Printing F# discriminated union

Question

I am writing a F# program which parses a string into a AST type which is a discriminated union.

When I use fsi (on Mono + Mac OS X) to run my code, the AST is printed out in a nice format. But when I use printfn "%s" <| ast.ToString() I get something like FSI_0002.Absyn+clazz. Writing a ToString method for all the discriminated union types would be a big chore.

How do I make the value print the way fsi does it?

Answer 1

Have you tried printfn "%A" ast? The %A specifier takes into consideration the StructuredFormatDisplayAttribute^[MSDN], if present.