In F#, How do I customize output of a custom type using printf?

Question

I've read through a good chunk of Expert F# and am working on building an actual application. While debugging, I've grown accustomed to passing fsi commands like this to make things legible in the repl window:

fsi.AddPrinter(fun (x : myType) -> myType.ToString())

I would like to extend this to work with the printf formatter, so I could type e.g.

printf "%A" instanceOfMyType

and control the output for a custom type. The book implies that this can be done (p 93, "Generic structural formatting can be extended to work with any user-defined data types, a topic covered on the F# website"), but I have failed to find any references as to how to actually accomplish this. Does anyone know how? Is it even possible?

Edit:

I should have included a code sample, it's a record type that I'm dealing with, e.g.

type myType = 
    {a: int}        
    override m.ToString() = "hello"

let t = {a=5}
printfn "%A" t
printfn "%A" (box t)

both print statements yield:

{a = 5;}

Answer 1

Hmm... I vaguely recall some changes to this, but I forget if they happened before or after the CTP (1.9.6.2).

In any case, on the CTP, I see that

type MyType() =
    override this.ToString() = "hi"
let x = new MyType()
let xs = Array.create 25 x
printfn "%A" x
printfn "%A" xs

when evaluated in the VFSI window does what I would want, and that

x;;
xs;;

also prints nicely. So, I guess I am unclear how this differs from what is desired?

Answer 2

If you override ToString method, that should do.

Answer 3

Imo, this is a bug in the ctp, but if you want to get it to work right now, the only way I know of is to override a deprecated interface, IFormattable. This allows you to control directly what is printed using structural formatting.

See also this thread on hubfs.

Like I said, expect this to change "soon". If you can do without, save yourself the hassle.