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.

Answer 2

To convert a discriminated union into a string, you should use sprintf "%A" ast instead of ast.ToString().

If you want Enum.GetName, you can use Microsoft.FSharp.Reflection namespace. See What is the Enum.GetName equivalent for F# union member?.

Answer 3

In addition to Daniel's comment, here is a good blog article explaining how to format it in whatever way you'd wish:

http://blogs.msdn.com/b/dsyme/archive/2010/01/08/some-tips-and-tricks-for-formatting-data-in-f-interactive-and-a-in-sprintf-printf-fprintf.aspx (web.archive.org)