[CmonDev]

"I think that using a custom build step is "cheating" to some degree in that you can add arbitrary features by adding language-external post build processing"

I guess, but: source code is still standard C#, it gives you pre-runtime static verification. As far as I understand Roslyn will support this kind of extensibility in a straightforward way. It is still better than unit testing same stuff in my opinion (except for specifying units in comments).

"By being built into the language" - do you mean F# code that uses units of measure wouldn't compile to OCAML (I am talking about simple compilability rather than proper support)?

[kvb]

It's very nice to be able to check annotations in a build step. I'm just saying that adding language-external checks makes it fairly meaningless to say that "C#" supports it; would it be fair to say that C# supports all of Haskell's features if I require annotating C# constructs with Haskell code and then use a modified Haskell compiler in a post-build step to verify that the pieces are composed in a valid way? None of this is to say that the C# code you linked to is not useful - it's just not possible to use "vanilla" C# to achieve what "vanilla" F# can do.

To your later question, while F# and OCaml share a common core, there's lots of F# code that won't compile as OCaml and vice versa, and units of measure are one such example. The syntax for measure-annotated types is not valid OCaml syntax.