[Gavino]

Firstly I must apologise, as it was me who added the second example.
I see now that it's not quite right. It should be:

...
Function CombingInfo(clip c) {
        file = "interlace.log"

        c = FrameEvaluate(c,"
                diff = 0.50*YDifferenceFromPrevious() + 0.25*UDifferenceFromPrevious() + 0.25*VDifferenceFromPrevious()
                b = IsMoving(diff)
                a = IsCombed(combedthreshold)
        ")
        c = WriteFile(c, file, "a", "sep", "b")
        return c
}
...

That is, the WriteFile call should come after the FrameEvaluate().
As it was, the values of a and b were not defined on the first frame (and on later frames wrongly referred to the previous frame). That's because the YDifferenceFromPrevious() requests a frame from WriteFile before the variables have been assigned a value.

The first example works OK because there the input to YDifferenceFromPrevious (etc) is the global clip, and a frame is not requested from WriteFile until all the variables have been assigned.

So your function should work if you move the WriteFile call to be after FrameEvaluate().