I reported earlier that the N-Queens problem on F# was too slow. I translated the benchmark in C# and it ran 2X faster than F#. It was essentially the same program… lots of recursion and list processing. So how can F# be 2X slower than C# for the same code? One simple instruction: isinst. I learned long ago that type checking on the CLR is too slow for dynamic languages. Though F# is statically typed, it still performs type checking on discriminated union types. Specifically, operations on lists must always check if the list is null. F# does this by checking if the object is a Nil class: “obj is Nil” in C#. Type checking is extremely slow and should be avoided whenever possible in hotspots. List processing is definitely a hot spot.
When I rewrote the N-Queens problem I used my own list class which used null to represent Nil. Checking “obj == null” is extremely fast. I then modified my list to use a Nil class and performance dropped down to F# speed. The solution I used with my Scheme compiler was to use null to represent the end of the list. An alternative is skip the nil check and wrap the list access in a try-catch. It only fails in the exceptional situation anyway, since properly written code will check for nil before it calls hd. This should speed up list performance dramatically.