Building an F# console app via CLI using the .NET 5 SDK. Image by Author.

F# Compiler Messages

Well 2020, it’s time for my annual (F#) blog post.

I wrote my first FsAdvent post in 2018.

And then I followed up with another post in 2019.

So, what’s happened in my F# world since then? For one, I joined the amazing Quicken Loans so I could do F# full time. It’s been awesome! I also signed up for the most recent round of the F# Software Foundation’s Mentorship Program again — this time as both a mentor and mentee. I think I’ll do one or the other next time!

This year, I’d like to share a small contribution I made to F# documentation and invite you to participate.

Backstory

I was reviewing some code at work recently and I noticed that one of our F# project files was ignoring a few warnings, but I didn’t know what they were. It looked something like this:

F# project file ignoring FS0025

Now imagine we had a code file like this:

F# implementation file with incomplete pattern match

The F# compiler normally yields a warning when it finds an incomplete pattern (not all cases are covered). However, because of the <NoWarn>0025</NoWarn> node in the project file, the build output does not include the warning:

Build succeeded.
    0 Warning(s)
    0 Error(s)

Without the <NoWarn> node, the build output is:

/somepath/FsAdvent2020.fs(8,11): warning FS0025: Incomplete pattern matches on this expression. For example, the value 'Hot' may indicate a case not covered by the pattern(s). [/somepath/FsAdvent2020.fsproj]
    1 Warning(s)
    0 Error(s)

Now, how would an F# developer know what FS0025 is? Well, they’re in luck …sort of. If they visit the F# compiler messages page, they’ll see a list of compiler messages. However, it’s far from complete. At the time of this writing, the table of contents looks like this:

Table of Contents on the F# compiler messages page

How I Contributed

I didn’t know about the F# compiler messages page I mentioned above. I did a web search for a few F# specific errors and landed on a GitHub issue from December 2019, Documentation pages for F# compiler errors like C#. Perfect! Chet Husk did all of the hard work in January 2020. Then, with a little encouragement and guidance, I made some enhancements to the helper script and added message 0025 on incomplete pattern matching:

FS0025 by brettrowberry · Pull Request #21424 · dotnet/docs

How You Can Contribute

1. Use https://github.com/dotnet/docs/pull/21424 as a guide.
2. See https://github.com/dotnet/docs/issues/16177#issuecomment-724426546 for tips on where to find the error numbers.

Good luck, and thank you for contributing to making F# better each and every year!

This post is part of #FsAdvent 2020 organized by Sergey Tihon.
For a listing of all posts, see the F# Advent Calendar in English 2020.
Thanks, Sergey!