Because it’s great, and people that think otherwise are dead to me.
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It's the one programming language that changed how I think about programming.
I'm only talking about the version before type providers. Then it got messy.
Before that, we could (and I did) recompile fsi.exe to do some custom prompt manipulation. It was a slog, but it worked, but then Microsoft faded from my life. Still, that early version (I believe 2.0) F# is just magnificent.