Question

I’ve built an F# app and now I’m using EF4.1 (code first) as a data store. I’ve stumbled upon a really quirky issue around typing.

Take the following snippet:

                let result = context
                                .SearchResults
                                .Where((fun (r:SearchResult) -> 
                                        r.Program = request.Program))
                                .OrderByDescending((fun r -> r.AcquisitionDate))
                                .FirstOrDefault()
                match result with
                    | price -> 
                        cacheProvider.Set(result)
                        Some(price)
                    | _ ->
                        ignore(context.SearchRequests.Add(request))
                        ignore(context.SaveChanges())
                        None

when the search result does not exist, the .FirstOrDefault() call returns null. So result is bound to null during runtime. But at compile time the F# compiler expects that result will never be null so I can’t do pattern checking around null (price when price <> null).

What am I missing here? Is there a proper way to do this?

Answer 1

You can test for equality against Unchecked.defaultof<_> instead of null.