How do I customize output of a custom type using printf?

It looks like the Right Way to do this in F# 2.0 is by using the StructuredFormatDisplay attribute, for example:

[<StructuredFormatDisplay("hello {a}")>]
type myType = {a: int}

In this example, instead of the default {a = 42;}, you would get hello 42.

This works the same way for object, record, and union types. And although the pattern must be of the format "PreText {PropertyName} PostText" (PreText and PostText being optional), this is actually more powerful than ToString() because:

1. PropertyName can be a property of any type. If it is not a string, then it will also be subject to structured formatting. Don Syme’s blog gives an example of recursively formatting a tree in this way.

2. It may be a calculated property. So you could actually get ToString() to work for record and union types, though in a rather round-about way:

   [<StructuredFormatDisplay("{AsString}")>]
   type myType = 
       {a: int}
       override m.ToString() = "hello"
       member m.AsString = m.ToString()  // a property that calls a method
   

By the way, ToString() will always be used (even for record and union types) if you call printfn "%O" instead of printfn "%A".