Inconsistent behaviour of Seq.sort and Seq.sortBy for sequence of floats containing a NaN

Question

When sorting a sequence of floats containing a NaN, Seq.sort puts the NaNs at the head of the result:

> [ 0.0; nan; 1.0; nan; -1.0 ] |> Seq.sort
val it : seq<float> = seq [nan; nan; -1.0; 0.0; ...]

However, Seq.sortBy appears to fail, and simply passes the sequence on, unsorted. Presumably this is because NaN violates basic ordering principles.

> [ 0.0; nan; 1.0; nan; -1.0 ] |> Seq.sortBy id
val it : seq<float> = seq [0.0; nan; 1.0; nan; ...]

No exception is thrown which might indicate that sortBy failed to produce a sorted list, and this can lead to surprising behaviour in code that relies on it. It's easy to code around the problem once you find it, but harder to anticipate, and so likely to cause bugs.

Is there a good reason for sort and sortBy being inconsistent in this way?

Answer 1

This issue is fixed upstream: https://github.com/Microsoft/visualfsharp/issues/370