scriptfile
    • Let’s see how you install F# in the modern .NET Core world…
    • I’m pretty sure I already have .NET Core installed via Homebrew
    • Yup, got the dotnet-sdk cask
  • There’s the ionide-f# extension, but its website feels outdated, like it talks about needing Mono to get F#, which I’m pretty sure isn’t true. I want to do the most minimal and most up-to-date thing
  • Ok, this Microsoft doc says “Ionide will generate .NET Framework F# projects, not dotnet core, which can have cross-platform compatibility issues. If you are running on Linux or OSX, a simpler way to get started is to use the command-line tools.”
  • Ah, dotnet --help includes “Additional commands from bundled tools”, which includes fsi, but fsi is not a command on my path currently. So I have to figure out how to execute it… (I wonder if Homebrew Cask installed it, but only put dotnet in my path…)
  • Ohhh… the first post in this thread inadvertently demonstrates that you use it like this: dotnet fsi 😏
  • When I run it, this message appears:
    > Failed to install ctrl-c handler - Ctrl-C handling will not be available. Error was:
          Could not load file or assembly 'Mono.Posix, Version=, Culture=neutral, PublicKeyToken=0738eb9f132ed756'. The system cannot find the file specified.
  • Nonetheless, if I write printfn "Hellow World!";;, it obediently prints it, and its evaluation that val it : unit = ()
  • From here, we should be able to just provide the filename of an FSX file to FSI, and it’ll run it. Let’s try
  • > dotnet fsi ~/dev/dotnet-projects/fsharp/test.fsx
  • Despite that error message up there, hitting ^C did indeed terminate the interactive prompt
  • Now let’s try that command above to run the script
  • Yup, it prints Hello World! Success! 🤩
  • Ah, from the FSharp tutorial I linked above, you can run commands from the interactive prompt by prefixing them with #, e.g. #quit quits the prompt
  • Ok, cool: I can write scratch code in an FSX file and then run it at the built-in terminal. This will come in useful as I read the book version of FSharpForFunAndProfit


  • It looks to me like this is what you need the Ionide extension for VSCode for. Let’s try installing it and see if it does anything to my FSX file
  • Ionide-fsharp - “This extension is recommended based on files you recently opened”, I reckon that’s a good sign.
    • Its last update was six days ago, which is also a good sign
  • Ah, I have a little button in the upper-right of the tab bar that lets me run the script or send to FSI. When I try to run script, it opens the terminal and says “fsharpi: command not found”, so evidently it’s trying to use some runner that I don’t have 🤔
  • Not sure how to get it to show me Intellisense errors, either. Might only work on files inside the typical .NET Solution/Project structure?
  • Annoying: There are a set of commands added to the VSCode command bar prefixed with F#:, but if you type F#: into the bar, it ignores the non-alphanumeric characters and finds every fucking command with an F in it, and of course the F#, perhaps because they’re newest, are all at the bottom of the list 🙄
  • Intellisense is clearly not working, because I wrote the word “inference” in a comment, declared a value i, and when I typed i elsewhere (to assign the value as one of the values in a tuple), VSCode’s default Intellisense helpfully suggested “inference” instead of the, you know, actual symbol…
  • Well, let’s try creating a project from the command bar and see what Ionide does with it
  • It complains there’s no open folder. True! Let’s open ~/dev/dotnet-projects/fsharp
  • I wonder if it can’t evaluate my file because of the missing fsharpi
  • Ooookay: the New Project thingy asked for a folder name and a project name and then said it succeeded, but I don’t see a new folder or project anywhere that I’d expect…
  • The F#: Get Help command does sweet fuck all
  • Maybe I should restart Visual Studio?
  • Nah that didn’t make a difference
    • This extension’s lack of feedback is frustrating. It just doesn’t work and doesn’t seem to indicate why
  • This troubleshooting section has some things to try
    • this suggests I need to install mono…

Time Passes…

Getting F# Projects Working

  • Get Started with F# in Visual Studio Code - Microsoft Docs looks like a promising walkthrough
  • I already have Ionide installed in VSCode (see above), so let’s see if I can just create a project on the CLI and load it
  • Cool, that’s working. I’ve got an Ionide icon in the sidebar now that shows me the solution explorer; looks like Intellisense works!
  • Nice: the scripting example works, too. If I save a file as .fsx (I’m saving it in the project’s directory, I don’t know if this is required, or if it’ll parse any FSX file)
    • I just tested it: it parses an FSX on my Desktop just fine 👍🏼
  • Sweet: I now have a programming environment I can use to learn more about F# and write some useful programs with it.