Getting the types in an F# union type II -- the perils of relying on undocumented implementation details
Recalling this example --
It was adequate for the use case I had of it at the time; but then I tried a different union type, at which point a bug showed up
FSI_0003+Card System.NullReferenceException: Object reference not set to an instance of an object. at FSI_0005.it@24-1.Invoke(Type x) at Microsoft.FSharp.Collections.SeqModule.Iterate[T](FSharpFunc`2 action, IEnumerable`1 source) at
.$FSI_0005.main@() Stopped due to error
And, of course, that is exactly what I deserved for relying on the undocumented internals.
A quick decompilation reveals this to be because union cases that carry no data are actually concrete base type instances, bearing the tag number as data, like --
rather than being redundant empty subtypes.
So we need to tweak our function to be
and all is well. For the moment. Until the representation changes.