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From: =?ISO-8859-1?Q?J=E9r=E9mie?= Dimino <jeremie@dimino.org>
To: Alex Rubinsteyn <alex.rubinsteyn@gmail.com>
Cc: Caml List <caml-list@inria.fr>
Date: Wed, 07 Dec 2011 18:10:58 +0100
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Subject: Re: [Caml-list] Generic printer patch

Le mercredi 07 décembre 2011 à 11:46 -0500, Alex Rubinsteyn a écrit :
> Cool! Thanks for writing this. Can you explain how defining a custom
> printer works? The new language features still bewilder me.

If you want to define a custom printer for a type named foo, you have to
define a function named string_of_foo. It must be in the same module as
foo. It takes as arguments printers for its type variables, the value to
print and returns a string.

For example, if your type is:

  type ('a, 'b) foo

a custom printer for foo will have the signature:

  val string_of_foo : ('a -> string) -> ('b -> string) -> ('a, 'b) foo -> string

For example:

  module Id : sig
    type t
    val string_of_t : t -> string
    val create : unit -> t
  end = struct
    type t = int
    let string_of_t x = "<id=" ^ string_of_int x ^ ">"
    let next = ref 0
    let create () = incr next; !next
  end

You may also want to use "show" in your custom printer. In order to tell
show to use the printers given as arguments, you must use type
parameters like that:

  let string_of_foo (type a) (type b) string_of_a string_of_b (x : (a, b) foo) =
    ...

For example to export a generated printer for an abstract type:

  module M : sig
    type ('a, 'b) t
    val string_of_t : ('a -> string) -> ('b -> string) -> ('a, 'b) t -> string
  end = struct
    type ('a, 'b) t = A of 'a | B of 'b
    let string_of_t (type a) (type b) string_of_a string_of_b (x : (a, b) t) = show x
  end

Cheers,

-- 
Jérémie