Constraining an F# generic function to a union type?

Question

As per the title, is there any way to constrain an F# generic function to a union type? So far I am using:

let toDomain<'T> external: 'T option =
    assert FSharpType.IsUnion(typeof<'T>)
    ...

Which fails at runtime with a System.ArgumentException if I attempt to use a non-union, but I would prefer the check earlier.

Answer 1

No.

If you peek at the implementation of IsUnion and follow the code a bit, it boils down to checking for the presence of the attribute/argument [<CompilationMapping(SourceConstructFlags.SumType)>].

For now there is no support for purely attribute-based constraints, either in F# or in .NET.