R1D23 Reverse Polish Hello World

I took a brief break from C# and cracked open a book I got a few months ago about the F# programming language. Functional programming is making a comeback it seems. I’ve done some work in Clojure and Lisp in the last few years. In addition I heard Javascript referred to as “Lisp in C’s Clothing” so that might count as well.

If JS is Lisp in C’s Clothing, I have no idea what clothing F# is wearing.

Rather than the traditional “Hello World” the first bits of code that we wrote was an implementation of a reverse polish notation calculator program.

Reverse Polish Notation Calculator in F#

The code for the function itself reminds me of writing grammars for ANTRL. I have no idea what “|” “|>” or “::” are doing in this context, but I can’t wait to find out. I was starting to get discouraged, but then following this code example the author reassures us.

“Don’t be discouraged if the RPN calculator code doesn’t make much sense right now; that’s the point! ”

Excerpt From: Dave Fancher. “The Book of F#: Breaking Free with Managed Functional Programming.” iBooks.

I am excited that I got all of this working out of the box on my Macbook. F# comes baked into the latest version of the dotnet core SDK. You can start a new F# project with the following incantation:

dotnet new console -lang F# -n MyFirstFSharpProject

You can run it with:

dotnet run

Like any good Lisp, F# comes with a built in REPL. It seems you need to install mono in order to get this to work. I was able to do it with homebrew.

brew install mono

Then you can fire up an F# repl with


You can test it out and make sure it works with a simple example.

> let greeting = "Hello from the F# REPL!"
- greeting;;

// output should be 
val greeting : string = "Hello from the F# REPL!"
val it : string = "Hello from the F# REPL!"

I’m looking forward to learning a bit more F#. It is one of the out of the box supported languages on Azure Notebooks.

Leave a Reply