F# type inference runs left -> right, top -> bottom. In the first snippet, the lambda is encountered first, and the compiler has no knowledge of what the type of "d" is, so it can't assume d has a ".Hour" member. In the second snippet, the compiler has already seen the input list and knows it contains DateTimes, so when it encounters the lambda, "d" is known to be a DateTime and everything is groovy.
Ah, I see, thanks for that. So it's not running Hindley Milner (or at least, not unless it's quite heavily modified). That's not surprising, I guess, since it takes no small amount of effort to get HM to work in an OO/imperative setting.
