Why is this F# code so slow?

Solution 1:

The problem is that the min3 function is compiled as a generic function that uses generic comparison (I thought this uses just IComparable, but it is actually more complicated - it would use structural comparison for F# types and it's fairly complex logic).

> let min3(a, b, c) = min a (min b c);;
val min3 : 'a * 'a * 'a -> 'a when 'a : comparison

In the C# version, the function is not generic (it just takes int). You can improve the F# version by adding type annotations (to get the same thing as in C#):

let min3(a:int, b, c) = min a (min b c)

...or by making min3 as inline (in which case, it will be specialized to int when used):

let inline min3(a, b, c) = min a (min b c);;

For a random string str of length 300, I get the following numbers:

> levenshtein str ("foo" + str);;
Real: 00:00:03.938, CPU: 00:00:03.900, GC gen0: 275, gen1: 1, gen2: 0
val it : int = 3

> levenshtein_inlined str ("foo" + str);;
Real: 00:00:00.068, CPU: 00:00:00.078, GC gen0: 0, gen1: 0, gen2: 0
val it : int = 3